xref: /frameworks/support/media-compat/java/android/support/v4/media/MediaBrowserServiceCompat.java

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.support.v4.media;

import android.app.Service;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Binder;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.Parcel;
import android.os.RemoteException;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.BundleCompat;
import android.support.v4.media.session.MediaSessionCompat;
import android.support.v4.os.ResultReceiver;
import android.support.v4.util.ArrayMap;
import android.text.TextUtils;
import android.util.Log;

import java.io.FileDescriptor;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

import static android.support.v4.media.MediaBrowserProtocol.*;

/**
 * Base class for media browse services.
 * <p>
 * Media browse services enable applications to browse media content provided by an application
 * and ask the application to start playing it.  They may also be used to control content that
 * is already playing by way of a {@link MediaSessionCompat}.
 * </p>
 *
 * To extend this class, you must declare the service in your manifest file with
 * an intent filter with the {@link #SERVICE_INTERFACE} action.
 *
 * For example:
 * </p><pre>
 * &lt;service android:name=".MyMediaBrowserServiceCompat"
 *          android:label="&#64;string/service_name" >
 *     &lt;intent-filter>
 *         &lt;action android:name="android.media.browse.MediaBrowserService" />
 *     &lt;/intent-filter>
 * &lt;/service>
 * </pre>
 */
public abstract class MediaBrowserServiceCompat extends Service {
    private static final String TAG = "MediaBrowserServiceCompat";
    private static final boolean DBG = false;

    private MediaBrowserServiceImpl mImpl;

    /**
     * The {@link Intent} that must be declared as handled by the service.
     */
    public static final String SERVICE_INTERFACE = "android.media.browse.MediaBrowserService";

    /**
     * A key for passing the MediaItem to the ResultReceiver in getItem.
     *
     * @hide
     */
    public static final String KEY_MEDIA_ITEM = "media_item";

    private final ArrayMap<IBinder, ConnectionRecord> mConnections = new ArrayMap();
    private final ServiceHandler mHandler = new ServiceHandler();
    MediaSessionCompat.Token mSession;

    interface MediaBrowserServiceImpl {
        void onCreate();
        IBinder onBind(Intent intent);
    }

    class MediaBrowserServiceImplBase implements MediaBrowserServiceImpl {
        private Messenger mMessenger;

        @Override
        public void onCreate() {
            mMessenger = new Messenger(mHandler);
        }

        @Override
        public IBinder onBind(Intent intent) {
            if (SERVICE_INTERFACE.equals(intent.getAction())) {
                return mMessenger.getBinder();
            }
            return null;
        }
    }

    class MediaBrowserServiceImplApi21 implements MediaBrowserServiceImpl {
        private Object mServiceObj;

        @Override
        public void onCreate() {
            mServiceObj = MediaBrowserServiceCompatApi21.createService();
            MediaBrowserServiceCompatApi21.onCreate(mServiceObj, new ServiceImplApi21());
        }

        @Override
        public IBinder onBind(Intent intent) {
            return MediaBrowserServiceCompatApi21.onBind(mServiceObj, intent);
        }
    }

    class MediaBrowserServiceImplApi23 implements MediaBrowserServiceImpl {
        private Object mServiceObj;

        @Override
        public void onCreate() {
            mServiceObj = MediaBrowserServiceCompatApi23.createService();
            MediaBrowserServiceCompatApi23.onCreate(mServiceObj, new ServiceImplApi23());
        }

        @Override
        public IBinder onBind(Intent intent) {
            return MediaBrowserServiceCompatApi23.onBind(mServiceObj, intent);
        }
    }

    private final class ServiceHandler extends Handler {
        private final ServiceImpl mServiceImpl = new ServiceImpl();

