Build tools for Flutter GPU shader bundles/libraries.

Features

Use native asset build hooks to import Flutter GPU shader bundle assets.

Getting started

1. Place some Flutter GPU shaders in your project. For this example, we'll assume the existence of two shaders: shaders/my_cool_shader.vert and shaders/my_cool_shader.frag.
2. Create a shader bundle manifest file in your project. The filename must end with .shaderbundle.json. For this example, we'll assume the following file is saved as my_cool_bundle.shaderbundle.json:

   {
       "CoolVertex": {
           "type": "vertex",
           "file": "shaders/my_cool_shader.vert"
       },
       "CoolFragment": {
           "type": "fragment",
           "file": "shaders/my_cool_shader.frag"
       }
   }

3. Next, define a build hook in your project that builds the shader bundle using buildShaderBundleJson. The build hook must be named hook/build.dart in your project; this script will be automatically invoked by Flutter:

   import 'package:hooks/hooks.dart';
   import 'package:flutter_gpu_shaders/build.dart';
   
   void main(List<String> args) async {
     await build(args, (config, output) async {
       await buildShaderBundleJson(
           buildInput: config,
           buildOutput: output,
           manifestFileName: 'my_cool_bundle.shaderbundle.json');
     });
   }

   buildShaderBundleJson always declares the manifest and directly listed shader files as build dependencies. With Flutter SDKs whose impellerc supports --depfile for shader bundles, it also declares transitive #include dependencies from the generated depfile. Older impellerc builds continue to use the manifest scan fallback.
4. In your project's pubspec.yaml, add the built shader bundle as an asset:

   flutter:
     assets:
       - build/shaderbundles/my_cool_bundle.shaderbundle

5. If your Flutter toolchain supports Dart DataAssets, you can opt in to registering the generated bundle with the Flutter asset bundle instead of listing it in pubspec.yaml:

   await buildShaderBundleJson(
     buildInput: config,
     buildOutput: output,
     manifestFileName: 'my_cool_bundle.shaderbundle.json',
     assetMode: ShaderBundleAssetMode.dataAssetsIfAvailable,
   );

   With DataAssets enabled, the result's flutterAssetKey is the key to pass to gpu.ShaderLibrary.fromAsset. When DataAssets are unavailable, dataAssetsIfAvailable falls back to the legacy file output. Use ShaderBundleAssetMode.dataAssetsRequired to fail fast with guidance instead.
6. You can now import the built shader bundle as a library using gpu.ShaderLibrary.fromAsset in your project. For example:

   import 'package:flutter_gpu/gpu.dart' as gpu;
   
   final String _kBaseShaderBundlePath =
       'build/shaderbundles/my_cool_bundle.shaderbundle';
   
   gpu.ShaderLibrary? _baseShaderLibrary;
   gpu.ShaderLibrary get baseShaderLibrary {
     if (_baseShaderLibrary != null) {
       return _baseShaderLibrary!;
     }
     _baseShaderLibrary = gpu.ShaderLibrary.fromAsset(_kBaseShaderBundlePath);
     if (_baseShaderLibrary != null) {
       return _baseShaderLibrary!;
     }
   
     throw Exception(
         "Failed to load base shader bundle! ($_kBaseShaderBundlePath)");
   }