src/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/trees/layer_tree_host_common.h"
  10
  11 namespace cc {
  12
  13 // These classes provide means to iterate over the
  14 // RenderSurface-Layer tree.
  15
  16 // Example code follows, for a tree of Layer/RenderSurface objects.
  17 // See below for details.
  18 //
  19 // void DoStuffOnLayers(
  20 //     const RenderSurfaceLayerList& render_surface_layer_list) {
  21 //   typedef LayerIterator<Layer> LayerIteratorType;
  22 //
  23 //   LayerIteratorType end =
  24 //       LayerIteratorType::End(&render_surface_layer_list);
  25 //   for (LayerIteratorType
  26 //            it = LayerIteratorType::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 // Non-templated constants
  83 struct LayerIteratorValue {
  84   static const int kInvalidTargetRenderSurfaceLayerIndex = -1;
  85   // This must be -1 since the iterator action code assumes that this value can
  86   // be reached by subtracting one from the position of the first layer in the
  87   // current target surface's child layer list, which is 0.
  88   static const int kLayerIndexRepresentingTargetRenderSurface = -1;
  89 };
  90
  91 // The position of a layer iterator that is independent
  92 // of its many template types.
  93 template <typename LayerType> struct LayerIteratorPosition {
  94   bool represents_target_render_surface;
  95   bool represents_contributing_render_surface;
  96   bool represents_itself;
  97   LayerType* target_render_surface_layer;
  98   LayerType* current_layer;
  99 };
 100
 101 // An iterator class for walking over layers in the
 102 // RenderSurface-Layer tree.
 103 template <typename LayerType>
 104 class LayerIterator {
 105   typedef LayerIterator<LayerType> LayerIteratorType;
 106   typedef typename LayerType::RenderSurfaceListType LayerList;
 107   typedef typename LayerType::RenderSurfaceType RenderSurfaceType;
 108
 109  public:
 110   LayerIterator() : render_surface_layer_list_(NULL) {}
 111
 112   static LayerIteratorType Begin(const LayerList* render_surface_layer_list) {
 113     return LayerIteratorType(render_surface_layer_list, true);
 114   }
 115   static LayerIteratorType End(const LayerList* render_surface_layer_list) {
 116     return LayerIteratorType(render_surface_layer_list, false);
 117   }
 118
 119   LayerIteratorType& operator++() {
 120     MoveToNext();
 121     return *this;
 122   }
 123   bool operator==(const LayerIterator& other) const {
 124     return target_render_surface_layer_index_ ==
 125            other.target_render_surface_layer_index_ &&
 126            current_layer_index_ == other.current_layer_index_;
 127   }
 128   bool operator!=(const LayerIteratorType& other) const {
 129     return !(*this == other);
 130   }
 131
 132   LayerType* operator->() const { return current_layer(); }
 133   LayerType* operator*() const { return current_layer(); }
 134
 135   bool represents_target_render_surface() const {
 136     return current_layer_represents_target_render_surface();
 137   }
 138   bool represents_contributing_render_surface() const {
 139     return !represents_target_render_surface() &&
 140            current_layer_represents_contributing_render_surface();
 141   }
 142   bool represents_itself() const {
 143     return !represents_target_render_surface() &&
 144            !represents_contributing_render_surface();
 145   }
 146
 147   LayerType* target_render_surface_layer() const {
 148     return render_surface_layer_list_->at(target_render_surface_layer_index_);
 149   }
 150
 151   operator const LayerIteratorPosition<LayerType>() const {
 152     LayerIteratorPosition<LayerType> position;
 153     position.represents_target_render_surface =
 154         represents_target_render_surface();
 155     position.represents_contributing_render_surface =
 156         represents_contributing_render_surface();
 157     position.represents_itself = represents_itself();
 158     position.target_render_surface_layer = target_render_surface_layer();
 159     position.current_layer = current_layer();
 160     return position;
 161   }
 162
 163  private:
 164   LayerIterator(const LayerList* render_surface_layer_list, bool start)
 165       : render_surface_layer_list_(render_surface_layer_list),
 166         target_render_surface_layer_index_(0) {
 167     for (size_t i = 0; i < render_surface_layer_list->size(); ++i) {
 168       if (!render_surface_layer_list->at(i)->render_surface()) {
 169         NOTREACHED();
 170         MoveToEnd();
 171         return;
 172       }
 173     }
 174
 175     if (start && !render_surface_layer_list->empty())
 176       MoveToBegin();
 177     else
 178       MoveToEnd();
 179   }
 180
 181   void MoveToBegin() {
 182     target_render_surface_layer_index_ = 0;
 183     current_layer_index_ = target_render_surface_children().size() - 1;
 184     MoveToHighestInSubtree();
 185   }
 186
 187   void MoveToEnd() {
 188     target_render_surface_layer_index_ =
 189         LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex;
 190     current_layer_index_ = 0;
 191   }
 192
 193   void MoveToNext() {
 194     // Moves to the previous layer in the current RS layer list.
