Add feature flags to the framework #168437

loic-sharma · 2025-05-07T03:12:21Z

Motivation

We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example:

Example feature flags:

Android sensitive content: [Android] Implement setting content sensitivity on Android #158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password.
Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window.

Use case

Users will be able to turn on features by:

Option 1: Run flutter config --enable-my-feature. This enables the feature for all projects on the machine
Option 2: Add enable-my-feature: true in their pubspec.yaml, under the flutter section. This would enable the for a single project on the machine.

Turning on a feature affects both development-time (flutter run) and deployment-time (flutter build x). For example, I can flutter build windows to create an .exe with multi-window features enabled.

How this works

This adds a new runtimeId property to the tool's Feature class. If a feature is on and has a runtimeId, its runtimeId will be stamped into the Dart application as a Dart define. The framework uses this Dart define to determine which features are enabled.

Multi-window example

#168697 shows how this new feature flag system can be used to add a multi-window feature flag:

It adds a new multi-window feature to the Flutter tool. This can be turned on using flutter config --enable-multi-window or by putting enable-multi-window: true in an app's .pubspec, under the flutter section.
It adds a new isMultiWindowEnabled property to the framework.
The Material library can use this new property to determine whether it should create a new window. Example.

Limitations

Tool and framework only

For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine.

For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the AndroidManifest.xml or Info.plist files.

This is a fixable limitation, we just need to invest in this plumbing :)

Tree shaking

Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps.

Pre-launch Checklist

I read the Contributor Guide and followed the process outlined there for submitting PRs.
I read the Tree Hygiene wiki page, which explains my responsibilities.
I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
I signed the CLA.
I listed at least one issue that this PR fixes in the description above.
I updated/added relevant documentation (doc comments with ///).
I added new tests to check the change I am making, or this PR is test-exempt.
I followed the breaking change policy and added Data Driven Fixes where supported.
All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

matanlurey

Let's revisit adding 5 device-lab integration tests for this feature.

.ci.yaml

packages/flutter_tools/lib/src/build_system/targets/common.dart

justinmc

I'm excited about this. I left a bunch of comments and questions, plus a few below.

How will we communicate to people how to create and use feature flags? I'm wondering if we need more docs in this PR, or maybe just a separate doc or guide somewhere.
We should also communicate the limitations that we discussed for this approach, like that it can't be used in the engine at startup time.
Does the pubspec.yaml setting override the flutter config setting if they conflict?
Currently is the idea to try to prevent app developers from using these feature flags? It looks like that's not 100% possible. Are we should we don't want app developers to use them?
What if we (or maybe an app developer) wants to conditionally import some dart code depending on whether or not a feature flag is set, is that possible?

packages/flutter/lib/src/services/features.dart

packages/flutter_tools/lib/src/build_system/targets/common.dart

packages/flutter_tools/test/general.shard/build_system/targets/common_test.dart

justinmc · 2025-05-16T17:41:09Z

packages/flutter/lib/src/services/features.dart

Would this make more sense to put in the foundation library? I think services has some dependencies like gestures, which wouldn't be able to access the feature flags.

I also think that we should avoid importing this file relatively and should import it from its library. There's something about this in the styleguide: https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md#import-conventions

I moved this down to the foundation library!

For relative file imports, see reply here: #168437 (comment)

justinmc · 2025-05-16T17:46:00Z

packages/flutter/lib/src/services/features.dart

+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+


I was trying to think of a way around needing to add a getter here for each feature flag (e.g. isMultiWindowEnabled), but that's probably the best. Better than needing to access the feature flag by string. It will allow us to really easily find all uses of any feature flag.

Also RE: using @internal for isMultiWindowEnabled. It looks like that would be the first time that the framework has used @internal. I'm not aware of any hard rule against it though.

Users could work around this by accessing debugEnabledFeatureFlags or the environment variable directly.

Are we sure that we don't want users to use our feature flags?

@justinmc I'm leaning towards being able to remove feature flags at any point without it being a breaking change. If we accept that requirement, using @internal is preferable over @visibleForTesting as @visibleForTesting makes it OK to use this member in a test.

However, @internal has the requirement that the member is not exported. In other words, using @internal requires us to use relative imports instead of import from its library.

