Move inval dedup to Window for wider usages.

tools/viewer/sk_app/Window.h

Index: tools/viewer/sk_app/Window.h
diff --git a/tools/viewer/sk_app/Window.h b/tools/viewer/sk_app/Window.h
index 63d5e19e5594716c9bc95742b0fac51fe8b9af75..0ebaf2ef1e39873bbe1dc0e2d6e02e3a7d1aad9c 100644
--- a/tools/viewer/sk_app/Window.h
+++ b/tools/viewer/sk_app/Window.h
@@ -27,7 +27,9 @@ public:
 
     virtual void setTitle(const char*) = 0;
     virtual void show() = 0;
-    virtual void inval() = 0;
+
+    // Check if the current window has any pending inval; call inval if not.

[djsollen, 2016/05/23 17:06:48]

update the comment...

// shedules an invalidation event for window if one is not currently pending.

+    void inval();
 
     virtual bool scaleContentToFit() const { return false; }
     virtual bool supportsContentRect() const { return false; }
@@ -164,6 +166,16 @@ protected:
     void*        fPaintUserData;
 
     WindowContext* fWindowContext = nullptr;
+
+    // Make sure that either onPaint or markInvalReceived is called in the subclasses'

[djsollen, 2016/05/23 17:06:48]

I think this comment is outdated.  the client window doesn't need to worry about
this unless they don't call onPaint as part of handling the inval event.

[liyuqian, 2016/05/23 17:29:51]

In case that the client window doesn't call onPaint (e.g., Android), this
comment reminds it to call markInvalProcessed. I'll update the comment to
reflect the name change of inval to onInval.

+    // implementations of inval. They unset fIsContentInvalided which allow invalIfNeeded
+    // to invalidate the content in the future.
+    virtual void onInval() = 0;
+
+    // Uncheck fIsContentInvalided to allow future inval/onInval.
+    void markInvalProcessed();
+
+    bool fIsContentInvalidated = false;  // use this to avoid duplicate invalidate events
 };
 
 }   // namespace sk_app