Support focus highlight in Android window

components/arc/common/accessibility_helper.mojom

+// 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.
+
+// Next MinVersion: 1
+
+module arc.mojom;
+
+import "screen_rect.mojom";
+
+// Next method ID: 1
+interface AccessibilityHelperHost {
+  OnViewFocused@0(ScreenRect rect);

[David Tseng, 2017/01/11 19:26:53]

Just FYI. We will eventually want to pass through not only the rectangle of
focus events, but all of the data contained in theAccessibilityNodeInfo on the
Android side for all event types.

With that in mind, would you mind making this into
OnAccessibilityEvent(EventType type, AccessibilityNodeInfoData data);

where
EventType is an enum of all Android events (you can just put focus at this
point)
and
AccessibilityNodeInfoData is a struct containing data about the
AccessibilityNodeInfo
(you can just put 
boundsInScreen
 for now).
https://developer.android.com/reference/android/view/accessibility/Accessibil...

[David Tseng, 2017/01/11 19:34:55]

Quick correction. We want an array<AccessibilityNodeInfoData> above. It will be
a serialized collection of data.

[yawano, 2017/01/17 02:29:21]

Sorry, why we need to send it as an array? I thought the accessibility node info
data in the second parameter is about event source.

Is it for send accessibility node info tree of a screen? If so, I prefer to add
another method for it as we don't always need accessibility node info tree.

[David Tseng, 2017/01/17 18:46:32]

It would be nice to have just one api.

Yes, an array is needed because it will be used for serialization of the tree.
It also wouldn't prevent us from simply placing one node in the array whenever
focus highlight is on as opposed to ChromeVox.

However, this seems fine for now and I can make any necessary changes.

[yawano, 2017/01/18 01:33:38]

I feel it costly to serialize accessibility node info tree every time an event
has dispatched.

Yes, using array doesn't mean that we always need to pass node info tree and we
can pass single node info, but it would be confusing as sometimes tree is passed
and sometimes not.

Anyway, let's review this CL with current interface, the single accessibility
node info version. We can change this later if we need to pass accessibility
node info tree in onAccessibilityEvent.

[David Tseng, 2017/01/18 18:54:09]

Yes clearly we'd prefer to send over diffs of the tree minimizing the ipc call
-- which I think would be the bulk of the cost; also note that the framework
already serializes the ANI tree to send to the accessibility service. However,
serialization of the whole tree would only be for the initial implementation
(see the design doc I sent that discusses optimizations).
Could be a root with one child or just the ancestry chain of the focused item.
Depending on the client, we would / can prune unneeded nodes. At this point
though, I'd like to see something working before trying to tackle optimizing.
Hope you agree with this especially since ChromeVox is the primary consumer of
focus highlight anyway.
Agreed; let's land this cl first and I'll make the above changes and yes it will
be needed. Thanks for the preemptive changes!

+};
+
+// Next method ID: 1
+interface AccessibilityHelperInstance {
+  Init@0(AccessibilityHelperHost host);
+};