Merge pull request #476 from abarth/forced_layer

Reduce record time in Stocks drawer animation by 55%

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

@@ -59,10 +59,6 @@ abstract class Layer {
     _nextSibling = null;
     _previousSibling = null;
     _parent = null;
-    _dispose();
-  }
-
-  void _dispose() {
   }
 
   /// Override this function to upload this layer to the engine
@@ -88,11 +84,6 @@ class PictureLayer extends Layer {
   /// The picture's coodinate system matches this layer's coodinate system
   ui.Picture picture;
 
-  void _dispose() {
-    super._dispose();
-    picture.dispose();
-  }
-
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
     builder.addPicture(offset + layerOffset, picture, paintBounds);
   }
@@ -190,7 +181,6 @@ class ContainerLayer extends Layer {
     child._previousSibling = null;
     child._nextSibling = null;
     child._parent = null;
-    child._dispose();
   }
 
   /// Removes all of this layer's children from its child list
@@ -201,23 +191,12 @@ class ContainerLayer extends Layer {
       child._previousSibling = null;
       child._nextSibling = null;
       child._parent = null;
-      child._dispose();
       child = next;
     }
     _firstChild = null;
     _lastChild = null;
   }
 
-  void _dispose() {
-    super._dispose();
-    Layer child = _firstChild;
-    while (child != null) {
-      Layer next = child.nextSibling;
-      child._dispose();
-      child = next;
-    }
-  }
-
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
     addChildrenToScene(builder, offset + layerOffset);
   }

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

@@ -1052,6 +1052,19 @@ class RenderPointerListener extends RenderProxyBox {
   }
 }
 
+/// Force this subtree to have a layer
+///
+/// This render object creates a separate display list for its child, which
+/// can improve performance if the subtree repaints at different times than
+/// the surrounding parts of the tree. Specifically, when the child does not
+/// repaint but its parent does, we can re-use the display list we recorded
+/// previously. Similarly, when the child repaints but the surround tree does
+/// not, we can re-record its display list without re-recording the display list
+/// for the surround tree.
+class RenderForcedLayer extends RenderProxyBox {
+  bool get hasLayer => true;
+}
+
 /// Is invisible during hit testing.
 ///
 /// When [ignoring] is true, this render object (and its subtree) is invisible

packages/flutter/lib/src/widgets/basic.dart

@@ -1321,6 +1321,11 @@ class Listener extends OneChildRenderObjectWidget {
   }
 }
 
+class ForcedLayer extends OneChildRenderObjectWidget {
+  ForcedLayer({ Key key, Widget child }) : super(key: key, child: child);
+  RenderForcedLayer createRenderObject() => new RenderForcedLayer();
+}
+
 class IgnorePointer extends OneChildRenderObjectWidget {
   IgnorePointer({ Key key, Widget child, this.ignoring: true })
     : super(key: key, child: child);

packages/flutter/lib/src/widgets/routes.dart

@@ -39,7 +39,9 @@ class OverlayRoute extends Route {
 
   void didPush(OverlayState overlay, OverlayEntry insertionPoint) {
     for (WidgetBuilder builder in builders) {
-      _overlayEntries.add(new OverlayEntry(builder: builder));
+      _overlayEntries.add(new OverlayEntry(builder: (BuildContext context) {
+        return new ForcedLayer(child: builder(context));
+      }));
       overlay?.insert(_overlayEntries.last, above: insertionPoint);
       insertionPoint = _overlayEntries.last;
     }