While I prefer @internal, I'd be OK with just sticking to @visibleForTesting, exporting this file, and using library imports. The drawback is that feature flags will then fall under the normal deprecation policy. I don't think that's the end of the world though.

For others reading this thread, we decided to switch to @internal.

packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart

matanlurey

LGTM with test nit.

bkonyi

LGTM with some nits.

packages/flutter_tools/lib/src/runner/flutter_command.dart

packages/flutter_tools/test/general.shard/features_test.dart

justinmc

Overall this approach looks good. See my questions in my first review by the way: #168437 (review). A few more below.

Recently @\jonahwilliams mentioned possibly wanting to use a feature flag in the rasterizer, and that this approach wouldn't work for that. Does that change anything here? I know these feature flags also can't be in the engine at startup time either so probably irrelevant to this proposal, but wanted to double check.

Testing the framework in the era of feature flags

It's possible that someone opens a PR in flutter/flutter that breaks a feature that is behind a feature flag without realizing it.

When we land feature flags, will we have to test each PR with all combinations of feature flags in order to be sure that it works? In practicality I think the answer is probably that we'll either test with all of them or none of them. Is that acceptable, and which one is it? If it's all of them, should there be an easy way to enable all feature flags?

justinmc · 2025-06-07T00:15:17Z

packages/flutter/lib/src/foundation/_features.dart

Just double checking, the underscore in the filename has no functional meaning to Dart right?

Yup, I confirmed with the Dart team that a _ file name prefix does not affect semantics.

packages/flutter/lib/src/foundation/_features.dart

packages/flutter_tools/lib/src/build_info.dart

matanlurey · 2025-06-11T17:37:40Z

It's possible that someone opens a PR in flutter/flutter that breaks a feature that is behind a feature flag without realizing it.

When we land feature flags, will we have to test each PR with all combinations of feature flags in order to be sure that it works? In practicality I think the answer is probably that we'll either test with all of them or none of them. Is that acceptable, and which one is it? If it's all of them, should there be an easy way to enable all feature flags?

Let's handle this discussion outside of this PR, we could use a doc (Feature-Flags.md) helping the team here.

The short answer is no, unless there is a framework wide disruptive flag (like --enable-impeller), we would not increase our test matrix infinitely, and there would not be a plan to "enable all feature flags" - that would be a signal the feature flags are too intertwined.

#170456.

loic-sharma · 2025-06-12T22:23:33Z

Responding to @justinmc's top-level review comments:

See my questions in my first review

Oops I missed those!

How will we communicate to people how to create ... feature flags?

Good question, we can add guidance for this as part of the Feature-Flags.md doc Matan suggested :)

How will we communicate to people how to ... use feature flags?

This can be documented on a feature-by-feature basis. For example, we've had separate docs / instructions for how to enable Impeller, how to enable thread merging, etc. I think that model has worked well so far.

Does the pubspec.yaml setting override the flutter config setting if they conflict?

Yes that's correct. Feature flags have the following precedence:

Your project's .pubspec file
Your global config settings (set using flutter config)
Environment variables

Currently is the idea to try to prevent app developers from using these feature flags? It looks like that's not 100% possible. Are we should we don't want app developers to use them?

Agreed it's not 100% possible. The idea is to allow ourselves to remove feature flags once a feature has shipped. Removing feature flags is key to ensuring we don't have a combinatorial explosion of feature flag combinations to test. If needed, we can loosen this restriction in the future.

What if we (or maybe an app developer) wants to conditionally import some dart code depending on whether or not a feature flag is set, is that possible?

Could you expand on what scenarios you have in mind for this?

I believe that's not possible right now. I believe neither Android sensitive content or desktop multi-window would need this.

Recently @\jonahwilliams mentioned possibly wanting to use a feature flag in the rasterizer, and that this approach wouldn't work for that. Does that change anything here?

I added a "Limitations" section to this pull request's description to call this out.

To address that limitation, for each platform we'll need to stand up another path to flow feature flags information to the platform's embedder and the engine. The exact details haven't been finalized yet. One option is we might generate platform-specific configuration (like AndroidManifest.xml for Android or Info.plist for iOS) to pass feature flags to the embedder. Another option is we might generate a platform-agnostic configuration file that the embedder/engine can read to determine feature's state. Anyhow, whatever we do there would be additive and would not change anything in this PR.

justinmc · 2025-06-13T20:59:10Z

Thank you for all of the answers!

