Link MediaSessionTabHelper with native MediaSession [CL is going to be split]

content/public/android/java/src/org/chromium/content/browser/ChromeMediaSession.java

+// 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.browser;
+
+import org.chromium.base.ObserverList;
+import org.chromium.base.ObserverList.RewindableIterator;
+import org.chromium.base.VisibleForTesting;
+import org.chromium.base.annotations.CalledByNative;
+import org.chromium.base.annotations.JNINamespace;
+import org.chromium.content_public.browser.WebContents;
+import org.chromium.content_public.common.MediaMetadata;
+
+/**
+ * The ChromeMediaSession Java wrapper to allow communicating with the native MediaSession object.
+ * ChromeMediaSession is created and owned by native MediaSessionAndroid.
+ */
+@JNINamespace("content")
+public class ChromeMediaSession {
+    private long mNativeMediaSessionAndroid;
+    // The observers are stored only Java and are proxied via ChromeMediaSession to observe native
+    // MediaSession.
+    private ObserverList<MediaSessionObserver> mObservers;
+    private RewindableIterator<MediaSessionObserver> mObserversIterator;
+
+    /**
+     * @returns The {@link ChromeMediaSession} for |contents|.
+     */
+    public static ChromeMediaSession fromWebContents(WebContents contents) {
+        return nativeGetMediaSessionFromWebContents(contents);
+    }
+
+    /**
+     * Add an observer. This method can only be called from {@link MediaSessionObserver}
+     * constructor.
+     */
+    protected void addObserver(MediaSessionObserver observer) {
+        mObservers.addObserver(observer);
+    }
+
+    /**
+     * Remove an observer.
+     */
+    public void removeObserver(MediaSessionObserver observer) {
+        mObservers.removeObserver(observer);
+    }
+
+    /**
+     * Wrapper for native MediaSession::Resume().
+     */
+    public void resume() {
+        nativeResume(mNativeMediaSessionAndroid);
+    }
+
+    /**
+     * Wrapper for native MediaSession::Suspend();
+     */
+    public void suspend() {
+        nativeSuspend(mNativeMediaSessionAndroid);
+    }
+
+    /**
+     * Wrapper for native MediaSession::Stop();
+     */
+    public void stop() {
+        nativeStop(mNativeMediaSessionAndroid);
+    }
+
+    @CalledByNative
+    private boolean hasObservers() {
+        return !mObservers.isEmpty();
+    }
+
+    @CalledByNative
+    private void mediaSessionDestroyed() {
+        for (mObserversIterator.rewind(); mObserversIterator.hasNext();) {
+            mObserversIterator.next().mediaSessionDestroyed();
+        }
+    }
+
+    @VisibleForTesting
+    @CalledByNative
+    public void mediaSessionStateChanged(boolean isControllable, boolean isSuspended) {
+        for (mObserversIterator.rewind(); mObserversIterator.hasNext();) {
+            mObserversIterator.next().mediaSessionStateChanged(isControllable, isSuspended);
+        }
+    }
+
+    @VisibleForTesting
+    @CalledByNative
+    public void mediaSessionMetadataChanged(MediaMetadata metadata) {
+        for (mObserversIterator.rewind(); mObserversIterator.hasNext();) {
+            mObserversIterator.next().mediaSessionMetadataChanged(metadata);
+        }
+    }
+
+    /**
+     * Create a ChromeMediaSession object for a native MediaSession.
+     */
+    @CalledByNative
+    private static ChromeMediaSession create(long nativeChromeMediaSession) {
+        return new ChromeMediaSession(nativeChromeMediaSession);
+    }
+
+    /**
+     * Called when the native MediaSessionAndroid (the owner) is destroyed.
+     */
+    @CalledByNative
+    private void tearDown() {
+        mNativeMediaSessionAndroid = 0;
+        // All observers should stop observing when receiving MediaSessionDestroyed.
+        assert mObservers.isEmpty();
+        mObservers.clear();
+    }
+
+    private ChromeMediaSession(long nativeChromeMediaSession) {
+        mNativeMediaSessionAndroid = nativeChromeMediaSession;
+        mObservers = new ObserverList<MediaSessionObserver>();
+        mObserversIterator = mObservers.rewindableIterator();
+    }
+
+    private static native ChromeMediaSession nativeGetMediaSessionFromWebContents(
+            WebContents contents);
+    private native void nativeResume(long nativeMediaSessionAndroid);
+    private native void nativeSuspend(long nativeMediaSessionAndroid);
+    private native void nativeStop(long nativeMediaSessionAndroid);
+}