Add support for deferred components (#576)

## What does this change?

This PR include support for [Flutter Deferred Components](https://docs.flutter.dev/perf/deferred-components).

Deferred components allow developers to the app into multiple apk to reduce its size. This can be used to optimize the initial download and download components only on necessary but it is also mandatory for apk with more than 200MB.

This library initially looks for assets listed on `flutter.assets` of `pubspec.yaml`, with this change the library will continue looking for the assets lists under `flutter.assets` but will merge with the list of assets included on `flutter.deferred-components.$`. This will be merged in a single list.

Ideally in the future assets should have different classname, so users will easily know when dealing with deferred components. This wasn't implemented at this moment because it would requeire a significative changes.

Fixes #577 🎯

## Type of change

- [x] New feature (non-breaking change which adds functionality)
- [x] This change requires a documentation update

---------

Co-authored-by: Ian Maciel <[email protected]>
Co-authored-by: Alex Li <[email protected]>

Author: 3 people

README.md

@@ -212,6 +212,14 @@ flutter:
     - assets/flare/Penguin.flr
     - assets/rive/vehicles.riv
     - pictures/ocean_view.jpg
+
+  # Also include assets from deferred components
+  # https://docs.flutter.dev/perf/deferred-components
+  deferred-components:
+    - name: myDeferredComponent
+      assets:
+        - assets/images/another_image.jps
+        - assets/videos/a_large_video.mp4
 ```
 
 These configurations will generate **`assets.gen.dart`** under the **`lib/gen/`** directory by default.

packages/core/lib/generators/assets_generator.dart

@@ -33,7 +33,7 @@ class AssetsGenConfig {
       pubspecFile.parent.absolute.path,
       config.pubspec.packageName,
       config.pubspec.flutterGen,
-      config.pubspec.flutter.assets,
+      _buildFlutterAssetsList(config.pubspec.flutter),
       config.pubspec.flutterGen.assets.exclude.map(Glob.new).toList(),
     );
   }
@@ -48,6 +48,19 @@ class AssetsGenConfig {
       flutterGen.assets.outputs.packageParameterEnabled ? _packageName : '';
 }
 
+/// Build assets from the main list and the deferred components.
+List<Object> _buildFlutterAssetsList(Flutter flutter) {
+  final flutterAssets = flutter.assets;
+  // We may have several deferred components, with a list of assets for each.
+  // So before spreading the list of deferred components, we need to spread
+  // the list of assets for each deferred component.
+  final deferredComponents = flutter.deferredComponents ?? [];
+  return deferredComponents.fold(
+    flutterAssets,
+    (list, component) => list + (component.assets ?? []),
+  );
+}
+
 Future<String> generateAssets(
   AssetsGenConfig config,
   DartFormatter formatter,

packages/core/lib/settings/pubspec.dart

@@ -76,6 +76,7 @@ class Flutter {
   const Flutter({
     required this.assets,
     required this.fonts,
+    required this.deferredComponents,
   });
 
   factory Flutter.fromJson(Map json) => _$FlutterFromJson(json);
@@ -85,6 +86,9 @@ class Flutter {
 
   @JsonKey(name: 'fonts', required: true)
   final List<FlutterFonts> fonts;
+
+  @JsonKey(name: 'deferred-components', required: false)
+  final List<FlutterDeferredComponents>? deferredComponents;
 }
 
 @JsonSerializable(disallowUnrecognizedKeys: false)
@@ -311,3 +315,20 @@ class FlutterGenElementFontsOutputs extends FlutterGenElementOutputs {
   @JsonKey(name: 'package_parameter_enabled', defaultValue: false)
   final bool packageParameterEnabled;
 }
+
+@JsonSerializable()
+class FlutterDeferredComponents {
+  const FlutterDeferredComponents({
+    required this.name,
+    required this.assets,
+  });
+
+  factory FlutterDeferredComponents.fromJson(Map json) =>
+      _$FlutterDeferredComponentsFromJson(json);
+
+  @JsonKey(name: 'name', required: true)
+  final String name;
+
+  @JsonKey(name: 'assets', required: false)
+  final List<Object>? assets;
+}

packages/core/lib/settings/pubspec.g.dart

@@ -102,6 +102,11 @@ void main() {
       await expectedAssetsGen(pubspec);
     });
 
+    test('Assets with deferred components assets', () async {
+      const pubspec = 'test_resources/pubspec_assets_deferred_components.yaml';
+      await expectedAssetsGen(pubspec);
+    });
+
     test('Assets with duplicate flavoring entries', () async {
       const pubspec =
           'test_resources/pubspec_assets_flavored_duplicate_entry.yaml';