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diff --git a/gfx/layers/ReadbackLayer.h b/gfx/layers/ReadbackLayer.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_READBACKLAYER_H
+#define GFX_READBACKLAYER_H
+
+#include <stdint.h>             // for uint64_t
+#include "Layers.h"             // for Layer, etc
+#include "mozilla/gfx/Rect.h"   // for gfxRect
+#include "mozilla/gfx/Point.h"  // for IntPoint
+#include "mozilla/mozalloc.h"   // for operator delete
+#include "nsCOMPtr.h"           // for already_AddRefed
+#include "nsDebug.h"            // for NS_ASSERTION
+#include "nsPoint.h"            // for nsIntPoint
+#include "nscore.h"             // for nsACString
+
+class gfxContext;
+
+namespace mozilla {
+namespace layers {
+
+class ReadbackProcessor;
+
+namespace layerscope {
+class LayersPacket;
+}  // namespace layerscope
+
+/**
+ * A ReadbackSink receives a stream of updates to a rectangle of pixels.
+ * These update callbacks are always called on the main thread, either during
+ * EndTransaction or from the event loop.
+ */
+class ReadbackSink {
+ public:
+  ReadbackSink() = default;
+  virtual ~ReadbackSink() = default;
+
+  /**
+   * Sends an update to indicate that the background is currently unknown.
+   */
+  virtual void SetUnknown(uint64_t aSequenceNumber) = 0;
+  /**
+   * Called by the layer system to indicate that the contents of part of
+   * the readback area are changing.
+   * @param aRect is the rectangle of content that is being updated,
+   * in the coordinate system of the ReadbackLayer.
+   * @param aSequenceNumber updates issued out of order should be ignored.
+   * Only use updates whose sequence counter is greater than all other updates
+   * seen so far. Return null when a non-fresh sequence value is given.
+   * @return a context into which the update should be drawn. This should be
+   * set up to clip to aRect. Zero should never be passed as a sequence number.
+   * If this returns null, EndUpdate should NOT be called. If it returns
+   * non-null, EndUpdate must be called.
+   *
+   * We don't support partially unknown backgrounds. Therefore, the
+   * first BeginUpdate after a SetUnknown will have the complete background.
+   */
+  virtual already_AddRefed<gfx::DrawTarget> BeginUpdate(
+      const gfx::IntRect& aRect, uint64_t aSequenceNumber) = 0;
+  /**
+   * EndUpdate must be called immediately after BeginUpdate, without returning
+   * to the event loop.
+   * @param aContext the context returned by BeginUpdate
+   * Implicitly Restore()s the state of aContext.
+   */
+  virtual void EndUpdate(const gfx::IntRect& aRect) = 0;
+};
+
+/**
+ * A ReadbackLayer never renders anything. It enables clients to extract
+ * the rendered contents of the layer tree below the ReadbackLayer.
+ * The rendered contents are delivered asynchronously via calls to a
+ * ReadbackSink object supplied by the client.
+ *
+ * This is a "best effort" API; it is possible for the layer system to tell
+ * the ReadbackSink that the contents of the readback area are unknown.
+ *
+ * This API exists to work around the limitations of transparent windowless
+ * plugin rendering APIs. It should not be used for anything else.
+ */
+class ReadbackLayer : public Layer {
+ public:
+  MOZ_LAYER_DECL_NAME("ReadbackLayer", TYPE_READBACK)
+
+  virtual void ComputeEffectiveTransforms(
+      const gfx::Matrix4x4& aTransformToSurface) override {
+    // Snap our local transform first, and snap the inherited transform as well.
+    // This makes our snapping equivalent to what would happen if our content
+    // was drawn into a PaintedLayer (gfxContext would snap using the local
+    // transform, then we'd snap again when compositing the PaintedLayer).
+    mEffectiveTransform =
+        SnapTransform(GetLocalTransform(),
+                      gfxRect(0, 0, mSize.width, mSize.height), nullptr) *
+        SnapTransformTranslation(aTransformToSurface, nullptr);
+  }
+
+  /**
+   * CONSTRUCTION PHASE ONLY
+   * Set the callback object to which readback updates will be delivered.
