Bug 764188 - Expose a global frame tree generation number. r=dbaron

This generation number will be incremented every time that the frame
tree for a presshell changes. Callers need to deal with the case where
this value overflows. We use this to avoid a bunch of Android
screenshotting.

layout/base/nsCSSFrameConstructor.cpp

@@ -8265,6 +8265,7 @@ nsCSSFrameConstructor::BeginUpdate() {
     rootPresContext->IncrementDOMGeneration();
   }
 
+  ++sGlobalGenerationNumber;
   ++mUpdateCount;
 }
 

layout/base/nsFrameManager.cpp

@@ -1993,3 +1993,5 @@ nsFrameManagerBase::UndisplayedMap::Clear(void)
   mLastLookup = nsnull;
   PL_HashTableEnumerateEntries(mTable, RemoveUndisplayedEntry, 0);
 }
+
+PRUint32 nsFrameManagerBase::sGlobalGenerationNumber;

layout/base/nsFrameManagerBase.h

@@ -53,6 +53,8 @@ public:
     mRootFrame = aRootFrame;
   }
 
+  static PRUint32 GetGlobalGenerationNumber() { return sGlobalGenerationNumber; }
+
 protected:
   class UndisplayedMap;
 
@@ -64,6 +66,16 @@ protected:
   PLDHashTable                    mPlaceholderMap;
   UndisplayedMap*                 mUndisplayedMap;
   bool                            mIsDestroyingFrames;  // The frame manager is destroying some frame(s).
+
+  // The frame tree generation number
+  // We use this to avoid unnecessary screenshotting
+  // on Android. Unfortunately, this is static to match
+  // the single consumer which is also static. Keeping
+  // this the same greatly simplifies lifetime issues and
+  // makes sure we always using the correct number.
+  // A per PresContext generation number is available
+  // via nsPresContext::GetDOMGeneration
+  static PRUint32                 sGlobalGenerationNumber;
 };
 
 #endif

layout/base/nsPresContext.h

@@ -1350,6 +1350,9 @@ public:
 
   /**
    * Get the current DOM generation counter.
+   *
+   * See nsFrameManagerBase::GetGlobalGenerationNumber() for a
+   * global generation number.
    */
   PRUint32 GetDOMGeneration() { return mDOMGeneration; }
 

widget/android/nsAppShell.cpp

@@ -24,6 +24,7 @@
 #include "nsIDOMClientRect.h"
 #include "nsIDOMWakeLockListener.h"
 #include "nsIPowerManagerService.h"
+#include "nsFrameManager.h"
 
 #include "mozilla/Services.h"
 #include "mozilla/unused.h"
@@ -117,6 +118,15 @@ class AfterPaintListener : public nsIDOMEventListener {
     }
 
     virtual nsresult HandleEvent(nsIDOMEvent* aEvent) {
+        PRUint32 generation = nsFrameManager::GetGlobalGenerationNumber();
+        if (mLastGeneration == generation) {
+            // the frame tree has not changed since our last AfterPaint
+            // so we can drop this event.
+            return NS_OK;
+        }
+
+        mLastGeneration = generation;
+
         nsCOMPtr<nsIDOMNotifyPaintEvent> paintEvent = do_QueryInterface(aEvent);
         if (!paintEvent)
             return NS_OK;
@@ -139,6 +149,7 @@ class AfterPaintListener : public nsIDOMEventListener {
     }
 
   private:
+    PRUint32 mLastGeneration;
     nsCOMPtr<nsIDOMEventTarget> mEventTarget;
 };