Bug 1429932 - Part 3: Refactor RetainedDisplayListBuilder::AttemptPartialUpdate to have an early return instead of a nested scope. r=miko

MozReview-Commit-ID: L91euwUeJ5x

layout/painting/RetainedDisplayListBuilder.cpp

@@ -987,15 +987,30 @@ ClearFrameProps(nsTArray<nsIFrame*>& aFrames)
   }
 }
 
+class AutoClearFramePropsArray
+{
+public:
+  AutoClearFramePropsArray() = default;
+
+  ~AutoClearFramePropsArray()
+  {
+    ClearFrameProps(mFrames);
+  }
+
+  nsTArray<nsIFrame*>& Frames() { return mFrames; }
+
+  bool IsEmpty() const { return mFrames.IsEmpty(); }
+
+private:
+  nsTArray<nsIFrame*> mFrames;
+};
+
 void
 RetainedDisplayListBuilder::ClearFramesWithProps()
 {
-  nsTArray<nsIFrame*> modifiedFrames;
-  nsTArray<nsIFrame*> framesWithProps;
-  GetModifiedAndFramesWithProps(&mBuilder, &modifiedFrames, &framesWithProps);
-
-  ClearFrameProps(modifiedFrames);
-  ClearFrameProps(framesWithProps);
+  AutoClearFramePropsArray modifiedFrames;
+  AutoClearFramePropsArray framesWithProps;
+  GetModifiedAndFramesWithProps(&mBuilder, &modifiedFrames.Frames(), &framesWithProps.Frames());
 }
 
 bool
@@ -1008,24 +1023,27 @@ RetainedDisplayListBuilder::AttemptPartialUpdate(nscolor aBackstop)
 
   mBuilder.EnterPresShell(mBuilder.RootReferenceFrame());
 
-  nsTArray<nsIFrame*> modifiedFrames;
-  nsTArray<nsIFrame*> framesWithProps;
-  GetModifiedAndFramesWithProps(&mBuilder, &modifiedFrames, &framesWithProps);
+  // We set the override dirty regions during ComputeRebuildRegion or in
+  // nsLayoutUtils::InvalidateForDisplayPortChange. The display port change also
+  // marks the frame modified, so those regions are cleared here as well.
+  AutoClearFramePropsArray modifiedFrames;
+  AutoClearFramePropsArray framesWithProps;
+  GetModifiedAndFramesWithProps(&mBuilder, &modifiedFrames.Frames(), &framesWithProps.Frames());
 
   // Do not allow partial builds if the retained display list is empty, or if
   // ShouldBuildPartial heuristic fails.
-  const bool shouldBuildPartial = !mList.IsEmpty() && ShouldBuildPartial(modifiedFrames);
+  const bool shouldBuildPartial = !mList.IsEmpty() && ShouldBuildPartial(modifiedFrames.Frames());
 
   if (mPreviousCaret != mBuilder.GetCaretFrame()) {
     if (mPreviousCaret) {
       if (mBuilder.MarkFrameModifiedDuringBuilding(mPreviousCaret)) {
-        modifiedFrames.AppendElement(mPreviousCaret);
+        modifiedFrames.Frames().AppendElement(mPreviousCaret);
       }
     }
 
     if (mBuilder.GetCaretFrame()) {
       if (mBuilder.MarkFrameModifiedDuringBuilding(mBuilder.GetCaretFrame())) {
-        modifiedFrames.AppendElement(mBuilder.GetCaretFrame());
+        modifiedFrames.Frames().AppendElement(mBuilder.GetCaretFrame());
       }
     }
 
@@ -1034,10 +1052,13 @@ RetainedDisplayListBuilder::AttemptPartialUpdate(nscolor aBackstop)
 
   nsRect modifiedDirty;
   AnimatedGeometryRoot* modifiedAGR = nullptr;
-  bool merged = false;
-  if (shouldBuildPartial &&
-      ComputeRebuildRegion(modifiedFrames, &modifiedDirty,
-                           &modifiedAGR, framesWithProps)) {
+  if (!shouldBuildPartial ||
+      !ComputeRebuildRegion(modifiedFrames.Frames(), &modifiedDirty,
+                           &modifiedAGR, framesWithProps.Frames())) {
+    mBuilder.LeavePresShell(mBuilder.RootReferenceFrame(), &mList);
+    return false;
+  }
+
   modifiedDirty.IntersectRect(modifiedDirty, mBuilder.RootReferenceFrame()->GetVisualOverflowRectRelativeToSelf());
 
   PreProcessDisplayList(&mList, modifiedAGR);
@@ -1075,16 +1096,6 @@ RetainedDisplayListBuilder::AttemptPartialUpdate(nscolor aBackstop)
   //printf_stderr("Painting --- Merged list:\n");
   //nsFrame::PrintDisplayList(&mBuilder, mList);
 
-    merged = true;
-  }
-
   mBuilder.LeavePresShell(mBuilder.RootReferenceFrame(), &mList);
-
-  // We set the override dirty regions during ComputeRebuildRegion or in
-  // nsLayoutUtils::InvalidateForDisplayPortChange. The display port change also
-  // marks the frame modified, so those regions are cleared here as well.
-  ClearFrameProps(modifiedFrames);
-  ClearFrameProps(framesWithProps);
-
-  return merged;
+  return true;
 }