+   * This also resets the "needed rectangle" so that on the next layer tree
+   * transaction we will try to deliver the full contents of the readback
+   * area to the sink.
+   * This layer takes ownership of the sink. It will be deleted when the
+   * layer is destroyed or when a new sink is set.
+   * Initially the contents of the readback area are completely unknown.
+   */
+  void SetSink(ReadbackSink* aSink) {
+    SetUnknown();
+    mSink = mozilla::WrapUnique(aSink);
+  }
+  ReadbackSink* GetSink() { return mSink.get(); }
+
+  /**
+   * CONSTRUCTION PHASE ONLY
+   * Set the size of content that should be read back. The readback area
+   * has its top-left at 0,0 and has size aSize.
+   * Can only be called while the sink is null!
+   */
+  void SetSize(const gfx::IntSize& aSize) {
+    NS_ASSERTION(!mSink, "Should have no sink while changing size!");
+    mSize = aSize;
+  }
+  const gfx::IntSize& GetSize() { return mSize; }
+  gfx::IntRect GetRect() { return gfx::IntRect(gfx::IntPoint(0, 0), mSize); }
+
+  bool IsBackgroundKnown() {
+    return mBackgroundLayer || mBackgroundColor.a == 1.f;
+  }
+
+  void NotifyRemoved() {
+    SetUnknown();
+    mSink = nullptr;
+  }
+
+  void NotifyPaintedLayerRemoved(PaintedLayer* aLayer) {
+    if (mBackgroundLayer == aLayer) {
+      mBackgroundLayer = nullptr;
+    }
+  }
+
+  const nsIntPoint& GetBackgroundLayerOffset() {
+    return mBackgroundLayerOffset;
+  }
+
+  uint64_t AllocateSequenceNumber() { return ++mSequenceCounter; }
+
+  void SetUnknown() {
+    if (IsBackgroundKnown()) {
+      if (mSink) {
+        mSink->SetUnknown(AllocateSequenceNumber());
+      }
+      mBackgroundLayer = nullptr;
+      mBackgroundColor = gfx::DeviceColor();
+    }
+  }
+
+ protected:
+  friend class ReadbackProcessor;
+
+  ReadbackLayer(LayerManager* aManager, void* aImplData)
+      : Layer(aManager, aImplData),
+        mSequenceCounter(0),
+        mSize(0, 0),
+        mBackgroundLayer(nullptr),
+        mBackgroundLayerOffset(0, 0),
+        mBackgroundColor(gfx::DeviceColor()) {}
+
+  virtual void PrintInfo(std::stringstream& aStream,
+                         const char* aPrefix) override;
+
+  virtual void DumpPacket(layerscope::LayersPacket* aPacket,
+                          const void* aParent) override;
+
+  uint64_t mSequenceCounter;
+  UniquePtr<ReadbackSink> mSink;
+  gfx::IntSize mSize;
+
+  // This can refer to any (earlier) sibling PaintedLayer. That PaintedLayer
+  // must have mUsedForReadback set on it. If the PaintedLayer is removed
+  // for the container, this will be set to null by NotifyPaintedLayerRemoved.
+  // This PaintedLayer contains the contents which have previously been reported
+  // to mSink. The PaintedLayer had only an integer translation transform,
+  // and it covered the entire readback area. This layer also had only an
+  // integer translation transform.
+  PaintedLayer* mBackgroundLayer;
+  // When mBackgroundLayer is non-null, this is the offset to add to
+  // convert from the coordinates of mBackgroundLayer to the coordinates
+  // of this layer.
+  nsIntPoint mBackgroundLayerOffset;
+  // When mBackgroundColor is opaque, this is the color of the ColorLayer
+  // that contained the contents we reported to mSink, which covered the
+  // entire readback area.
+  gfx::DeviceColor mBackgroundColor;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* GFX_READBACKLAYER_H */