Description

Standard*Codec allows for extensions to support arbitrary types; this
had not previously been implemented for the C++ version.

Overview of changes:

• EncodableValue's std::variant type now allows for a new CustomEncodableValue, which is a thin wrapper around std::any, to store arbitrary extension types.
• ByteBufferStream* has been split into an interface class and the buffer-based implementation, with the former now part of the public API surface to be used in standard codec extensions.
  • They also gained utility methods for some common data types to simplify writing extensions.
• StandardCodecSerializer is now part of the public API surface, and is subclassable.
• StandardCodecSerializer's ReadValue has been split into ReadValue and ReadValueOfType to match the structure used when subclassing on the Dart side, for easier porting of custom extensions across languages.
• Standard*Codec now optionally accepts a non-default serializer in GetInstance, providing a shared instance using that serializer.

Related Issues

Fixes flutter/flutter#31174

Tests

I added the following tests: Round-trip testing of method codec and message codec with custom types.

Checklist

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• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change.