        @Override
        public void handleMessage(Message msg) {
            Bundle data = msg.getData();
            switch (msg.what) {
                case CLIENT_MSG_CONNECT:
                    mServiceImpl.connect(data.getString(DATA_PACKAGE_NAME),
                            data.getInt(DATA_CALLING_UID), data.getBundle(DATA_ROOT_HINTS),
                            new ServiceCallbacksCompat(msg.replyTo));
                    break;
                case CLIENT_MSG_DISCONNECT:
                    mServiceImpl.disconnect(new ServiceCallbacksCompat(msg.replyTo));
                    break;
                case CLIENT_MSG_ADD_SUBSCRIPTION:
                    mServiceImpl.addSubscription(data.getString(DATA_MEDIA_ITEM_ID),
                            new ServiceCallbacksCompat(msg.replyTo));
                    break;
                case CLIENT_MSG_REMOVE_SUBSCRIPTION:
                    mServiceImpl.removeSubscription(data.getString(DATA_MEDIA_ITEM_ID),
                            new ServiceCallbacksCompat(msg.replyTo));
                    break;
                case CLIENT_MSG_GET_MEDIA_ITEM:
                    mServiceImpl.getMediaItem(data.getString(DATA_MEDIA_ITEM_ID),
                            (ResultReceiver) data.getParcelable(DATA_RESULT_RECEIVER));
                    break;
                default:
                    Log.w(TAG, "Unhandled message: " + msg
                            + "\n  Service version: " + SERVICE_VERSION_CURRENT
                            + "\n  Client version: " + msg.arg1);
            }
        }

        @Override
        public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
            // Binder.getCallingUid() in handleMessage will return the uid of this process.
            // In order to get the right calling uid, Binder.getCallingUid() should be called here.
            Bundle data = msg.getData();
            data.setClassLoader(MediaBrowserCompat.class.getClassLoader());
            data.putInt(DATA_CALLING_UID, Binder.getCallingUid());
            return super.sendMessageAtTime(msg, uptimeMillis);
        }

        public void postOrRun(Runnable r) {
            if (Thread.currentThread() == getLooper().getThread()) {
                r.run();
            } else {
                post(r);
            }
        }

        public ServiceImpl getServiceImpl() {
            return mServiceImpl;
        }
    }

    /**
     * All the info about a connection.
     */
    private class ConnectionRecord {
        String pkg;
        Bundle rootHints;
        ServiceCallbacks callbacks;
        BrowserRoot root;
        HashSet<String> subscriptions = new HashSet();
    }

    /**
     * Completion handler for asynchronous callback methods in {@link MediaBrowserServiceCompat}.
     * <p>
     * Each of the methods that takes one of these to send the result must call
     * {@link #sendResult} to respond to the caller with the given results.  If those
     * functions return without calling {@link #sendResult}, they must instead call
     * {@link #detach} before returning, and then may call {@link #sendResult} when
     * they are done.  If more than one of those methods is called, an exception will
     * be thrown.
     *
     * @see MediaBrowserServiceCompat#onLoadChildren
     * @see MediaBrowserServiceCompat#onLoadItem
     */
    public static class Result<T> {
        private Object mDebug;
        private boolean mDetachCalled;
        private boolean mSendResultCalled;

        Result(Object debug) {
            mDebug = debug;
        }

        /**
         * Send the result back to the caller.
         */
        public void sendResult(T result) {
            if (mSendResultCalled) {
                throw new IllegalStateException("sendResult() called twice for: " + mDebug);
            }
            mSendResultCalled = true;
            onResultSent(result);
        }

        /**
         * Detach this message from the current thread and allow the {@link #sendResult}
         * call to happen later.
         */
        public void detach() {
            if (mDetachCalled) {
                throw new IllegalStateException("detach() called when detach() had already"
                        + " been called for: " + mDebug);
            }
            if (mSendResultCalled) {
                throw new IllegalStateException("detach() called when sendResult() had already"
                        + " been called for: " + mDebug);
            }
            mDetachCalled = true;
        }

        boolean isDone() {
            return mDetachCalled || mSendResultCalled;
        }

        /**
         * Called when the result is sent, after assertions about not being called twice
         * have happened.
         */
        void onResultSent(T result) {
        }
    }

    private class ServiceImpl {
        public void connect(final String pkg, final int uid, final Bundle rootHints,
                final ServiceCallbacks callbacks) {

            if (!isValidPackage(pkg, uid)) {
                throw new IllegalArgumentException("Package/uid mismatch: uid=" + uid
                        + " package=" + pkg);
            }

            mHandler.postOrRun(new Runnable() {
                @Override
                public void run() {
                    final IBinder b = callbacks.asBinder();

