cc/layers/layer_iterator.h

   1 // Copyright 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef CC_LAYERS_LAYER_ITERATOR_H_
   6 #define CC_LAYERS_LAYER_ITERATOR_H_
   7
   8 #include "cc/base/cc_export.h"
   9 #include "cc/layers/layer_impl.h"
  10 #include "cc/trees/layer_tree_host_common.h"
  11
  12 namespace cc {
  13
  14 // These classes provide means to iterate over the
  15 // RenderSurfaceImpl-LayerImpl tree.
  16
  17 // Example code follows, for a tree of LayerImpl/RenderSurfaceImpl objects.
  18 // See below for details.
  19 //
  20 // void DoStuffOnLayers(
  21 //     const LayerImplList& render_surface_layer_list) {
  22 //
  23 //   LayerIterator end =
  24 //       LayerIterator::End(&render_surface_layer_list);
  25 //   for (LayerIterator
  26 //            it = LayerIterator::Begin(&render_surface_layer_list);
  27 //        it != end;
  28 //        ++it) {
  29 //     // Only one of these will be true
  30 //     if (it.represents_target_render_surface())
  31 //       foo(*it);  // *it is a layer representing a target RenderSurface
  32 //     if (it.represents_contributing_render_surface())
  33 //       bar(*it);  // *it is a layer representing a RenderSurface that
  34 //                  // contributes to the layer's target RenderSurface
  35 //     if (it.represents_itself())
  36 //       baz(*it);  // *it is a layer representing itself,
  37 //                  // as it contributes to its own target RenderSurface
  38 //   }
  39 // }
  40
  41 // A RenderSurface R may be referred to in one of two different contexts.
  42 // One RenderSurface is "current" at any time, for whatever operation
  43 // is being performed. This current surface is referred to as a target surface.
  44 // For example, when R is being painted it would be the target surface.
  45 // Once R has been painted, its contents may be included into another
  46 // surface S. While S is considered the target surface when it is being
  47 // painted, R is called a contributing surface in this context as it
  48 // contributes to the content of the target surface S.
  49 //
  50 // The iterator's current position in the tree always points to some layer.
  51 // The state of the iterator indicates the role of the layer,
  52 // and will be one of the following three states.
  53 // A single layer L will appear in the iteration process in at least one,
  54 // and possibly all, of these states.
  55 // 1. Representing the target surface: The iterator in this state,
  56 // pointing at layer L, indicates that the target RenderSurface
  57 // is now the surface owned by L. This will occur exactly once for each
  58 // RenderSurface in the tree.
  59 // 2. Representing a contributing surface: The iterator in this state,
  60 // pointing at layer L, refers to the RenderSurface owned
  61 // by L as a contributing surface, without changing the current
  62 // target RenderSurface.
  63 // 3. Representing itself: The iterator in this state, pointing at layer L,
  64 // refers to the layer itself, as a child of the
  65 // current target RenderSurface.
  66 //
  67 // The FrontToBack iterator will iterate over children layers of a surface
  68 // before the layer representing the surface as a target surface.
  69 //
  70 // To use the iterators:
  71 //
  72 // Create a stepping iterator and end iterator by calling
  73 // LayerIterator::Begin() and LayerIterator::End() and passing in the
  74 // list of layers owning target RenderSurfaces. Step through the tree
  75 // by incrementing the stepping iterator while it is != to
  76 // the end iterator. At each step the iterator knows what the layer
  77 // is representing, and you can query the iterator to decide
  78 // what actions to perform with the layer given what it represents.
  79
  80 ////////////////////////////////////////////////////////////////////////////////
  81
  82 struct LayerIteratorValue {
  83   static const int kInvalidTargetRenderSurfaceLayerIndex = -1;
  84   // This must be (size_t)-1 since the iterator action code assumes that this
  85   // value can be reached by subtracting one from the position of the first
  86   // layer in the current target surface's child layer list, which is 0.
