[WebView] Add a new flushVisualState API to AwContents.

android_webview/java/src/org/chromium/android_webview/AwContents.java

Index: android_webview/java/src/org/chromium/android_webview/AwContents.java
diff --git a/android_webview/java/src/org/chromium/android_webview/AwContents.java b/android_webview/java/src/org/chromium/android_webview/AwContents.java
index 3ee10deafe17e003768787f98b007027fe113eda..7fee065f83a1a62786342e0de1b05ddd04647703 100644
--- a/android_webview/java/src/org/chromium/android_webview/AwContents.java
+++ b/android_webview/java/src/org/chromium/android_webview/AwContents.java
@@ -92,6 +92,8 @@ public class AwContents implements SmartClipProvider {
     // produce little visible difference.
     private static final float ZOOM_CONTROLS_EPSILON = 0.007f;
 
+    private static final String BLANK_URL = "about:blank";
+
     /**
      * WebKit hit test related data strcutre. These are used to implement
      * getHitTestResult, requestFocusNodeHref, requestImageRef methods in WebView.
@@ -254,6 +256,9 @@ public class AwContents implements SmartClipProvider {
     // when in this state.
     private boolean mTemporarilyDetached;
 
+    // This flag indicates that we're currently loading BLANK_URL from loadBlankImmediately.
+    private boolean mIsLoadingBlankImmediately = false;
+
     // True when this AwContents has been destroyed.
     // Do not use directly, call isDestroyed() instead.
     private boolean mIsDestroyed = false;
@@ -833,7 +838,7 @@ public class AwContents implements SmartClipProvider {
         if (mWebContentsObserver != null) {
             mWebContentsObserver.detachFromWebContents();
         }
-        mWebContentsObserver = new AwWebContentsObserver(mWebContents, mContentsClient);
+        mWebContentsObserver = new AwWebContentsObserver(mWebContents, this, mContentsClient);
     }
 
     /**
@@ -1078,6 +1083,24 @@ public class AwContents implements SmartClipProvider {
                 mScrollOffsetManager.computeVerticalScrollRange()));
     }
 
+    void onPageFinished(String url) {
+        if (mIsLoadingBlankImmediately && BLANK_URL.equals(url)) {

[mkosiba (inactive), 2015/01/14 18:17:12]

this seems race-prone as we might end up running this code as a response to some
previous 'random' navigation to about:blank. I guess that shouldn't be a problem
in practice unless there is some code in the renderer that navigates to
about:blank and then immediately somewhere else.

[Ignacio Solla, 2015/01/16 16:02:07]

I doubt that the race that you describe will actually happen in practice, so
I'be tempted to not worry about it.

If you feel that we should fix this potential problem, a solution would be to
pass a token with the about:blank load to the renderer, and only clear the state
when the token is received back, but I think that that would be overkill for
sth. that we are not even sure that it occurs in practice. 

[mkosiba (inactive), 2015/01/19 11:15:57]

yeath, sorry. I just wanted to say "at first glance this looks like a race, but
I don't think it is". Probably worth a comment explaining why we're not using
that extra token you described.

[Ignacio Solla, 2015/01/19 18:26:40]

Acknowledged.

+            // Waiting for onPageFinished before flashing ensures that the flush callback will not
+            // be received before BLANK_URL has made it into the screen.
+            flushVisualState(new VisualStateFlushCallback() {
+                @Override
+                public void onFailure(int reason) {
+                    mIsLoadingBlankImmediately = false;
+                }
+
+                @Override
+                public void onComplete() {
+                    mIsLoadingBlankImmediately = false;
+                }
+            });
+        }
+    }
+
     public void clearView() {
         if (!isDestroyed()) nativeClearView(mNativeAwContents);
     }
@@ -1142,6 +1165,19 @@ public class AwContents implements SmartClipProvider {
     }
 
     /**
+     * Loads "about:blank" and draws the background color specified by
+     * {@link AwContents#setBackgroundColor(int)} immediately without having to wait for the
+     * asynchronous load to complete ({@link AwContentsClient#onPageFinished(String)}).
+     *
+     * <p>This API is useful when recycling container views and want to clear the contents from the
+     * old page immediately.
+     */
+    public void loadBlankImmediately() {

[mkosiba (inactive), 2015/01/14 18:17:12]

maybe ignore any extra calls for as long as mIsLoadingBlankImmediately is true?