                    // Clear out the old subscriptions.  We are getting new ones.
                    mConnections.remove(b);

                    final ConnectionRecord connection = new ConnectionRecord();
                    connection.pkg = pkg;
                    connection.rootHints = rootHints;
                    connection.callbacks = callbacks;

                    connection.root =
                            MediaBrowserServiceCompat.this.onGetRoot(pkg, uid, rootHints);

                    // If they didn't return something, don't allow this client.
                    if (connection.root == null) {
                        Log.i(TAG, "No root for client " + pkg + " from service "
                                + getClass().getName());
                        try {
                            callbacks.onConnectFailed();
                        } catch (RemoteException ex) {
                            Log.w(TAG, "Calling onConnectFailed() failed. Ignoring. "
                                    + "pkg=" + pkg);
                        }
                    } else {
                        try {
                            mConnections.put(b, connection);
                            if (mSession != null) {
                                callbacks.onConnect(connection.root.getRootId(),
                                        mSession, connection.root.getExtras());
                            }
                        } catch (RemoteException ex) {
                            Log.w(TAG, "Calling onConnect() failed. Dropping client. "
                                    + "pkg=" + pkg);
                            mConnections.remove(b);
                        }
                    }
                }
            });
        }

        public void disconnect(final ServiceCallbacks callbacks) {
            mHandler.postOrRun(new Runnable() {
                @Override
                public void run() {
                    final IBinder b = callbacks.asBinder();

                    // Clear out the old subscriptions.  We are getting new ones.
                    final ConnectionRecord old = mConnections.remove(b);
                    if (old != null) {
                        // TODO
                    }
                }
            });
        }


        public void addSubscription(final String id, final ServiceCallbacks callbacks) {
            mHandler.postOrRun(new Runnable() {
                @Override
                public void run() {
                    final IBinder b = callbacks.asBinder();

                    // Get the record for the connection
                    final ConnectionRecord connection = mConnections.get(b);
                    if (connection == null) {
                        Log.w(TAG, "addSubscription for callback that isn't registered id="
                                + id);
                        return;
                    }

                    MediaBrowserServiceCompat.this.addSubscription(id, connection);
                }
            });
        }

        public void removeSubscription(final String id, final ServiceCallbacks callbacks) {
            mHandler.postOrRun(new Runnable() {
                @Override
                public void run() {
                    final IBinder b = callbacks.asBinder();

                    ConnectionRecord connection = mConnections.get(b);
                    if (connection == null) {
                        Log.w(TAG, "removeSubscription for callback that isn't registered id="
                                + id);
                        return;
                    }
                    if (!connection.subscriptions.remove(id)) {
                        Log.w(TAG, "removeSubscription called for " + id
                                + " which is not subscribed");
                    }
                }
            });
        }

        public void getMediaItem(final String mediaId, final ResultReceiver receiver) {
            if (TextUtils.isEmpty(mediaId) || receiver == null) {
                return;
            }

            mHandler.postOrRun(new Runnable() {
                @Override
                public void run() {
                    performLoadItem(mediaId, receiver);
                }
            });
        }
    }

    private class ServiceImplApi21 implements MediaBrowserServiceCompatApi21.ServiceImplApi21 {
        final ServiceImpl mServiceImpl;

        ServiceImplApi21() {
            mServiceImpl = mHandler.getServiceImpl();
        }

        @Override
        public void connect(final String pkg, final Bundle rootHints,
                final MediaBrowserServiceCompatApi21.ServiceCallbacks callbacks) {
            mServiceImpl.connect(pkg, Binder.getCallingUid(), rootHints,
                    new ServiceCallbacksApi21(callbacks));
        }

        @Override
        public void disconnect(final MediaBrowserServiceCompatApi21.ServiceCallbacks callbacks) {
            mServiceImpl.disconnect(new ServiceCallbacksApi21(callbacks));
        }


        @Override
        public void addSubscription(
                final String id, final MediaBrowserServiceCompatApi21.ServiceCallbacks callbacks) {
            mServiceImpl.addSubscription(id, new ServiceCallbacksApi21(callbacks));
        }

