cc: Refactored UpdateTilingsToCurrentRasterSource a bit; added comments

This patch is just a small refactor with a few more comments explaining
what this function is meant to do. I've been confused in the past when
trying to remember what this function does, so hopefully this helps...

R=danakj, enne

Review URL: https://codereview.chromium.org/973713002

Cr-Commit-Position: refs/heads/master@{#318931}

cc/resources/picture_layer_tiling_set.cc

@@ -53,48 +53,53 @@ PictureLayerTilingSet::PictureLayerTilingSet(
 PictureLayerTilingSet::~PictureLayerTilingSet() {
 }
 
-void PictureLayerTilingSet::UpdateTilingsToCurrentRasterSource(
-    scoped_refptr<RasterSource> raster_source,
-    const PictureLayerTilingSet* twin_set,
-    const Region& layer_invalidation,
-    float minimum_contents_scale,
-    float maximum_contents_scale) {
-  RemoveTilingsBelowScale(minimum_contents_scale);
-  RemoveTilingsAboveScale(maximum_contents_scale);
-
-  // Copy over tilings that are shared with the |twin_set| tiling set (if it
-  // exists).
-  if (twin_set) {
-    if (twin_set->tilings_.empty()) {
+void PictureLayerTilingSet::CopyTilingsFromPendingTwin(
+    const PictureLayerTilingSet* pending_twin_set,
+    const scoped_refptr<RasterSource>& raster_source) {
+  if (pending_twin_set->tilings_.empty()) {
     // If the twin (pending) tiling set is empty, it was not updated for the
     // current frame. So we drop tilings from our set as well, instead of
     // leaving behind unshared tilings that are all non-ideal.
     RemoveAllTilings();
   }
 
-    for (PictureLayerTiling* twin_tiling : twin_set->tilings_) {
-      float contents_scale = twin_tiling->contents_scale();
-      DCHECK_GE(contents_scale, minimum_contents_scale);
-      DCHECK_LE(contents_scale, maximum_contents_scale);
-
+  for (PictureLayerTiling* pending_twin_tiling : pending_twin_set->tilings_) {
+    float contents_scale = pending_twin_tiling->contents_scale();
     PictureLayerTiling* this_tiling = FindTilingWithScale(contents_scale);
     if (!this_tiling) {
       scoped_ptr<PictureLayerTiling> new_tiling = PictureLayerTiling::Create(
-            contents_scale, raster_source, client_,
-            max_tiles_for_interest_area_, skewport_target_time_in_seconds_,
+          contents_scale, raster_source, client_, max_tiles_for_interest_area_,
+          skewport_target_time_in_seconds_,
           skewport_extrapolation_limit_in_content_pixels_);
       tilings_.push_back(new_tiling.Pass());
       this_tiling = tilings_.back();
     }
-      this_tiling->CloneTilesAndPropertiesFrom(*twin_tiling);
-    }
+    this_tiling->CloneTilesAndPropertiesFrom(*pending_twin_tiling);
   }
+}
+
+void PictureLayerTilingSet::UpdateTilingsToCurrentRasterSource(
+    scoped_refptr<RasterSource> raster_source,
+    const PictureLayerTilingSet* pending_twin_set,
+    const Region& layer_invalidation,
+    float minimum_contents_scale,
+    float maximum_contents_scale) {
+  RemoveTilingsBelowScale(minimum_contents_scale);
+  RemoveTilingsAboveScale(maximum_contents_scale);
 
-  // For unshared tilings, invalidate tiles and update them to the new raster
-  // source.
+  // Copy over tilings that are shared with the |pending_twin_set| tiling set
+  // (if it exists).
+  if (pending_twin_set)
+    CopyTilingsFromPendingTwin(pending_twin_set, raster_source);
+
+  // If the tiling is not shared (FindTilingWithScale returns nullptr) or if
+  // |this| is the sync set (pending_twin_set is nullptr), then invalidate
+  // tiles and update them to the new raster source.
   for (PictureLayerTiling* tiling : tilings_) {
-    if (twin_set && twin_set->FindTilingWithScale(tiling->contents_scale()))
+    if (pending_twin_set &&
+        pending_twin_set->FindTilingWithScale(tiling->contents_scale())) {
       continue;
+    }
 
     tiling->SetRasterSourceAndResize(raster_source);
     tiling->Invalidate(layer_invalidation);
@@ -104,9 +109,9 @@ void PictureLayerTilingSet::UpdateTilingsToCurrentRasterSource(
     // raster source.
     tiling->CreateMissingTilesInLiveTilesRect();
 
-    // If |twin_set| is present, use the resolutions from there. Otherwise leave
-    // all resolutions as they are.
-    if (twin_set)
+    // If |pending_twin_set| is present, then |this| is active and |tiling| is
+    // not in the pending set, which means it is now NON_IDEAL_RESOLUTION.
+    if (pending_twin_set)
       tiling->set_resolution(NON_IDEAL_RESOLUTION);
   }
 
@@ -126,11 +131,12 @@ void PictureLayerTilingSet::UpdateTilingsToCurrentRasterSource(
     DCHECK_LE(NumHighResTilings(), 1);
     // When commiting from the main thread the high res tiling may get dropped,
     // but when cloning to the active tree, there should always be one.
-    if (twin_set) {
+    if (pending_twin_set) {
       DCHECK_EQ(1, NumHighResTilings())
           << " num tilings on active: " << tilings_.size()
-          << " num tilings on pending: " << twin_set->tilings_.size()
-          << " num high res on pending: " << twin_set->NumHighResTilings()
+          << " num tilings on pending: " << pending_twin_set->tilings_.size()
+          << " num high res on pending: "
+          << pending_twin_set->NumHighResTilings()
           << " are on active tree: " << (client_->GetTree() == ACTIVE_TREE);
     }
   }

cc/resources/picture_layer_tiling_set.h

@@ -55,9 +55,15 @@ class CC_EXPORT PictureLayerTilingSet {
                       PictureLayerTilingSet* recycled_twin_set);
   void RemoveNonIdealTilings();
 
+  // This function can be called on both the active and pending tree.
+  // |pending_twin_set| represents the current pending twin. In situations where
+  // this is called on the active tree in two trees situations,
+  // |pending_twin_set| represents the tiling set from the pending twin layer.
+  // In situations where this is called on the sync tree (whether it's pending
+  // or active in cases of one tree), |pending_twin_set| should be nullptr.
   void UpdateTilingsToCurrentRasterSource(
       scoped_refptr<RasterSource> raster_source,
-      const PictureLayerTilingSet* twin_set,
+      const PictureLayerTilingSet* pending_twin_set,
       const Region& layer_invalidation,
       float minimum_contents_scale,
       float maximum_contents_scale);
@@ -163,6 +169,10 @@ class CC_EXPORT PictureLayerTilingSet {
       float skewport_target_time_in_seconds,
       int skewport_extrapolation_limit_in_content_pixels);
 
+  void CopyTilingsFromPendingTwin(
+      const PictureLayerTilingSet* pending_twin_set,
+      const scoped_refptr<RasterSource>& raster_source);
+
   // Remove one tiling.
   void Remove(PictureLayerTiling* tiling);