Move inval dedup to Window for wider usages.

tools/viewer/sk_app/Window.cpp

Index: tools/viewer/sk_app/Window.cpp
diff --git a/tools/viewer/sk_app/Window.cpp b/tools/viewer/sk_app/Window.cpp
index 0a7bcf8a70b75c0f5d61a4dfdb2b36408275e13d..bf4117bac80bab5e25cf641b11bfb594d7cd265b 100644
--- a/tools/viewer/sk_app/Window.cpp
+++ b/tools/viewer/sk_app/Window.cpp
@@ -63,6 +63,8 @@ bool Window::onTouch(int owner, InputState state, float x, float y) {
 }
 
 void Window::onPaint() {
+    SkAutoMutexAcquire ama(fMutex);

[jvanverth1, 2016/05/23 14:22:32]

How long does the mutex take? On single-threaded systems we'd want to run as
fast as possible. Maybe place in SKIA_BUILD_ANDROID block?

+    fIsContentInvalidated = false;

[djsollen, 2016/05/23 14:44:26]

putting this here won't work on Android as the inval message calls
paintIfNeeded() which checks the status of the backing window before getting
here.  So you could end up in a situation like...

1) post inval message
2) receive inval but don't call onPaint b/c the surface is not ready
3) all subsequent attempts to inval will be ignored
4) nothing ever draws. :(

[liyuqian, 2016/05/23 14:58:24]

Good catch! Revised.

     sk_sp<SkSurface> backbuffer = fWindowContext->getBackbufferSurface();
     if (backbuffer) {
         // draw into the canvas of this surface
@@ -92,4 +94,12 @@ void Window::setDisplayParams(const DisplayParams& params) {
     fWindowContext->setDisplayParams(params);
 }
 
+void Window::checkAndInval() {
+    SkAutoMutexAcquire ama(fMutex);
+    if (!fIsContentInvalidated) {
+        fIsContentInvalidated = true;
+        inval();
+    }
+}
+
 }   // namespace sk_app