        @Override
        public void removeSubscription(final String id,
                final MediaBrowserServiceCompatApi21.ServiceCallbacks callbacks) {
            mServiceImpl.removeSubscription(id, new ServiceCallbacksApi21(callbacks));
        }
    }

    private class ServiceImplApi23 extends ServiceImplApi21
            implements MediaBrowserServiceCompatApi23.ServiceImplApi23 {
        @Override
        public void getMediaItem(final String mediaId,
                final MediaBrowserServiceCompatApi23.ItemCallback cb) {
            ResultReceiver receiverCompat = new ResultReceiver(mHandler) {
                @Override
                protected void onReceiveResult(int resultCode, Bundle resultData) {
                    MediaBrowserCompat.MediaItem item = resultData.getParcelable(KEY_MEDIA_ITEM);
                    Parcel itemParcel = null;
                    if (item != null) {
                        itemParcel = Parcel.obtain();
                        item.writeToParcel(itemParcel, 0);
                    }
                    cb.onItemLoaded(resultCode, resultData, itemParcel);
                }
            };
            mServiceImpl.getMediaItem(mediaId, receiverCompat);
        }
    }

    private interface ServiceCallbacks {
        IBinder asBinder();
        void onConnect(String root, MediaSessionCompat.Token session, Bundle extras)
                throws RemoteException;
        void onConnectFailed() throws RemoteException;
        void onLoadChildren(String mediaId, List<MediaBrowserCompat.MediaItem> list)
                throws RemoteException;
    }

    private class ServiceCallbacksCompat implements ServiceCallbacks {
        final Messenger mCallbacks;

        ServiceCallbacksCompat(Messenger callbacks) {
            mCallbacks = callbacks;
        }

        public IBinder asBinder() {
            return mCallbacks.getBinder();
        }

        public void onConnect(String root, MediaSessionCompat.Token session, Bundle extras)
                throws RemoteException {
            if (extras == null) {
                extras = new Bundle();
            }
            extras.putInt(EXTRA_SERVICE_VERSION, SERVICE_VERSION_CURRENT);
            Bundle data = new Bundle();
            data.putString(DATA_MEDIA_ITEM_ID, root);
            data.putParcelable(DATA_MEDIA_SESSION_TOKEN, session);
            data.putBundle(DATA_ROOT_HINTS, extras);
            sendRequest(SERVICE_MSG_ON_CONNECT, data);
        }

        public void onConnectFailed() throws RemoteException {
            sendRequest(SERVICE_MSG_ON_CONNECT_FAILED, null);
        }

        public void onLoadChildren(String mediaId, List<MediaBrowserCompat.MediaItem> list)
                throws RemoteException {
            Bundle data = new Bundle();
            data.putString(DATA_MEDIA_ITEM_ID, mediaId);
            if (list != null) {
                data.putParcelableArrayList(DATA_MEDIA_ITEM_LIST,
                        list instanceof ArrayList ? (ArrayList) list : new ArrayList<>(list));
            }
            sendRequest(SERVICE_MSG_ON_LOAD_CHILDREN, data);
        }

        private void sendRequest(int what, Bundle data) throws RemoteException {
            Message msg = Message.obtain();
            msg.what = what;
            msg.arg1 = SERVICE_VERSION_CURRENT;
            msg.setData(data);
            mCallbacks.send(msg);
        }
    }

    private class ServiceCallbacksApi21 implements ServiceCallbacks {
        final MediaBrowserServiceCompatApi21.ServiceCallbacks mCallbacks;
        Messenger mMessenger;

        ServiceCallbacksApi21(MediaBrowserServiceCompatApi21.ServiceCallbacks callbacks) {
            mCallbacks = callbacks;
        }

        public IBinder asBinder() {
            return mCallbacks.asBinder();
        }

        public void onConnect(String root, MediaSessionCompat.Token session, Bundle extras)
                throws RemoteException {
            if (extras == null) {
                extras = new Bundle();
            }
            mMessenger = new Messenger(mHandler);
            BundleCompat.putBinder(extras, EXTRA_MESSENGER_BINDER, mMessenger.getBinder());
            extras.putInt(EXTRA_SERVICE_VERSION, SERVICE_VERSION_CURRENT);
            mCallbacks.onConnect(root, session.getToken(), extras);
        }

        public void onConnectFailed() throws RemoteException {
            mCallbacks.onConnectFailed();
        }

        public void onLoadChildren(String mediaId, List<MediaBrowserCompat.MediaItem> list)
                throws RemoteException {
            List<Parcel> parcelList = null;
            if (list != null) {
                parcelList = new ArrayList<>();
                for (MediaBrowserCompat.MediaItem item : list) {
                    Parcel parcel = Parcel.obtain();
                    item.writeToParcel(parcel, 0);
                    parcelList.add(parcel);
                }
            }
            mCallbacks.onLoadChildren(mediaId, parcelList);
        }
    }

