One issue worth pointing out is that if (during flutter run)

1. an asset's transformers are changed and
2. there is a hot reload

then the asset won't be re-transformed. This is because the only thing we check during a DevFS update is the last-modified time of the asset file.

One issue worth pointing out is that if (during flutter run)

1. an asset's transformers are changed and
2. there is a hot reload

then the asset won't be re-transformed. This is because the only thing we check during a DevFS update is the last-modified time of the asset file.

What is the "asset file" in this context? the output of the transformer?

And is this a temporary bug, or a known limitation that we should document?

One issue worth pointing out is that if (during flutter run)

1. an asset's transformers are changed and
2. there is a hot reload

then the asset won't be re-transformed. This is because the only thing we check during a DevFS update is the last-modified time of the asset file.

What is the "asset file" in this context? the output of the transformer?

And is this a temporary bug, or a known limitation that we should document?

The original file. This is a limitation of how we implement hot reload. More research and another PR likely to follow.

One issue worth pointing out is that if (during flutter run)

1. an asset's transformers are changed and
2. there is a hot reload

then the asset won't be re-transformed. This is because the only thing we check during a DevFS update is the last-modified time of the asset file.

What is the "asset file" in this context? the output of the transformer?
And is this a temporary bug, or a known limitation that we should document?

The original file. This is a limitation of how we implement hot reload. More research and another PR likely to follow.

Ahh, I see what you mean. If the input asset is the same, but the transformer itself was changed.

Is the timestamp on the application's pubspec.yaml used as an input to bundling assets?

I believe I've fixed the hot reload issue. Changes are in this commit: fix asset not being updated when transformers are changed before hot reload.

This works primarily by utilizing the previously dormant bundleDirty parameter on ResidentRunner::updateDevFS. However, this means that, if the asset bundle becomes dirty for whatever reason, we'll end up re-writing all assets.

I am not sure this is acceptable, as any modification to a pubspec basically means all assets needs to be processed and rewritten to the device, which could be slow given the right circumstances (or basically any case with transformers). I want to experiment with more granular caching, though this will likely make all this code even more complicated than it already is.

I believe I've fixed the hot reload issue. Changes are in this commit: fix asset not being updated when transformers are changed before hot reload.

This works primarily by utilizing the previously dormant bundleDirty parameter on ResidentRunner::updateDevFS. However, this means that, if the asset bundle becomes dirty for whatever reason, we'll end up re-writing all assets.

I am not sure this is acceptable, as any modification to a pubspec basically means all assets needs to be processed and rewritten to the device, which could be slow given the right circumstances (or basically any case with transformers). I want to experiment with more granular caching, though this will likely make all this code even more complicated than it already is.

I thought about this more and brought this up during this week's tool PR review. I'm OK with punting on this by documenting this feature as not being supported in hot-reload. To make this feature work with hot reload, we'd need to do some significant refactoring to asset bundling+writing code (to avoid adding more spaghetti than there already is). We need to overhaul what makes an asset "dirty" so we know when to retransform and rewrite it to the device. Currently, this mechanism is implemented as a function of the last-modified-time on the underlying asset file (or for non-files, a boolean that returns true the first time it's read and then false afterwards). It is unclear if this is worth overhauling at this time, but perhaps such a decision would be easier if we had another asset related feature that would require a similar refactoring to support hot reload.

@christopherfujino, after this PR lands, I'll make a documentative PR that demonstrates a duct-tape/hacky solution to make this work for hot reload. It'll include comments that point out the obstacles in the way of making hot reload work well for this. This could be referenced in the future when we come back to this.

@christopherfujino, after this PR lands, I'll make a documentative PR that demonstrates a duct-tape/hacky solution to make this work for hot reload. It'll include comments that point out the obstacles in the way of making hot reload work well for this. This could be referenced in the future when we come back to this.

SGTM

@christopherfujino poke

More thoughts about hot-reload:

Another challenge with making this fully-support hot reload is that dart run <package> seems prone to failing when running concurrently with a pub get:

User-defined transformation of asset "/Users/andrewkolos/Desktop/asset_transformers_test/assets/svg.svg" failed.
Transformer process terminated with non-zero exit code: 255
Transformer package: vector_graphics_compiler
Full command: /Users/andrewkolos/Documents/GitHub/flutter/bin/cache/dart-sdk/bin/dart run vector_graphics_compiler:vector_graphics_compiler --input=/var/folders/qh/m_ckdxls0xvfc7ccjj13yjjw010s5h/T/flutter_tools.wGISPs/svg.svg-transformOutput0.svg
--output=/var/folders/qh/m_ckdxls0xvfc7ccjj13yjjw010s5h/T/flutter_tools.wGISPs/svg.svg-transformOutput1.svg
stdout:

stderr:
Unhandled exception:
PathNotFoundException: Cannot open file, path = '/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/tmpSnp7d7/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot.incremental.temp' (OS Error: No such file or directory, errno = 2)
#0      _checkForErrorResponse (dart:io/common.dart:55)
#1      _File.open.<anonymous closure> (dart:io/file_impl.dart:381)
<asynchronous suspension>
#2      _FileStreamConsumer.addStream.<anonymous closure> (dart:io/file_impl.dart:205)
<asynchronous suspension>
Compilation did not produce any result. Expected file at `/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/tmpSnp7d7/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot.incremental.temp`
Cannot rename file to '/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot', path =
'/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/tmpSnp7d7/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot.incremental.temp' (OS Error: No such file or directory, errno = 2)
PathNotFoundException: Cannot rename file to '/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot', path =
'/Users/andrewkolos/Desktop/asset_transformers_test/.dart_tool/pub/bin/vector_graphics_compiler/tmpSnp7d7/vector_graphics_compiler.dart-3.4.0-187.0.dev.snapshot.incremental.temp' (OS Error: No such file or directory, errno = 2)

This is problematic because the Dart VSCode extension seems to run flutter pub get automatically whenever pubspec.yaml is modified. I obtained the above error message by 1) I adding a new asset with a transformer to my pubspec, 2) saving the file, and then 3) quickly (within a few seconds) initiating a hot reload.