[Ignacio Solla, 2015/01/16 16:02:07]

Done.

+        loadUrl(new LoadUrlParams(BLANK_URL));
+        mIsLoadingBlankImmediately = true;
+    }
+
+    /**
      * Load url without fixing up the url string. Consumers of ContentView are responsible for
      * ensuring the URL passed in is properly formatted (i.e. the scheme has been added if left
      * off during user input).
@@ -1243,7 +1279,9 @@ public class AwContents implements SmartClipProvider {
         // Do not ask the ContentViewCore for the background color, as it will always
         // report white prior to initial navigation or post destruction,  whereas we want
         // to use the client supplied base value in those cases.
-        if (isDestroyed() || !mContentsClient.isCachedRendererBackgroundColorValid()) {
+        // When loading BLANK_URL we want to draw the client supplied base color immediately.
+        if (isDestroyed() || !mContentsClient.isCachedRendererBackgroundColorValid()
+                || mIsLoadingBlankImmediately) {
             return mBaseBackgroundColor;
         }
         return mContentsClient.getCachedRendererBackgroundColor();
@@ -2020,6 +2058,31 @@ public class AwContents implements SmartClipProvider {
         if (!isDestroyed()) nativeSetJsOnlineProperty(mNativeAwContents, networkUp);
     }
 
+    /**
+     * Callback used when flushing the visual state, see {@link #flushVisualState}.
+     */
+    @VisibleForTesting
+    public abstract static class VisualStateFlushCallback {

[mkosiba (inactive), 2015/01/14 18:17:12]

these usually go at the top of the file

[Ignacio Solla, 2015/01/16 16:02:07]

Done.

+        public abstract void onComplete();
+        public abstract void onFailure(int reason);
+    }
+
+    /**
+     * Flush the visual state.
+     *
+     * Flushing the visual state means queuing a callback in Blink that will be invoked when the
+     * contents of the DOM tree at the moment that the callback was enqueued (or later) are drawn
+     * into the screen. In other words, the following events need to happen before the callback is
+     * invoked:
+     * 1. The DOM tree is committed becoming the pending tree - see ThreadProxy::BeginMainFrame
+     * 2. The pending tree is activated becoming the active tree
+     * 3. A frame swap happens that draws the active tree into the screen
+     */
+    @VisibleForTesting
+    public void flushVisualState(VisualStateFlushCallback callback) {
+        nativeFlushVisualState(mNativeAwContents, callback);
+    }
+
     //--------------------------------------------------------------------------------------------
     //  Methods called from native via JNI
     //--------------------------------------------------------------------------------------------
@@ -2123,6 +2186,15 @@ public class AwContents implements SmartClipProvider {
         mContentsClient.getCallbackHelper().postOnNewPicture(mPictureListenerContentProvider);
     }
 
+    @CalledByNative
+    public static void flushVisualStateCallback(VisualStateFlushCallback callback, int value) {
+        if (value == 0) {
+            callback.onComplete();
+        } else {
+            callback.onFailure(value);
+        }
+    }
+
     // Called as a result of nativeUpdateLastHitTestData.
     @CalledByNative
     private void updateHitTestData(
@@ -2337,8 +2409,9 @@ public class AwContents implements SmartClipProvider {
 
         @Override
         public void onDraw(Canvas canvas) {
-            if (isDestroyed()) {
-                TraceEvent.instant("EarlyOut_destroyed");
+            if (isDestroyed() || mIsLoadingBlankImmediately) {
+                String trace = isDestroyed() ? "EarlyOut_destroyed" : "EarlyOut_loadingBlank";
+                TraceEvent.instant(trace);
                 canvas.drawColor(getEffectiveBackgroundColor());
                 return;
             }
@@ -2688,6 +2761,8 @@ public class AwContents implements SmartClipProvider {
     private native long nativeGetAwDrawGLViewContext(long nativeAwContents);
     private native long nativeCapturePicture(long nativeAwContents, int width, int height);
     private native void nativeEnableOnNewPicture(long nativeAwContents, boolean enabled);
+    private native void nativeFlushVisualState(long nativeAwContents,
+            VisualStateFlushCallback callback);
     private native void nativeClearView(long nativeAwContents);
     private native void nativeSetExtraHeadersForUrl(long nativeAwContents,
             String url, String extraHeaders);