    @Override
    public void onCreate() {
        super.onCreate();
        if (Build.VERSION.SDK_INT >= 23) {
            mImpl = new MediaBrowserServiceImplApi23();
        } else if (Build.VERSION.SDK_INT >= 21) {
            mImpl = new MediaBrowserServiceImplApi21();
        } else {
            mImpl = new MediaBrowserServiceImplBase();
        }
        mImpl.onCreate();
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mImpl.onBind(intent);
    }

    @Override
    public void dump(FileDescriptor fd, PrintWriter writer, String[] args) {
    }

    /**
     * Called to get the root information for browsing by a particular client.
     * <p>
     * The implementation should verify that the client package has permission
     * to access browse media information before returning the root id; it
     * should return null if the client is not allowed to access this
     * information.
     * </p>
     *
     * @param clientPackageName The package name of the application which is
     *            requesting access to browse media.
     * @param clientUid The uid of the application which is requesting access to
     *            browse media.
     * @param rootHints An optional bundle of service-specific arguments to send
     *            to the media browse service when connecting and retrieving the
     *            root id for browsing, or null if none. The contents of this
     *            bundle may affect the information returned when browsing.
     * @return The {@link BrowserRoot} for accessing this app's content or null.
     */
    public abstract @Nullable BrowserRoot onGetRoot(@NonNull String clientPackageName,
            int clientUid, @Nullable Bundle rootHints);

    /**
     * Called to get information about the children of a media item.
     * <p>
     * Implementations must call {@link Result#sendResult result.sendResult}
     * with the list of children. If loading the children will be an expensive
     * operation that should be performed on another thread,
     * {@link Result#detach result.detach} may be called before returning from
     * this function, and then {@link Result#sendResult result.sendResult}
     * called when the loading is complete.
     *
     * @param parentId The id of the parent media item whose children are to be
     *            queried.
     * @param result The Result to send the list of children to, or null if the
     *            id is invalid.
     */
    public abstract void onLoadChildren(@NonNull String parentId,
            @NonNull Result<List<MediaBrowserCompat.MediaItem>> result);

    /**
     * Called to get information about a specific media item.
     * <p>
     * Implementations must call {@link Result#sendResult result.sendResult}. If
     * loading the item will be an expensive operation {@link Result#detach
     * result.detach} may be called before returning from this function, and
     * then {@link Result#sendResult result.sendResult} called when the item has
     * been loaded.
     * <p>
     * The default implementation sends a null result.
     *
     * @param itemId The id for the specific
     *            {@link MediaBrowserCompat.MediaItem}.
     * @param result The Result to send the item to, or null if the id is
     *            invalid.
     */
    public void onLoadItem(String itemId, Result<MediaBrowserCompat.MediaItem> result) {
        result.sendResult(null);
    }

    /**
     * Call to set the media session.
     * <p>
     * This should be called as soon as possible during the service's startup.
     * It may only be called once.
     *
     * @param token The token for the service's {@link MediaSessionCompat}.
     */
    public void setSessionToken(final MediaSessionCompat.Token token) {
        if (token == null) {
            throw new IllegalArgumentException("Session token may not be null.");
        }
        if (mSession != null) {
            throw new IllegalStateException("The session token has already been set.");
        }
        mSession = token;
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                for (IBinder key : mConnections.keySet()) {
                    ConnectionRecord connection = mConnections.get(key);
                    try {
                        connection.callbacks.onConnect(connection.root.getRootId(), token,
                                connection.root.getExtras());
                    } catch (RemoteException e) {
                        Log.w(TAG, "Connection for " + connection.pkg + " is no longer valid.");
                        mConnections.remove(key);
                    }
                }
            }
        });
    }

    /**
     * Gets the session token, or null if it has not yet been created
     * or if it has been destroyed.
     */
    public @Nullable MediaSessionCompat.Token getSessionToken() {
        return mSession;
    }

