Decouple MediaSession messages from WebContents (full patch)

content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java

Index: content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
diff --git a/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java b/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
new file mode 100644
index 0000000000000000000000000000000000000000..fd789ded767928340da05c3081a929b24a390bbf
--- /dev/null
+++ b/content/public/android/java/src/org/chromium/content_public/browser/MediaSessionObserver.java
@@ -0,0 +1,66 @@
+// 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.content_public.browser;
+
+import android.support.annotation.Nullable;
+
+import org.chromium.content.browser.MediaSessionImpl;
+import org.chromium.content_public.common.MediaMetadata;
+
+/**
+ * This class is Java implementation of native MediaSessionObserver. The class receives media
+ * session messages from Java {@link MediaSession}, which acts acts as a proxy forwarding messages
+ * comming from the native MediaSession.
+ */
+public abstract class MediaSessionObserver {
+    private MediaSessionImpl mMediaSession;
+
+    /**
+     * Construct a MediaSessionObserver and start observing |mediaSession|.
+     */
+    protected MediaSessionObserver(MediaSession mediaSession) {
+        mMediaSession = (MediaSessionImpl) mediaSession;
+        mMediaSession.addObserver(this);
+    }
+
+    /**
+     * @return The observed media session.
+     */
+    @Nullable
+    public final MediaSession getMediaSession() {
+        return mMediaSession;
+    }
+
+    /**
+     * Called when the observed media session is destroyed.
+     *
+     * Gives subclass a chance to do clean up. After the whole loop over
+     * {@link MediaSessionObserver}s has been finished, {@link MediaSessionObserver#getMediaSession}
+     * will return null.
+     */
+    public void mediaSessionDestroyed() {}
+
+    /**
+     * Called when the observed {@link MediaSession} state has changed.
+     * @param isControllable If the session can be resumed or suspended.
+     * @param isSuspended if the session currently suspended or not.
+     */
+    public void mediaSessionStateChanged(boolean isControllable, boolean isSuspended) {}
+
+    /**
+     * Called when the {@link MediaSession} metadata has changed.
+     * @param metadata the new MediaMetadata after change.
+     */
+    public void mediaSessionMetadataChanged(MediaMetadata metadata) {}
+
+    /**
+     * Stop observing the media session. Users must explicitly call this before dereferencing the
+     * observer.
+     */
+    public final void stopObserving() {
+        if (mMediaSession == null) return;
+        mMediaSession.removeObserver(this);
+    }
+}