  87   static const size_t kLayerIndexRepresentingTargetRenderSurface =
  88       static_cast<size_t>(-1);
  89 };
  90
  91 // The position of a layer iterator that is independent
  92 // of its many template types.
  93 struct LayerIteratorPosition {
  94   bool represents_target_render_surface;
  95   bool represents_contributing_render_surface;
  96   bool represents_itself;
  97   LayerImpl* target_render_surface_layer;
  98   LayerImpl* current_layer;
  99 };
 100
 101 // An iterator class for walking over layers in the
 102 // RenderSurface-Layer tree.
 103 // TODO(enne): This class probably shouldn't be entirely inline and
 104 // should get moved to a .cc file where it makes sense.
 105 class LayerIterator {
 106  public:
 107   LayerIterator() : render_surface_layer_list_(nullptr) {}
 108
 109   static LayerIterator Begin(const LayerImplList* render_surface_layer_list) {
 110     return LayerIterator(render_surface_layer_list, true);
 111   }
 112   static LayerIterator End(const LayerImplList* render_surface_layer_list) {
 113     return LayerIterator(render_surface_layer_list, false);
 114   }
 115
 116   LayerIterator& operator++() {
 117     MoveToNext();
 118     return *this;
 119   }
 120   bool operator==(const LayerIterator& other) const {
 121     return target_render_surface_layer_index_ ==
 122            other.target_render_surface_layer_index_ &&
 123            current_layer_index_ == other.current_layer_index_;
 124   }
 125   bool operator!=(const LayerIterator& other) const {
 126     return !(*this == other);
 127   }
 128
 129   LayerImpl* operator->() const { return current_layer(); }
 130   LayerImpl* operator*() const { return current_layer(); }
 131
 132   bool represents_target_render_surface() const {
 133     return current_layer_represents_target_render_surface();
 134   }
 135   bool represents_contributing_render_surface() const {
 136     return !represents_target_render_surface() &&
 137            current_layer_represents_contributing_render_surface();
 138   }
 139   bool represents_itself() const {
 140     return !represents_target_render_surface() &&
 141            !represents_contributing_render_surface();
 142   }
 143
 144   LayerImpl* target_render_surface_layer() const {
 145     return render_surface_layer_list_->at(target_render_surface_layer_index_);
 146   }
 147
 148   operator const LayerIteratorPosition() const {
 149     LayerIteratorPosition position;
 150     position.represents_target_render_surface =
 151         represents_target_render_surface();
 152     position.represents_contributing_render_surface =
 153         represents_contributing_render_surface();
 154     position.represents_itself = represents_itself();
 155     position.target_render_surface_layer = target_render_surface_layer();
 156     position.current_layer = current_layer();
 157     return position;
 158   }
 159
 160  private:
 161   LayerIterator(const LayerImplList* render_surface_layer_list, bool start)
 162       : render_surface_layer_list_(render_surface_layer_list),
 163         target_render_surface_layer_index_(0) {
 164     for (size_t i = 0; i < render_surface_layer_list->size(); ++i) {
 165       if (!render_surface_layer_list->at(i)->render_surface()) {
 166         NOTREACHED();
 167         MoveToEnd();
 168         return;
 169       }
 170     }
 171
 172     if (start && !render_surface_layer_list->empty())
 173       MoveToBegin();
 174     else
 175       MoveToEnd();
 176   }
 177
 178   void MoveToBegin() {
 179     target_render_surface_layer_index_ = 0;
 180     current_layer_index_ = target_render_surface_children().size() - 1;
 181     MoveToHighestInSubtree();
 182   }
 183
 184   void MoveToEnd() {
 185     target_render_surface_layer_index_ =
 186         LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex;
 187     current_layer_index_ = 0;
 188   }
 189
 190   void MoveToNext() {
 191     // Moves to the previous layer in the current RS layer list.
 192     // Then we check if the new current layer has its own RS,
 193     // in which case there are things in that RS layer list that are higher,
 194     // so we find the highest layer in that subtree.