About conditionally importing based on a feature flag, I think I was just noting that code that is feature flagged off will still be imported into user's apps. That's probably fine but maybe just a limitation to be aware of.

About the order of precedence, consider documenting that order somewhere if you haven't already.

Everything else sounds good. I'll plan to re-review this after the dependent PRs are merged.

Closes #170456. There are still some TODOs I'd like to address, but in follow-up PRs: - Update the `# Framework` sub-sections after #168437 merges - Add known limitations (i.e. embedder/engine) - Add best practices (using a feature flag versus just using a parameter, named constructor, etc)

In the Flutter framework, only the `foundation` library is allowed to import `package:meta`. If other parts of the Flutter framework want to use `package:meta`, they must do so through the `foundation` library by exporting the desired `package:meta` member. This requirement is enforced through `dev/bots/analyze.dart` [here](https://github.com/flutter/flutter/blob/master/dev/bots/analyze.dart#L2749-L2750). This exports the `internal` attribute so that it can be used by the Flutter framework. This change was split off from #168437 as it might cause problems in google3 if there is code that imports both `package:meta/meta.dart` and `package:flutter/foundation.dart`. I will get a test exemption. Part of: #167668 ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 2. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 3. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

This relands #168437. The google3 fixes were landed in: #171547, https://critique.corp.google.com/cl/781275353, #171933. This PR is split into two commits: 1. d625379, code from #168437 without any changes 2. f35d29e, updates the PR to omit obvious types. Original PR description: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: #158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 3. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example #168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 4. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps.

Closes flutter#170456. There are still some TODOs I'd like to address, but in follow-up PRs: - Update the `# Framework` sub-sections after flutter#168437 merges - Add known limitations (i.e. embedder/engine) - Add best practices (using a feature flag versus just using a parameter, named constructor, etc)

In the Flutter framework, only the `foundation` library is allowed to import `package:meta`. If other parts of the Flutter framework want to use `package:meta`, they must do so through the `foundation` library by exporting the desired `package:meta` member. This requirement is enforced through `dev/bots/analyze.dart` [here](https://github.com/flutter/flutter/blob/master/dev/bots/analyze.dart#L2749-L2750). This exports the `internal` attribute so that it can be used by the Flutter framework. This change was split off from flutter#168437 as it might cause problems in google3 if there is code that imports both `package:meta/meta.dart` and `package:flutter/foundation.dart`. I will get a test exemption. Part of: flutter#167668 ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 2. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 3. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…r#171542)  Reverts: flutter#168437   Initiated by: chingjun   Reason for reverting: broke internal customers   Original PR Author: loic-sharma   Reviewed By: {bkonyi, justinmc, matanlurey}   This change reverts the following previous change: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 2. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 3. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md  Co-authored-by: auto-submit[bot] <[email protected]>

This relands flutter#168437. The google3 fixes were landed in: flutter#171547, https://critique.corp.google.com/cl/781275353, flutter#171933. This PR is split into two commits: 1. d625379, code from flutter#168437 without any changes 2. f35d29e, updates the PR to omit obvious types. Original PR description: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 3. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 4. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps.

## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 2. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 3. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

…r#171542)  Reverts: flutter#168437   Initiated by: chingjun   Reason for reverting: broke internal customers   Original PR Author: loic-sharma   Reviewed By: {bkonyi, justinmc, matanlurey}   This change reverts the following previous change: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 2. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 3. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps. ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord].  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md  Co-authored-by: auto-submit[bot] <[email protected]>

