Convert Gamepad IPC messages into mojo interface.

device/gamepad/public/interfaces/gamepad.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.
 
 module device.mojom;
 
 struct GamepadQuaternion {
   float x;
   float y;
   float z;
@@ skipping 31 matching lines @@
   bool connected;
   array<uint16> id;
   uint64 timestamp;
   array<double> axes;
   array<GamepadButton> buttons;
   array<uint16> mapping;
   GamepadPose? pose;
   GamepadHand hand;
   uint32 display_id;
 };
+
+interface GamepadObserver {
+  GamepadConnected(int32 index, Gamepad gamepad);
+  GamepadDisconnected(int32 index, Gamepad gamepad);
+};
+
+// Asks the browser process to start polling, and return a shared memory
+// handles that will hold the data from the hardware. See

[Tom Sepez, 2016/12/06 17:53:40]

nit: handle that

[ke.he, 2016/12/07 08:05:10]

Done.

+// gamepad_hardware_buffer.h for a description of how synchronization is
+// handled. The number of Starts should match the number of Stops.

[Tom Sepez, 2016/12/06 17:53:40]

nit: or what happens if they don't?  Does the browser poll forever?  Do we clean
up on child termination?

[ke.he, 2016/12/07 08:05:10]

Hi, Tom Sepez, Thanks very much for considering this.
The GamepadService owns a std::Set of consumers. The renderer does
register/unregister consumer in GamepadService by calling
StartPolling/StopPolling. If GamepadService finds the std::Set<consumers> are
not empty, it will ask GamepadProvider to do Polling in the polling thread, to
notify consumers if any updates.
So if a renderer calls Starts, and doesn't call Stops, the polling thread will
keep polling, it means that renderer wants to received updates before it calls
Stops.
We do cleanup on child termination. Each render process "StrongBinds" a
GamepadMonitor(on behalf a consumer) instance in browser side, GamepadMonitor
will un-register in its destructor to avoid cause unnecessary polling.
And, mismatch of Starts and Stops may cause DCHECK() complains too.
 
Actually mojoifiction didn't change the logic of the register/unregister and the
polling. Comments here are copied from the removed
"content/common/gamepad_messages.h".

+interface GamepadMonitor {
+  [Sync]
+  GamepadStartPolling() => (handle<shared_buffer> memory_handle);
+
+  [Sync]
+  GamepadStopPolling() => ();
+
+  SetObserver(GamepadObserver gamepad_observer);
+};