Move toImage() impl from RepaintRenderBoundary to OffsetLayer (#17060)

https://github.com/flutter/flutter/issues/16859

packages/flutter/lib/src/rendering/layer.dart

@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:collection';
-import 'dart:ui' as ui show ImageFilter, Picture, SceneBuilder;
+import 'dart:ui' as ui show Image, ImageFilter, Picture, Scene, SceneBuilder;
 import 'dart:ui' show Offset;
 
 import 'package:flutter/foundation.dart';
@@ -515,6 +516,41 @@ class OffsetLayer extends ContainerLayer {
     super.debugFillProperties(properties);
     properties.add(new DiagnosticsProperty<Offset>('offset', offset));
   }
+
+  /// Capture an image of the current state of this layer and its children.
+  ///
+  /// The returned [ui.Image] has uncompressed raw RGBA bytes, in the
+  /// dimensions [logicalSize] multiplied by the [pixelRatio].
+  ///
+  /// The [pixelRatio] describes the scale between the logical pixels and the
+  /// size of the output image. It is independent of the
+  /// [window.devicePixelRatio] for the device, so specifying 1.0 (the default)
+  /// will give you a 1:1 mapping between logical pixels and the output pixels
+  /// in the image.
+  ///
+  /// See also:
+  ///
+  ///  * [RenderRepaintBoundary.toImage] for a similar API at the render object level.
+  ///  * [dart:ui.Scene.toImage] for more information about the image returned.
+  Future<ui.Image> toImage(Size logicalSize, {double pixelRatio: 1.0}) async {
+    assert(pixelRatio != null);
+    final ui.SceneBuilder builder = new ui.SceneBuilder();
+    final Matrix4 transform = new Matrix4.diagonal3Values(pixelRatio, pixelRatio, 1.0);
+    transform.translate(-offset.dx, -offset.dy, 0.0);
+    builder.pushTransform(transform.storage);
+    addToScene(builder, Offset.zero);
+    final ui.Scene scene = builder.build();
+    try {
+      // Size is rounded up to the next pixel to make sure we don't clip off
+      // anything.
+      return await scene.toImage(
+        (pixelRatio * logicalSize.width).ceil(),
+        (pixelRatio * logicalSize.height).ceil(),
+      );
+    } finally {
+      scene.dispose();
+    }
+  }
 }
 
 /// A composite layer that clips its children using a rectangle.

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 
-import 'dart:ui' as ui show ImageFilter, Gradient, SceneBuilder, Scene, Image;
+import 'dart:ui' as ui show ImageFilter, Gradient, Image;
 
 import 'package:flutter/animation.dart';
 import 'package:flutter/foundation.dart';
@@ -2470,27 +2470,52 @@ class RenderRepaintBoundary extends RenderProxyBox {
   /// will give you a 1:1 mapping between logical pixels and the output pixels
   /// in the image.
   ///
+  /// ## Sample code
+  ///
+  /// The following is an example of how to go from a `GlobalKey` on a
+  /// `RepaintBoundary` to a PNG:
+  ///
+  /// ```dart
+  /// class PngHome extends StatefulWidget {
+  ///   PngHome({Key key}) : super(key: key);
+  ///
+  ///   @override
+  ///   _PngHomeState createState() => new _PngHomeState();
+  /// }
+  ///
+  /// class _PngHomeState extends State<PngHome> {
+  ///   GlobalKey globalKey = new GlobalKey();
+  ///
+  ///   Future<void> _capturePng() async {
+  ///     RenderRepaintBoundary boundary = globalKey.currentContext.findRenderObject();
+  ///     ui.Image image = await boundary.toImage();
+  ///     ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
+  ///     Uint8List pngBytes = byteData.buffer.asUint8List();
+  ///     print(pngBytes);
+  ///   }
+  ///
+  ///   @override
+  ///   Widget build(BuildContext context) {
+  ///     return RepaintBoundary(
+  ///       key: globalKey,
+  ///       child: Center(
+  ///         child: FlatButton(
+  ///           child: Text('Hello World', textDirection: TextDirection.ltr),
+  ///           onPressed: _capturePng,
+  ///         ),
+  ///       ),
+  ///     );
+  ///   }
+  /// }
+  /// ```
+  ///
   /// See also:
   ///
+  ///  * [OffsetLayer.toImage] for a similar API at the layer level.
   ///  * [dart:ui.Scene.toImage] for more information about the image returned.
-  Future<ui.Image> toImage({double pixelRatio: 1.0}) async {
+  Future<ui.Image> toImage({double pixelRatio: 1.0}) {
     assert(!debugNeedsPaint);
-    final ui.SceneBuilder builder = new ui.SceneBuilder();
-    final Matrix4 transform = new Matrix4.diagonal3Values(pixelRatio, pixelRatio, 1.0);
-    transform.translate(-layer.offset.dx, -layer.offset.dy, 0.0);
-    builder.pushTransform(transform.storage);
-    layer.addToScene(builder, Offset.zero);
-    final ui.Scene scene = builder.build();
-    try {
-      // Size is rounded up to the next pixel to make sure we don't clip off
-      // anything.
-      return await scene.toImage(
-        (pixelRatio * size.width).ceil(),
-        (pixelRatio * size.height).ceil(),
-      );
-    } finally {
-      scene.dispose();
-    }
+    return layer.toImage(size, pixelRatio: pixelRatio);
   }