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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 */
#include <stack>
#include <unordered_map>
#include "mozilla/Attributes.h"
#include "mozilla/webrender/WebRenderAPI.h"
namespace mozilla {
class nsDisplayItem;
struct ActiveScrolledRoot;
struct DisplayItemClipChain;
namespace wr {
class DisplayListBuilder;
namespace layers {
class StackingContextHelper;
class WebRenderLayerManager;
* This class manages creating and assigning scroll layers and clips in
* WebRender based on the gecko display list. It has a few public functions that
* are intended to be invoked while traversing the Gecko display list, and it
* uses the ASR and clip information from the display list to create the
* necessary clip state in WebRender.
* The structure of the clip state in WebRender ends up quite similar to how
* it is in Gecko. For each ASR in Gecko, we create a scroll layer (i.e. a
* scrolling clip) in WebRender; these form a tree structure similar to the
* ASR tree structure. Ancestors of scroll layers are always other scroll
* layers, or the root scroll node.
* The DisplayItemClipChain list of clips from the gecko display list is
* converted to a WR clip chain and pushed on the stack prior to creating
* any WR commands for that item, and is popped afterwards. In addition,
* the WR clip chain has a parent pointer, which points to the clip chain for
* any enclosing stacking context. This again results in a strucuture very
* similar to that in Gecko, where the clips from container display items get
* applied to the contained display items.
class ClipManager {
void BeginBuild(WebRenderLayerManager* aManager,
wr::DisplayListBuilder& aBuilder);
void EndBuild();
void BeginList(const StackingContextHelper& aStackingContext);
void EndList(const StackingContextHelper& aStackingContext);
wr::WrSpaceAndClipChain SwitchItem(nsDisplayListBuilder* aBuilder,
nsDisplayItem* aItem);
void PushOverrideForASR(const ActiveScrolledRoot* aASR,
const wr::WrSpatialId& aSpatialId);
void PopOverrideForASR(const ActiveScrolledRoot* aASR);
wr::WrSpatialId SpatialIdAfterOverride(const wr::WrSpatialId& aSpatialId);
wr::WrSpatialId GetScrollLayer(const ActiveScrolledRoot* aASR);
Maybe<wr::WrSpatialId> DefineScrollLayers(const ActiveScrolledRoot* aASR,
nsDisplayItem* aItem);
Maybe<wr::WrClipChainId> DefineClipChain(const DisplayItemClipChain* aChain,
int32_t aAppUnitsPerDevPixel);
WebRenderLayerManager* MOZ_NON_OWNING_REF mManager;
wr::DisplayListBuilder* mBuilder;
// Stack of clip caches. Each cache contains a map from gecko
// DisplayItemClipChain objects to webrender WrClipIds, which allows us to
// avoid redefining identical clips in WR. However, the gecko
// DisplayItemClipChain items get deduplicated quite aggressively, without
// regard to things like the enclosing reference frame or mask. On the WR
// side, we cannot deduplicate clips that aggressively. So what we do is
// any time we enter a new reference frame (for example) we create a new clip
// cache on mCacheStack. This ensures we continue caching stuff within a given
// reference frame, but disallow caching stuff across reference frames. In
// general we need to do this anytime PushOverrideForASR is called, as that is
// called for the same set of conditions for which we cannot deduplicate
// clips.
using ClipIdMap = std::unordered_map<const DisplayItemClipChain*,
AutoTArray<wr::WrClipId, 4>>;
std::stack<ClipIdMap> mCacheStack;
// A map that holds the cache overrides created by (a) "out of band" clips,
// i.e. clips that are generated by display items but that ClipManager
// doesn't know about and (b) stacking contexts that affect clip positioning.
// These are called "cache overrides" because while we're inside these things,
// we cannot use the ASR from the gecko display list as-is. Fundamentally this
// results from a mismatch between the ASR+clip items on the gecko side and
// the ClipScrollTree on the WR side; the WR side incorporates things like
// transforms and stacking context origins while the gecko side manages those
// differently.
// Any time ClipManager wants to define a new clip as a child of ASR X, it
// should first check the cache overrides to see if there is a cache override
// item ((a) or (b) above) that is already a child of X, and then define that
// clip as a child of Y instead. This map stores X -> Y, which allows
// ClipManager to do the necessary lookup. Note that there theoretically might
// be multiple different "Y" clips (in case of nested cache overrides), which
// is why we need a stack.
std::unordered_map<wr::WrSpatialId, std::stack<wr::WrSpatialId>> mASROverride;
// This holds some clip state for a single nsDisplayItem
struct ItemClips {
ItemClips(const ActiveScrolledRoot* aASR,
const DisplayItemClipChain* aChain, int32_t aAppUnitsPerDevPixel,
bool aSeparateLeaf);
// These are the "inputs" - they come from the nsDisplayItem
const ActiveScrolledRoot* mASR;
const DisplayItemClipChain* mChain;
int32_t mAppUnitsPerDevPixel;
bool mSeparateLeaf;
// These are the "outputs" - they are pushed to WR as needed
wr::WrSpatialId mScrollId;
Maybe<wr::WrClipChainId> mClipChainId;
void UpdateSeparateLeaf(wr::DisplayListBuilder& aBuilder,
int32_t aAppUnitsPerDevPixel);
bool HasSameInputs(const ItemClips& aOther);
wr::WrSpaceAndClipChain GetSpaceAndClipChain() const;
// A stack of ItemClips corresponding to the nsDisplayItem ancestry. Each
// time we recurse into a nsDisplayItem's child list, this stack size
// increases by one. The topmost item on the stack is for the display item
// we are currently processing and items deeper on the stack are for that
// display item's ancestors.
std::stack<ItemClips> mItemClipStack;
} // namespace layers
} // namespace mozilla