tor-browser

StackingContextHelper.h (6182B)

---

/* -*- 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_STACKINGCONTEXTHELPER_H
#define GFX_STACKINGCONTEXTHELPER_H

#include "mozilla/Attributes.h"
#include "mozilla/gfx/MatrixFwd.h"
#include "mozilla/webrender/WebRenderAPI.h"
#include "mozilla/webrender/WebRenderTypes.h"
#include "Units.h"

namespace mozilla {

class nsDisplayTransform;
struct ActiveScrolledRoot;

namespace layers {

/**
* This is a helper class that pushes/pops a stacking context, and manages
* some of the coordinate space transformations needed.
*/
class MOZ_RAII StackingContextHelper {
public:
 StackingContextHelper(const StackingContextHelper& aParentSC,
                       const ActiveScrolledRoot* aAsr,
                       nsIFrame* aContainerFrame,
                       nsDisplayItem* aContainerItem,
                       wr::DisplayListBuilder& aBuilder,
                       const wr::StackingContextParams& aParams,
                       const LayoutDeviceRect& aBounds = LayoutDeviceRect());

 // This version of the constructor should only be used at the root level
 // of the tree, so that we have a StackingContextHelper to pass down into
 // the RenderLayer traversal, but don't actually want it to push a stacking
 // context on the display list builder.
 StackingContextHelper();

 // Pops the stacking context, if one was pushed during the constructor.
 ~StackingContextHelper();

 // Export the inherited scale
 gfx::MatrixScales GetInheritedScale() const { return mScale; }

 const gfx::Matrix& GetInheritedTransform() const {
   return mInheritedTransform;
 }

 const gfx::Matrix& GetSnappingSurfaceTransform() const {
   return mSnappingSurfaceTransform;
 }

 nsDisplayTransform* GetDeferredTransformItem() const;
 Maybe<gfx::Matrix4x4> GetDeferredTransformMatrix() const;
 // Functions for temporarily clearing and restoring the deferred
 // transform item during WebRender display list building. These are
 // used to ensure deferred transforms are not applied in duplicate
 // to nested nodes in the WebRenderScrollData tree.
 void ClearDeferredTransformItem() const;
 void RestoreDeferredTransformItem(nsDisplayTransform* aItem) const;

 bool AffectsClipPositioning() const { return mAffectsClipPositioning; }
 Maybe<wr::WrSpatialId> ReferenceFrameId() const { return mReferenceFrameId; }

private:
 wr::DisplayListBuilder* mBuilder;
 gfx::MatrixScales mScale;
 gfx::Matrix mInheritedTransform;

 // The "snapping surface" defines the space that we want to snap in.
 // You can think of it as the nearest physical surface.
 // Animated transforms create a new snapping surface, so that changes to their
 // transform don't affect the snapping of their contents. Non-animated
 // transforms do *not* create a new snapping surface, so that for example the
 // existence of a non-animated identity transform does not affect snapping.
 gfx::Matrix mSnappingSurfaceTransform;
 bool mAffectsClipPositioning;
 Maybe<wr::WrSpatialId> mReferenceFrameId;
 Maybe<wr::SpaceAndClipChainHelper> mSpaceAndClipChainHelper;

 // The deferred transform item is used when building the WebRenderScrollData
 // structure. The backstory is that APZ needs to know about transforms that
 // apply to the different APZC instances. Prior to bug 1423370, we would do
 // this by creating a new WebRenderLayerScrollData for each nsDisplayTransform
 // item we encountered. However, this was unnecessarily expensive because it
 // turned out a lot of nsDisplayTransform items didn't have new ASRs defined
 // as descendants, so we'd create the WebRenderLayerScrollData and send it
 // over to APZ even though the transform information was not needed in that
 // case.
 //
 // In bug 1423370 and friends, this was optimized by "deferring" a
 // nsDisplayTransform item when we encountered it during display list
 // traversal. If we found a descendant of that transform item that had a
 // new ASR or otherwise was "relevant to APZ", we would then pluck the
 // transform matrix off the deferred item and put it on the
 // WebRenderLayerScrollData instance created for that APZ-relevant descendant.
 //
 // One complication with this is if there are multiple nsDisplayTransform
 // items in the ancestor chain for the APZ-relevant item. As we traverse the
 // display list, we will defer the outermost nsDisplayTransform item, and when
 // we encounter the next one we will need to merge it with the already-
 // deferred one somehow. What we do in this case is have
 // mDeferredTransformItem always point to the "innermost" deferred transform
 // item (i.e. the closest ancestor nsDisplayTransform item of the item that
 // created this StackingContextHelper). And then we use
 // mDeferredAncestorTransform to store the product of all the other transforms
 // that were deferred. Note that this means we only need to look at
 // mDeferredAncestorTransform if mDeferredTransformItem is set. Note that we
 // can only do this if the nsDisplayTransform items share the same ASR. If we
 // are processing an nsDisplayTransform item with a different ASR than the
 // previously-deferred item, we assume that the previously-deferred transform
 // will get sent to APZ as part of a separate WebRenderLayerScrollData item,
 // and so we don't need to bother with any merging. (The merging probably
 // wouldn't even make sense because the coordinate spaces might be different
 // in the face of async scrolling). This behaviour of forcing a
 // WebRenderLayerScrollData item to be generated when the ASR changes is
 // implemented in
 // WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList.
 mutable nsDisplayTransform* mDeferredTransformItem;
 Maybe<gfx::Matrix4x4> mDeferredAncestorTransform;

 bool mRasterizeLocally;
};

}  // namespace layers
}  // namespace mozilla

#endif /* GFX_STACKINGCONTEXTHELPER_H */