diff -r 000000000000 -r 6474c204b198 gfx/layers/ReadbackProcessor.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/layers/ReadbackProcessor.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef GFX_READBACKPROCESSOR_H
+#define GFX_READBACKPROCESSOR_H
+
+#include <stdint.h>                     // for uint64_t
+#include "nsRect.h"                     // for nsIntRect
+#include "nsTArray.h"                   // for nsTArray
+ 
+class nsIntRegion;
+
+namespace mozilla {
+namespace layers {
+
+class ContainerLayer;
+class ReadbackLayer;
+class ThebesLayer;
+
+class ReadbackProcessor {
+public:
+  /**
+   * Called by the container before processing any child layers. Call this
+   * if any child layer might have changed in any way (other than content-only
+   * changes to layers other than ColorLayers and ThebesLayers).
+   *
+   * This method recomputes the relationship between ReadbackLayers and
+   * sibling layers, and dispatches changes to ReadbackLayers. Except that
+   * if a ThebesLayer needs its contents sent to some ReadbackLayer, we'll
+   * just record that internally and later the ThebesLayer should call
+   * GetThebesLayerUpdates when it paints, to find out which rectangle needs
+   * to be sent, and the ReadbackLayer it needs to be sent to.
+   */
+  void BuildUpdates(ContainerLayer* aContainer);
+
+  struct Update {
+    /**
+     * The layer a ThebesLayer should send its contents to.
+     */
+    ReadbackLayer* mLayer;
+    /**
+     * The rectangle of content that it should send, in the ThebesLayer's
+     * coordinate system. This rectangle is guaranteed to be in the ThebesLayer's
+     * visible region. Translate it to mLayer's coordinate system
+     * by adding mLayer->GetBackgroundLayerOffset().
+     */
+    nsIntRect      mUpdateRect;
+    /**
+     * The sequence counter value to use when calling DoUpdate
+     */
+    uint64_t       mSequenceCounter;
+  };
+  /**
+   * Appends any ReadbackLayers that need to be updated, and the rects that
+   * need to be updated, to aUpdates. Only need to call this for ThebesLayers
+   * that have been marked UsedForReadback().
+   * Each Update's mLayer's mBackgroundLayer will have been set to aLayer.
+   * If a ThebesLayer doesn't call GetThebesLayerUpdates, then all the
+   * ReadbackLayers that needed data from that ThebesLayer will be marked
+   * as having unknown backgrounds.
+   * @param aUpdateRegion if non-null, this region is set to the union
+   * of the mUpdateRects.
+   */
+  void GetThebesLayerUpdates(ThebesLayer* aLayer,
+                             nsTArray<Update>* aUpdates,
+                             nsIntRegion* aUpdateRegion = nullptr);
+
+  ~ReadbackProcessor();
+
+protected:
+  void BuildUpdatesForLayer(ReadbackLayer* aLayer);
+
+  nsTArray<Update> mAllUpdates;
+};
+
+}
+}
+#endif /* GFX_READBACKPROCESSOR_H */