Refactor code listening to platform events in content/renderer/.

content/public/renderer/platform_event_observer.h

Index: content/public/renderer/platform_event_observer.h
diff --git a/content/public/renderer/platform_event_observer.h b/content/public/renderer/platform_event_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..0f73dbd5240eac721a420f2de8b3b9aa22359006
--- /dev/null
+++ b/content/public/renderer/platform_event_observer.h
@@ -0,0 +1,118 @@
+// Copyright 2014 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.
+
+#ifndef CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_
+#define CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_
+
+#include "base/logging.h"
+#include "content/public/renderer/render_process_observer.h"
+#include "content/public/renderer/render_thread.h"
+
+namespace blink {
+class WebPlatformEventListener;
+}
+
+namespace content {
+
+// This class is used as a base class for PlatformEventObserver<ListenerType> to
+// allow storing PlatformEventObserver<> with different typename in the same
+// place.
+class PlatformEventObserverBase {
+ public:
+  virtual ~PlatformEventObserverBase() { }
+
+  // Methods that need to be exposed in PlatformEventObserverBase. Their purpose
+  // is described in PlatformEventObserver<>.
+
+  virtual void Start(blink::WebPlatformEventListener* listener) = 0;
+  virtual void Stop() = 0;
+  virtual void SendFakeDataForTesting(void*) = 0;
+};
+
+// PlatformEventObserver<> defines the basic skeleton for an object requesting
+// the browser process to start/stop listening to some platform/hardware events
+// and observe the result.
+// The results are received via IPC, assuming that the object was correctly
+// registered as an observer via the constructor taking a RenderThread.
+template <typename ListenerType>
+class PlatformEventObserver : public PlatformEventObserverBase,
+                              public RenderProcessObserver {
+ public:
+  // Creates a PlatformEventObserver that doesn't listen to response from the

[Avi (use Gerrit), 2014/08/07 19:56:11]

... listen to responses ...

[mlamouri (slow - plz ping), 2014/08/08 18:08:35]

Done.

+  // browser process. Can be used for testing purposes or for observers that
+  // have other means to get their results.
+  PlatformEventObserver()
+      : is_observing_(false),
+        listener_(0) {
+  }
+
+  // Creates a PlatformEventObserver that registers to the RenderThread in order
+  // to intercept the received IPC messages (via OnControlMessageReceived). If
+  // |thread| is null, it will not register.
+  explicit PlatformEventObserver(RenderThread* thread)
+      : is_observing_(false),
+        listener_(0) {
+    if (thread)
+      thread->AddObserver(this);
+  }
+
+  // The observer will automatically stop observing when destroyed in case of it
+  // did not stop before.

[Avi (use Gerrit), 2014/08/07 19:56:11]

... in case it did not stop before.

[mlamouri (slow - plz ping), 2014/08/08 18:08:35]

Done.

+  virtual ~PlatformEventObserver() {
+    if (is_observing())
+      Stop();
+  }
+
+  // Called when a new IPC message is received. Must be used to listen to the
+  // responses from the browser process if any is expected.

[Avi (use Gerrit), 2014/08/07 19:56:11]

... if any are expected.

[mlamouri (slow - plz ping), 2014/08/08 18:08:36]

Done.

+  virtual bool OnControlMessageReceived(const IPC::Message& msg) OVERRIDE = 0;
+
+  // Start observing. Will request the browser process to start listening to the
+  // events. |listener| will receive any response from the browser process.
+  // Note: should not be called if already observing.
+  virtual void Start(blink::WebPlatformEventListener* listener) {

[Avi (use Gerrit), 2014/08/07 19:56:11]

Can you correctly type this parameter as ListenerType rather than
blink::WebPlatformEventListener?

[mlamouri (slow - plz ping), 2014/08/08 18:08:35]

Using ListenerType would require the callers to cast the listener to
ListenerType. RendererWebKitPlatformSupportImpl only has
blink::WebPlatformEventListener from ::startListening().

The goal is to keep the details of the implementation to the class implementing
this template.

+    DCHECK(!is_observing());
+    listener_ = static_cast<ListenerType*>(listener);
+    is_observing_ = true;
+
+    SendStartMessage();
+  }
+
+  // Stop observing. Will let the browser know that it doesn't need to observe
+  // anymore.
+  virtual void Stop() {
+    DCHECK(is_observing());
+    listener_ = 0;
+    is_observing_ = false;
+
+    SendStopMessage();
+  }
+
+ protected:
+  // This method is expected to send an IPC to the browser process to let it
+  // know that it should start observing.
+  virtual void SendStartMessage() = 0;

[Avi (use Gerrit), 2014/08/07 19:56:11]

It's "expected" to send? Do you mean that it should be overridden by subclasses
to do so? If so, say so, and if not, what do you mean?

[mlamouri (slow - plz ping), 2014/08/08 18:08:35]

Done.

+
+  // This method is expected to send an IPC to the browser process to let it
+  /// know that it should stop observing.

[Avi (use Gerrit), 2014/08/07 19:56:11]

Same question as above. Also, fix ///.

[mlamouri (slow - plz ping), 2014/08/08 18:08:35]

Done.

+  virtual void SendStopMessage() = 0;
+
+  bool is_observing() const {
+    return is_observing_;
+  }
+
+  ListenerType* listener() {
+    return listener_;
+  }
+
+ private:
+  bool is_observing_;
+  ListenerType* listener_;
+
+  DISALLOW_COPY_AND_ASSIGN(PlatformEventObserver);
+};
+
+} // namespace content
+
+#endif // CONTENT_PUBLIC_RENDERER_PLATFORM_EVENT_OBSERVER_H_