Introduce ViewRoot to forward input/view events to native

ui/android/java/src/org/chromium/ui/base/EventHandler.java

Index: ui/android/java/src/org/chromium/ui/base/EventHandler.java
diff --git a/ui/android/java/src/org/chromium/ui/base/EventHandler.java b/ui/android/java/src/org/chromium/ui/base/EventHandler.java
new file mode 100644
index 0000000000000000000000000000000000000000..e908d464792cbda6b42bc967d80033f16a5f7fbf
--- /dev/null
+++ b/ui/android/java/src/org/chromium/ui/base/EventHandler.java
@@ -0,0 +1,40 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.ui.base;
+
+import org.chromium.base.annotations.CalledByNative;
+import org.chromium.base.annotations.JNINamespace;
+
+/**
+ * Class used to forward view, input events down to native.
+ */
+@JNINamespace("ui")
+public class EventHandler {

[Ted C, 2016/12/15 23:36:44]

from the discussion yesterday, I think EventDisptacher or EventRouter is
probably better than Handler.  Handler to me implies it does something and this
is just an intermediary.

And talking with boliu@, I think EventForwarder is potentially a better
alternative.

I then struggled with the concept that this class should exist only once for
each conceptual root of a view tree (WindowAndroid on Chrome, the child
ViewAndroid's in WebView).  While I think the need for this discrepancy is
unfortunate, this certainly predates your change is as orthogonal as you can
get.

To make it even clearer that this should exist only at the "root" of these
views, I actually propose we change the name to ViewRootEventForwarder.  Then it
should be clear that you can't get one of these for each view and start sending
events (which is what I "thought" you could do without reading this
further/talking to Bo).

And talking further with dtrainor@ and boliu@, we think the name of this should
just be ViewRoot.

[Jinsuk Kim, 2016/12/16 02:04:17]

Thanks for the discussion for better naming. Done.

+    // The corresponding native ViewAndroid. This object can only be used while
+    // the native instance is alive.
+    private long mNativeView;
+
+    @CalledByNative
+    private static EventHandler create(long nativeView) {
+        return new EventHandler(nativeView);
+    }
+
+    private EventHandler(long nativeView) {
+        mNativeView = nativeView;
+    }
+
+    public void onPhysicalBackingSizeChanged(int width, int height) {

[Ted C, 2016/12/15 23:36:44]

can you add javadoc here...physical backing size isn't something that is
immediately obvious to those not in the know

[Jinsuk Kim, 2016/12/16 02:04:17]

Done.

+        assert mNativeView != 0;
+        nativeOnPhysicalBackingSizeChanged(mNativeView, width, height);
+    }
+
+    @CalledByNative
+    private void onDestroyNativeView() {
+        mNativeView = 0;
+    }
+
+    private static native void nativeOnPhysicalBackingSizeChanged(long viewAndroid,
+            int width, int height);
+}