    /**
     * Notifies all connected media browsers that the children of
     * the specified parent id have changed in some way.
     * This will cause browsers to fetch subscribed content again.
     *
     * @param parentId The id of the parent media item whose
     * children changed.
     */
    public void notifyChildrenChanged(@NonNull final String parentId) {
        if (parentId == null) {
            throw new IllegalArgumentException("parentId cannot be null in notifyChildrenChanged");
        }
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                for (IBinder binder : mConnections.keySet()) {
                    ConnectionRecord connection = mConnections.get(binder);
                    if (connection.subscriptions.contains(parentId)) {
                        performLoadChildren(parentId, connection);
                    }
                }
            }
        });
    }

    /**
     * Return whether the given package is one of the ones that is owned by the uid.
     */
    private boolean isValidPackage(String pkg, int uid) {
        if (pkg == null) {
            return false;
        }
        final PackageManager pm = getPackageManager();
        final String[] packages = pm.getPackagesForUid(uid);
        final int N = packages.length;
        for (int i=0; i<N; i++) {
            if (packages[i].equals(pkg)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Save the subscription and if it is a new subscription send the results.
     */
    private void addSubscription(String id, ConnectionRecord connection) {
        // Save the subscription
        connection.subscriptions.add(id);

        // send the results
        performLoadChildren(id, connection);
    }

    /**
     * Call onLoadChildren and then send the results back to the connection.
     * <p>
     * Callers must make sure that this connection is still connected.
     */
    private void performLoadChildren(final String parentId, final ConnectionRecord connection) {
        final Result<List<MediaBrowserCompat.MediaItem>> result
                = new Result<List<MediaBrowserCompat.MediaItem>>(parentId) {
            @Override
            void onResultSent(List<MediaBrowserCompat.MediaItem> list) {
                if (mConnections.get(connection.callbacks.asBinder()) != connection) {
                    if (DBG) {
                        Log.d(TAG, "Not sending onLoadChildren result for connection that has"
                                + " been disconnected. pkg=" + connection.pkg + " id=" + parentId);
                    }
                    return;
                }

                try {
                    connection.callbacks.onLoadChildren(parentId, list);
                } catch (RemoteException ex) {
                    // The other side is in the process of crashing.
                    Log.w(TAG, "Calling onLoadChildren() failed for id=" + parentId
                            + " package=" + connection.pkg);
                }
            }
        };

        onLoadChildren(parentId, result);

        if (!result.isDone()) {
            throw new IllegalStateException("onLoadChildren must call detach() or sendResult()"
                    + " before returning for package=" + connection.pkg + " id=" + parentId);
        }
    }

    private void performLoadItem(String itemId, final ResultReceiver receiver) {
        final Result<MediaBrowserCompat.MediaItem> result =
                new Result<MediaBrowserCompat.MediaItem>(itemId) {
            @Override
            void onResultSent(MediaBrowserCompat.MediaItem item) {
                Bundle bundle = new Bundle();
                bundle.putParcelable(KEY_MEDIA_ITEM, item);
                receiver.send(0, bundle);
            }
        };

        MediaBrowserServiceCompat.this.onLoadItem(itemId, result);

        if (!result.isDone()) {
            throw new IllegalStateException("onLoadItem must call detach() or sendResult()"
                    + " before returning for id=" + itemId);
        }
    }

    /**
     * Contains information that the browser service needs to send to the client
     * when first connected.
     */
    public static final class BrowserRoot {
        final private String mRootId;
        final private Bundle mExtras;

        /**
         * Constructs a browser root.
         * @param rootId The root id for browsing.
         * @param extras Any extras about the browser service.
         */
        public BrowserRoot(@NonNull String rootId, @Nullable Bundle extras) {
            if (rootId == null) {
                throw new IllegalArgumentException("The root id in BrowserRoot cannot be null. " +
                        "Use null for BrowserRoot instead.");
            }
            mRootId = rootId;
            mExtras = extras;
        }

        /**
         * Gets the root id for browsing.
         */
        public String getRootId() {
            return mRootId;
        }

        /**
         * Gets any extras about the browser service.
         */
        public Bundle getExtras() {
            return mExtras;
        }
    }
}