This relands flutter#168437. The google3 fixes were landed in: flutter#171547, https://critique.corp.google.com/cl/781275353, flutter#171933. This PR is split into two commits: 1. d625379, code from flutter#168437 without any changes 2. f35d29e, updates the PR to omit obvious types. Original PR description: ## Motivation We'd like to let users opt-in to experimental features so that they can give early feedback while we iterate on the feature. For example: Example feature flags: 1. Android sensitive content: flutter#158473. When enabled, Flutter will tell Android when the view contains sensitive content like a password. 3. Desktop multi-window. When enabled, Flutter will use child windows to allow things like a context menu to "escape" outside of the current window. ### Use case Users will be able to turn on features by: * **Option 1**: Run `flutter config --enable-my-feature`. This enables the feature for all projects on the machine * **Option 2**: Add `enable-my-feature: true` in their `pubspec.yaml`, under the `flutter` section. This would enable the for a single project on the machine. Turning on a feature affects _both_ development-time (`flutter run`) and deployment-time (`flutter build x`). For example, I can `flutter build windows` to create an `.exe` with multi-window features enabled. ## How this works This adds a new [`runtimeId`](https://github.com/flutter/flutter/pull/168437/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R243-R247) property to the tool's `Feature` class. If a feature is on and has a `runtimeId`, its `runtimeId` will be [stamped into the Dart application as a Dart define](https://github.com/flutter/flutter/pull/168437/files#diff-bd662448bdc2e6f50e47cd3b20b22b41a828561bce65cb4d54ea4f5011cc604eR293-R327). The framework uses this Dart define to [determine which features are enabled](https://github.com/flutter/flutter/pull/168437/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45). ### Multi-window example flutter#168697 shows how this new feature flag system can be used to add a multi-window feature flag: 1. It adds a new [multi-window feature](https://github.com/flutter/flutter/pull/168697/files#diff-0ded384225f19a4c34d43c7c11f7cb084ff3db947cfa82d8d52fc94c112bb2a7R189-R198) to the Flutter tool. This can be turned on using `flutter config --enable-multi-window` or by putting `enable-multi-window: true` in an app's .pubspec, under the `flutter` section. 2. It adds a new [`isMultiWindowEnabled`](https://github.com/flutter/flutter/pull/168697/files#diff-c8dbd5cd3103bc5be53c4ac5be8bdb9bf73e10cd5d8e4ac34e737fd1f8602d45R7-R11) property to the framework. 4. The Material library can use this new property to determine whether it should create a new window. [Example](https://github.com/flutter/flutter/pull/168697/files#diff-2cbc1634ed6b61d61dfa090e7bfbbb7c60b74c8abc3a28df6f79eee691fd1b73). ## Limitations ### Tool and framework only For now, these feature flags are available only to the Flutter tool and Flutter framework. The flags are not automatically available to the embedder or the engine. For example, embedders need to configure their surfaces differently if Impeller is enabled. This configuration must happen before the Dart isolate is launched. As a result, the framework's feature flags is not a viable solution for this scenario for now. For these kinds of scenarios, we should continue to use platform-specific configuration like the `AndroidManifest.xml` or `Info.plist` files. This is a fixable limitation, we just need to invest in this plumbing :) ### Tree shaking Feature flags are not designed to help tree shaking. For example, you cannot conditionally import Dart code depending on the enabled feature flags. Code that is feature flagged off will still be imported into user's apps.

github-actions bot added tool Affects the "flutter" command-line tool. See also t: labels. framework flutter/packages/flutter repository. See also f: labels. labels May 7, 2025

loic-sharma force-pushed the framework_feature_flags branch 2 times, most recently from 97ae572 to 00ed7d9 Compare May 13, 2025 17:18

matanlurey reviewed May 14, 2025

View reviewed changes

.ci.yaml Show resolved Hide resolved

packages/flutter_tools/lib/src/build_system/targets/common.dart Outdated Show resolved Hide resolved

justinmc reviewed May 16, 2025

View reviewed changes

justinmc mentioned this pull request May 16, 2025

Add multi-window feature flag #168697

Closed

9 tasks

loic-sharma force-pushed the framework_feature_flags branch from 00ed7d9 to 3616c48 Compare May 17, 2025 00:18

loic-sharma mentioned this pull request May 20, 2025

Let tests mock the list of feature flags #169140

Merged

9 tasks

loic-sharma force-pushed the framework_feature_flags branch 2 times, most recently from dd5edb6 to 04c3364 Compare May 29, 2025 23:38

matanlurey reviewed May 29, 2025

View reviewed changes

packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart Show resolved Hide resolved

matanlurey approved these changes May 29, 2025

View reviewed changes

loic-sharma marked this pull request as ready for review May 30, 2025 00:12

loic-sharma force-pushed the framework_feature_flags branch 2 times, most recently from 085b37b to 8121d41 Compare June 2, 2025 21:42