 195     // Then we check if the new current layer has its own RS,
 196     // in which case there are things in that RS layer list that are higher,
 197     // so we find the highest layer in that subtree.
 198     // If we move back past the front of the list,
 199     // we jump up to the previous RS layer list, picking up again where we
 200     // had previously recursed into the current RS layer list.
 201
 202     if (!current_layer_represents_target_render_surface()) {
 203       // Subtracting one here will eventually cause the current layer
 204       // to become that layer representing the target render surface.
 205       --current_layer_index_;
 206       MoveToHighestInSubtree();
 207     } else {
 208       while (current_layer_represents_target_render_surface()) {
 209         if (!target_render_surface_layer_index_) {
 210           // End of the list.
 211           target_render_surface_layer_index_ =
 212               LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex;
 213           current_layer_index_ = 0;
 214           return;
 215         }
 216         target_render_surface_layer_index_ =
 217             target_render_surface()->target_render_surface_layer_index_history_;
 218         current_layer_index_ =
 219             target_render_surface()->current_layer_index_history_;
 220       }
 221     }
 222   }
 223
 224   void MoveToHighestInSubtree() {
 225     if (current_layer_represents_target_render_surface())
 226       return;
 227     while (current_layer_represents_contributing_render_surface()) {
 228       // Save where we were in the current target surface, move to the next one,
 229       // and save the target surface that we came from there
 230       // so we can go back to it.
 231       target_render_surface()->current_layer_index_history_ =
 232           current_layer_index_;
 233       int previous_target_render_surface_layer =
 234           target_render_surface_layer_index_;
 235
 236       for (LayerType* layer = current_layer();
 237            target_render_surface_layer() != layer;
 238            ++target_render_surface_layer_index_) {
 239       }
 240       current_layer_index_ = target_render_surface_children().size() - 1;
 241
 242       target_render_surface()->target_render_surface_layer_index_history_ =
 243           previous_target_render_surface_layer;
 244     }
 245   }
 246
 247   inline LayerType* current_layer() const {
 248     return current_layer_represents_target_render_surface()
 249            ? target_render_surface_layer()
 250            : target_render_surface_children().at(current_layer_index_);
 251   }
 252
 253   inline bool current_layer_represents_contributing_render_surface() const {
 254     return LayerTreeHostCommon::RenderSurfaceContributesToTarget<LayerType>(
 255         current_layer(), target_render_surface_layer()->id());
 256   }
 257   inline bool current_layer_represents_target_render_surface() const {
 258     return current_layer_index_ ==
 259            LayerIteratorValue::kLayerIndexRepresentingTargetRenderSurface;
 260   }
 261
 262   inline RenderSurfaceType* target_render_surface() const {
 263     return target_render_surface_layer()->render_surface();
 264   }
 265   inline const LayerList& target_render_surface_children() const {
 266     return target_render_surface()->layer_list();
 267   }
 268
 269   const LayerList* render_surface_layer_list_;
 270
 271   // The iterator's current position.
 272
 273   // A position in the render_surface_layer_list. This points to a layer which
 274   // owns the current target surface. This is a value from 0 to n-1
 275   // (n = size of render_surface_layer_list = number of surfaces).
 276   // A value outside of this range
 277   // (for example, LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex)
 278   // is used to indicate a position outside the bounds of the tree.
 279   int target_render_surface_layer_index_;
 280   // A position in the list of layers that are children of the
 281   // current target surface. When pointing to one of these layers,
 282   // this is a value from 0 to n-1 (n = number of children).
 283   // Since the iterator must also stop at the layers representing
 284   // the target surface, this is done by setting the current_layerIndex
 285   // to a value of LayerIteratorValue::LayerRepresentingTargetRenderSurface.
 286   int current_layer_index_;
 287 };
 288
 289 }  // namespace cc
 290
 291 #endif  // CC_LAYERS_LAYER_ITERATOR_H_