 195     // If we move back past the front of the list,
 196     // we jump up to the previous RS layer list, picking up again where we
 197     // had previously recursed into the current RS layer list.
 198
 199     if (!current_layer_represents_target_render_surface()) {
 200       // Subtracting one here will eventually cause the current layer
 201       // to become that layer representing the target render surface.
 202       --current_layer_index_;
 203       MoveToHighestInSubtree();
 204     } else {
 205       while (current_layer_represents_target_render_surface()) {
 206         if (!target_render_surface_layer_index_) {
 207           // End of the list.
 208           target_render_surface_layer_index_ =
 209               LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex;
 210           current_layer_index_ = 0;
 211           return;
 212         }
 213         target_render_surface_layer_index_ =
 214             target_render_surface()->target_render_surface_layer_index_history_;
 215         current_layer_index_ =
 216             target_render_surface()->current_layer_index_history_;
 217       }
 218     }
 219   }
 220
 221   void MoveToHighestInSubtree() {
 222     if (current_layer_represents_target_render_surface())
 223       return;
 224     while (current_layer_represents_contributing_render_surface()) {
 225       // Save where we were in the current target surface, move to the next one,
 226       // and save the target surface that we came from there
 227       // so we can go back to it.
 228       target_render_surface()->current_layer_index_history_ =
 229           current_layer_index_;
 230       int previous_target_render_surface_layer =
 231           target_render_surface_layer_index_;
 232
 233       for (LayerImpl* layer = current_layer();
 234            target_render_surface_layer() != layer;
 235            ++target_render_surface_layer_index_) {
 236       }
 237       current_layer_index_ = target_render_surface_children().size() - 1;
 238
 239       target_render_surface()->target_render_surface_layer_index_history_ =
 240           previous_target_render_surface_layer;
 241     }
 242   }
 243
 244   inline LayerImpl* current_layer() const {
 245     return current_layer_represents_target_render_surface()
 246                ? target_render_surface_layer()
 247                : LayerTreeHostCommon::get_layer_as_raw_ptr(
 248                      target_render_surface_children(), current_layer_index_);
 249   }
 250
 251   inline bool current_layer_represents_contributing_render_surface() const {
 252     return LayerTreeHostCommon::RenderSurfaceContributesToTarget<LayerImpl>(
 253         current_layer(), target_render_surface_layer()->id());
 254   }
 255   inline bool current_layer_represents_target_render_surface() const {
 256     return current_layer_index_ ==
 257            LayerIteratorValue::kLayerIndexRepresentingTargetRenderSurface;
 258   }
 259
 260   inline RenderSurfaceImpl* target_render_surface() const {
 261     return target_render_surface_layer()->render_surface();
 262   }
 263   inline const LayerImplList& target_render_surface_children() const {
 264     return target_render_surface()->layer_list();
 265   }
 266
 267   const LayerImplList* render_surface_layer_list_;
 268
 269   // The iterator's current position.
 270
 271   // A position in the render_surface_layer_list. This points to a layer which
 272   // owns the current target surface. This is a value from 0 to n-1
 273   // (n = size of render_surface_layer_list = number of surfaces).
 274   // A value outside of this range
 275   // (for example, LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex)
 276   // is used to indicate a position outside the bounds of the tree.
 277   int target_render_surface_layer_index_;
 278   // A position in the list of layers that are children of the
 279   // current target surface. When pointing to one of these layers,
 280   // this is a value from 0 to n-1 (n = number of children).
 281   // Since the iterator must also stop at the layers representing
 282   // the target surface, this is done by setting the current_layer_index
 283   // to a value of
 284   // LayerIteratorValue::kLayerIndexRepresentingTargetRenderSurface.
 285   size_t current_layer_index_;
 286 };
 287
 288 }  // namespace cc
 289
 290 #endif  // CC_LAYERS_LAYER_ITERATOR_H_