loic-sharma requested a review from bkonyi June 2, 2025 21:44

bkonyi approved these changes Jun 3, 2025

View reviewed changes

packages/flutter_tools/lib/src/runner/flutter_command.dart Outdated Show resolved Hide resolved

packages/flutter_tools/test/general.shard/features_test.dart Outdated Show resolved Hide resolved

justinmc reviewed Jun 7, 2025

View reviewed changes

matanlurey mentioned this pull request Jun 11, 2025

Document expected practices for creating/using feature flags #170456

Closed

loic-sharma force-pushed the framework_feature_flags branch from 95519f7 to 8dc702a Compare June 13, 2025 00:06

loic-sharma mentioned this pull request Jun 13, 2025

Update foundation library to export internal #170563

Merged

9 tasks

matanlurey mentioned this pull request Jun 17, 2025

Add an initial "Using feature flags" doc for the team. #170767

Merged

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Jul 6, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

fb7d872

engine-flutter-autoroll mentioned this pull request Jul 6, 2025

Roll Flutter from 2773c0c8e15e to 160f2cbed84d (149 revisions) flutter/packages#9565

Closed

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Jul 6, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

a3c6aa2

engine-flutter-autoroll mentioned this pull request Jul 7, 2025

Roll Flutter from 2773c0c8e15e to d35bde5363d5 (152 revisions) flutter/packages#9573

Closed

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Jul 7, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

82f8b79

engine-flutter-autoroll mentioned this pull request Jul 7, 2025

Roll Flutter from 2773c0c8e15e to d35bde5363d5 (152 revisions) flutter/packages#9575

Closed

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Jul 7, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

04fe441

engine-flutter-autoroll mentioned this pull request Jul 7, 2025

Roll Flutter from 2773c0c8e15e to a04fb324be73 (153 revisions) flutter/packages#9577

Closed

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Jul 7, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

3f43f8c

Piinks mentioned this pull request Jul 19, 2025

Flutter Notable Commits - 2025 #161460

Open

mattkae mentioned this pull request Jul 21, 2025

Add the 'windowing' feature flag and use to wrap an implementation for regular windows that always throws #172478

Merged

9 tasks

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Aug 14, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

36af3d3

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Aug 14, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

1ed3ec8

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Aug 15, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

a346203

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Aug 15, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

202a3c2

engine-flutter-autoroll added a commit to engine-flutter-autoroll/packages that referenced this pull request Aug 16, 2025

c6f9e801b Add feature flags to the framework (flutter/flutter#168437)

6fb7259

		// found in the LICENSE file.

		import 'package:flutter/foundation.dart';

Add feature flags to the framework #168437

Add feature flags to the framework #168437

Uh oh!

Conversation

loic-sharma commented May 7, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Motivation

Use case

How this works

Multi-window example

Limitations

Tool and framework only

Tree shaking

Pre-launch Checklist

Uh oh!

matanlurey left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

justinmc left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

justinmc May 16, 2025

Choose a reason for hiding this comment

Uh oh!

justinmc May 16, 2025

Choose a reason for hiding this comment

Uh oh!

loic-sharma May 17, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

justinmc May 16, 2025

Choose a reason for hiding this comment

Uh oh!

justinmc May 16, 2025

Choose a reason for hiding this comment

Uh oh!

loic-sharma May 16, 2025

Choose a reason for hiding this comment

Uh oh!

loic-sharma Jun 26, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

matanlurey left a comment

Choose a reason for hiding this comment

Uh oh!

bkonyi left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

justinmc left a comment

Choose a reason for hiding this comment

Testing the framework in the era of feature flags

Uh oh!

justinmc Jun 7, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

loic-sharma Jun 25, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

matanlurey commented Jun 11, 2025

Uh oh!

loic-sharma commented Jun 12, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

justinmc commented Jun 13, 2025

Uh oh!

Reviewers

Assignees

Labels

loic-sharma commented May 7, 2025 •

edited

Loading

loic-sharma May 17, 2025 •

edited

Loading

justinmc Jun 7, 2025 •

edited

Loading

loic-sharma commented Jun 12, 2025 •

edited

Loading