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Explain why OpacityLayer has an offset field (#9713)

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According to the request by Amir and Chris.
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liyuqian 2019-07-08 15:02:18 -07:00 committed by GitHub
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10
flow/layers/opacity_layer.h
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@ -15,6 +15,16 @@ namespace flutter {
// |EnsureSingleChild| will assert if there are no children.
class OpacityLayer : public ContainerLayer {
public:
// An offset is provided here because OpacityLayer.addToScene method in the
// Flutter framework can take an optional offset argument.
//
// By default, that offset is always zero, and all the offsets are handled by
// some parent TransformLayers. But we allow the offset to be non-zero for
// backward compatibility. If it's non-zero, the old behavior is to propage
// that offset to all the leaf layers (e.g., PictureLayer). That will make
// the retained rendering inefficient as a small offset change could propagate
// to many leaf layers. Therefore we try to capture that offset here to stop
// the propagation as repainting the OpacityLayer is expensive.
OpacityLayer(int alpha, const SkPoint& offset);
~OpacityLayer() override;