[rfw] Add support for widget builders (#5907)

This PR adds support for widget builders.

flutter/flutter#141658

packages/rfw/CHANGELOG.md

@@ -1,3 +1,6 @@
+## 1.0.25
+* Adds support for wildget builders.
+
 ## 1.0.24
 
 * Adds `InkResponse` material widget.

packages/rfw/lib/src/dart/binary.dart

@@ -298,6 +298,8 @@ const int _msEvent = 0x0E;
 const int _msSwitch = 0x0F;
 const int _msDefault = 0x10;
 const int _msSetState = 0x11;
+const int _msWidgetBuilder = 0x12;
+const int _msWidgetBuilderArgReference = 0x13;
 
 /// API for decoding Remote Flutter Widgets binary blobs.
 ///
@@ -453,6 +455,10 @@ class _BlobDecoder {
         return _readSwitch();
       case _msSetState:
         return SetStateHandler(StateReference(_readPartList()), _readArgument());
+      case _msWidgetBuilder:
+        return _readWidgetBuilder();
+      case _msWidgetBuilderArgReference:
+        return WidgetBuilderArgReference(_readString(), _readPartList());
       default:
         return _parseValue(type, _readArgument);
     }
@@ -468,6 +474,16 @@ class _BlobDecoder {
     return ConstructorCall(name, _readMap(_readArgument)!);
   }
 
+  WidgetBuilderDeclaration _readWidgetBuilder() {
+    final String argumentName = _readString();
+    final int type = _readByte();
+    if (type != _msWidget && type != _msSwitch) {
+      throw FormatException('Unrecognized data type 0x${type.toRadixString(16).toUpperCase().padLeft(2, "0")} while decoding widget builder blob.');
+    }
+    final BlobNode widget = type == _msWidget ? _readWidget() : _readSwitch();
+    return WidgetBuilderDeclaration(argumentName, widget);
+  }
+
   WidgetDeclaration _readDeclaration() {
     final String name = _readString();
     final DynamicMap? initialState = _readMap(readValue, nullIfEmpty: true);
@@ -613,6 +629,10 @@ class _BlobEncoder {
       bytes.addByte(_msWidget);
       _writeString(value.name);
       _writeMap(value.arguments, _writeArgument);
+    } else if (value is WidgetBuilderDeclaration) {
+      bytes.addByte(_msWidgetBuilder);
+      _writeString(value.argumentName);
+      _writeArgument(value.widget);
     } else if (value is ArgsReference) {
       bytes.addByte(_msArgsReference);
       _writeInt64(value.parts.length);
@@ -621,6 +641,11 @@ class _BlobEncoder {
       bytes.addByte(_msDataReference);
       _writeInt64(value.parts.length);
       value.parts.forEach(_writePart);
+    } else if (value is WidgetBuilderArgReference) {
+      bytes.addByte(_msWidgetBuilderArgReference);
+      _writeString(value.argumentName);
+      _writeInt64(value.parts.length);
+      value.parts.forEach(_writePart);
     } else if (value is LoopReference) {
       bytes.addByte(_msLoopReference);
       _writeInt64(value.loop);

packages/rfw/lib/src/dart/model.dart

@@ -439,6 +439,28 @@ class ConstructorCall extends BlobNode {
   String toString() => '$name($arguments)';
 }
 
+/// Representation of functions that return widgets in Remote Flutter Widgets library blobs.
+class WidgetBuilderDeclaration extends BlobNode {
+  /// Represents a callback that takes a single argument [argumentName] and returns the [widget].
+  const WidgetBuilderDeclaration(this.argumentName, this.widget);
+
+  /// The callback single argument name.
+  ///
+  /// In `Builder(builder: (scope) => Container());`, [argumentName] is "scope".
+  final String argumentName;
+
+  /// The widget that will be returned when the builder callback is called.
+  ///
+  /// This is usually a [ConstructorCall], but may be a [Switch] (so long as
+  /// that [Switch] resolves to a [ConstructorCall]. Other values (or a [Switch]
+  /// that does not resolve to a constructor call) will result in an
+  /// [ErrorWidget] being used.
+  final BlobNode widget;
+
+  @override
+  String toString() => '($argumentName) => $widget';
+}
+
 /// Base class for various kinds of references in the RFW data structures.
 abstract class Reference extends BlobNode {
   /// Abstract const constructor. This constructor enables subclasses to provide
@@ -534,6 +556,31 @@ class DataReference extends Reference {
   String toString() => 'data.${parts.join(".")}';
 }
 
+/// Reference to the single argument of type [DynamicMap] passed into the widget builder.
+///
+/// This class is used to represent references to a function argument.
+/// In `(scope) => Container(width: scope.width)`, this represents "scope.width".
+///
+/// See also:
+///
+///   * [WidgetBuilderDeclaration], which represents a widget builder definition.
+class WidgetBuilderArgReference extends Reference {
+  /// Wraps the given [argumentName] and [parts] as a [WidgetBuilderArgReference].
+  ///
+  /// The parts must not be mutated after the object is created.
+  const WidgetBuilderArgReference(this.argumentName, super.parts);
+
+  /// A reference to a [WidgetBuilderDeclaration.argumentName].
+  ///
+  /// In `Builder(builder: (scope) => Text(text: scope.result.text));`,
+  /// "scope.result.text" is the [WidgetBuilderArgReference].
+  /// The [argumentName] is "scope" and its [parts] are `["result", "text"]`.
+  final String argumentName;
+
+  @override
+  String toString() => '$argumentName.${parts.join('.')}';
+}
+
 /// Unbound reference to a [Loop].
 class LoopReference extends Reference {
   /// Wraps the given [loop] and [parts] as a [LoopReference].