Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/presentation/PresentationController.cpp

+// Copyright 2015 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.
+
+#include "config.h"
+#include "modules/presentation/PresentationController.h"
+
+#include "core/frame/LocalFrame.h"
+#include "public/platform/WebPresentationClient.h"
+#include "public/platform/WebPresentationController.h"
+
+namespace blink {
+
+PresentationController::PresentationController(LocalFrame& frame, WebPresentationClient* client)
+    : m_client(client)
+{
+    if (m_client)
+        m_client->setController(this);
+}
+
+PresentationController::~PresentationController()
+{
+    if (m_client)
+        m_client->setController(nullptr);
+}
+
+// static
+PassOwnPtrWillBeRawPtr<PresentationController> PresentationController::create(LocalFrame& frame, WebPresentationClient* client)
+{
+    return adoptPtrWillBeNoop(new PresentationController(frame, client));
+}
+
+// static
+const char* PresentationController::supplementName()
+{
+    return "PresentationController";
+}
+
+// static
+PresentationController* PresentationController::from(LocalFrame& frame)
+{
+    return static_cast<PresentationController*>(WillBeHeapSupplement<LocalFrame>::from(frame, supplementName()));
+}
+
+// static
+void PresentationController::provideTo(LocalFrame& frame, WebPresentationClient* client)
+{
+    WillBeHeapSupplement<LocalFrame>::provideTo(frame, PresentationController::supplementName(), PresentationController::create(frame, client));
+}
+
+void PresentationController::trace(Visitor* visitor)
+{
+    visitor->trace(m_presentation);
+    WillBeHeapSupplement<LocalFrame>::trace(visitor);
+}
+
+void PresentationController::deviceAvailabilityChanged(bool available)
+{
+    if (m_presentation)

[Peter Beverloo, 2015/01/07 14:29:09]

Should we get this event if there is no m_presentation anymore?

Right now, when the Presentation object gets destroyed, it calls into
PresentationController::setPresentation(), but only in the destructor. Since
m_presentation is a traced Persistent/Member, the lifetimes of Presentation will
therefore be coupled to PresentationController, so that call isn't actually
required.

Furthermore, since PresentationController assumes there is only a single
Presentation instance, it would be good to ASSERT() in setPresentation() that we
don't accidentally create a second one per LocalFrame.

[whywhat, 2015/01/07 16:22:16]

Done.

+        m_presentation->deviceAvailabilityChanged(available);
+}
+
+void PresentationController::addDeviceAvailabilityListener()
+{
+    if (m_client)
+        m_client->addDeviceAvailabilityListener();

[Peter Beverloo, 2015/01/07 14:29:09]

Hm, so the embedder is responsible for doing some kind of reference counting for
active listeners per WebPresentationController*? Does the embedder care about
the exact number of active listeners, or only about the boolean "is there a
listener" question?

[whywhat, 2015/01/07 16:22:16]

In general, the embedder should care if there's any number listeners or none.
However, I'm not sure it makes sense to do the accounting for listeners number
here rather than delegating to the embedder implementing the Mojo interface.
Depending on the discovery methods supported, the UA might ignore the boolean
completely or on the contrary actually care about the number.

[Peter Beverloo, 2015/01/07 19:53:11]

In order to keep information local, we should only tell the embedder if there
are *any* listeners or *no* listeners - the embedder will not behave differently
if there are two event listeners rather than one.

[whywhat, 2015/01/08 16:05:28]

Done.

+}
+
+void PresentationController::removeDeviceAvailabilityListener()
+{
+    if (m_client)
+        m_client->removeDeviceAvailabilityListener();
+}
+
+void PresentationController::setPresentation(Presentation* presentation)

[Peter Beverloo, 2015/01/07 14:29:09]

Can we limit the scope of what PresentationController knows about |presentation|
by using the WebPresentationController type instead?

[whywhat, 2015/01/07 16:22:16]

Not sure I want to introduce another interface and I didn't see such dependency
injection in other modules.
PresentationController already implements WebPresentationController to abstract
PresentationDispatcher (WebPresentationClient implementation) from knowing
anything about PresentationController.

[Peter Beverloo, 2015/01/07 19:53:11]

Oh right, that's OK. Just make sure the names of the method on Presentation and
PresentationController stay in sync then :-).

[whywhat, 2015/01/08 16:05:28]

I assume that most of the time the code won't compile then :)

+{
+    m_presentation = presentation;
+}
+
+} // namespace blink