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Android example source code file (RemoteCallbackList.java)

This example Android source code file (RemoteCallbackList.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Android by Example" TM.

Java - Android tags/keywords

callback, e, hashmap, ibinder, iinterface, illegalstateexception, n, object, remotecallbacklist, remoteexception, util

The RemoteCallbackList.java Android example source code

/*
 * Copyright (C) 2008 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.os;

import java.util.HashMap;

/**
 * Takes care of the grunt work of maintaining a list of remote interfaces,
 * typically for the use of performing callbacks from a
 * {@link android.app.Service} to its clients.  In particular, this:
 *
 * <ul>
 * <li> Keeps track of a set of registered {@link IInterface} callbacks,
 * taking care to identify them through their underlying unique {@link IBinder}
 * (by calling {@link IInterface#asBinder IInterface.asBinder()}.
 * <li> Attaches a {@link IBinder.DeathRecipient IBinder.DeathRecipient} to
 * each registered interface, so that it can be cleaned out of the list if its
 * process goes away.
 * <li> Performs locking of the underlying list of interfaces to deal with
 * multithreaded incoming calls, and a thread-safe way to iterate over a
 * snapshot of the list without holding its lock.
 * </ul>
 *
 * <p>To use this class, simply create a single instance along with your
 * service, and call its {@link #register} and {@link #unregister} methods
 * as client register and unregister with your service.  To call back on to
 * the registered clients, use {@link #beginBroadcast},
 * {@link #getBroadcastItem}, and {@link #finishBroadcast}.
 *
 * <p>If a registered callback's process goes away, this class will take
 * care of automatically removing it from the list.  If you want to do
 * additional work in this situation, you can create a subclass that
 * implements the {@link #onCallbackDied} method.
 */
public class RemoteCallbackList<E extends IInterface> {
    /*package*/ HashMap<IBinder, Callback> mCallbacks
            = new HashMap<IBinder, Callback>();
    private Object[] mActiveBroadcast;
    private int mBroadcastCount = -1;
    private boolean mKilled = false;

    private final class Callback implements IBinder.DeathRecipient {
        final E mCallback;
        final Object mCookie;
        
        Callback(E callback, Object cookie) {
            mCallback = callback;
            mCookie = cookie;
        }

        public void binderDied() {
            synchronized (mCallbacks) {
                mCallbacks.remove(mCallback.asBinder());
            }
            onCallbackDied(mCallback, mCookie);
        }
    }

    /**
     * Simple version of {@link RemoteCallbackList#register(E, Object)}
     * that does not take a cookie object.
     */
    public boolean register(E callback) {
        return register(callback, null);
    }
    
    /**
     * Add a new callback to the list.  This callback will remain in the list
     * until a corresponding call to {@link #unregister} or its hosting process
     * goes away.  If the callback was already registered (determined by
     * checking to see if the {@link IInterface#asBinder callback.asBinder()}
     * object is already in the list), then it will be left as-is.
     * Registrations are not counted; a single call to {@link #unregister}
     * will remove a callback after any number calls to register it.
     *
     * @param callback The callback interface to be added to the list.  Must
     * not be null -- passing null here will cause a NullPointerException.
     * Most services will want to check for null before calling this with
     * an object given from a client, so that clients can't crash the
     * service with bad data.
     *
     * @param cookie Optional additional data to be associated with this
     * callback.
     * 
     * @return Returns true if the callback was successfully added to the list.
     * Returns false if it was not added, either because {@link #kill} had
     * previously been called or the callback's process has gone away.
     *
     * @see #unregister
     * @see #kill
     * @see #onCallbackDied
     */
    public boolean register(E callback, Object cookie) {
        synchronized (mCallbacks) {
            if (mKilled) {
                return false;
            }
            IBinder binder = callback.asBinder();
            try {
                Callback cb = new Callback(callback, cookie);
                binder.linkToDeath(cb, 0);
                mCallbacks.put(binder, cb);
                return true;
            } catch (RemoteException e) {
                return false;
            }
        }
    }

    /**
     * Remove from the list a callback that was previously added with
     * {@link #register}.  This uses the
     * {@link IInterface#asBinder callback.asBinder()} object to correctly
     * find the previous registration.
     * Registrations are not counted; a single unregister call will remove
     * a callback after any number calls to {@link #register} for it.
     *
     * @param callback The callback to be removed from the list.  Passing
     * null here will cause a NullPointerException, so you will generally want
     * to check for null before calling.
     *
     * @return Returns true if the callback was found and unregistered.  Returns
     * false if the given callback was not found on the list.
     *
     * @see #register
     */
    public boolean unregister(E callback) {
        synchronized (mCallbacks) {
            Callback cb = mCallbacks.remove(callback.asBinder());
            if (cb != null) {
                cb.mCallback.asBinder().unlinkToDeath(cb, 0);
                return true;
            }
            return false;
        }
    }

    /**
     * Disable this callback list.  All registered callbacks are unregistered,
     * and the list is disabled so that future calls to {@link #register} will
     * fail.  This should be used when a Service is stopping, to prevent clients
     * from registering callbacks after it is stopped.
     *
     * @see #register
     */
    public void kill() {
        synchronized (mCallbacks) {
            for (Callback cb : mCallbacks.values()) {
                cb.mCallback.asBinder().unlinkToDeath(cb, 0);
            }
            mCallbacks.clear();
            mKilled = true;
        }
    }

    /**
     * Old version of {@link #onCallbackDied(E, Object)} that
     * does not provide a cookie.
     */
    public void onCallbackDied(E callback) {
    }
    
    /**
     * Called when the process hosting a callback in the list has gone away.
     * The default implementation calls {@link #onCallbackDied(E)}
     * for backwards compatibility.
     * 
     * @param callback The callback whose process has died.  Note that, since
     * its process has died, you can not make any calls on to this interface.
     * You can, however, retrieve its IBinder and compare it with another
     * IBinder to see if it is the same object.
     * @param cookie The cookie object original provided to
     * {@link #register(E, Object)}.
     * 
     * @see #register
     */
    public void onCallbackDied(E callback, Object cookie) {
        onCallbackDied(callback);
    }

    /**
     * Prepare to start making calls to the currently registered callbacks.
     * This creates a copy of the callback list, which you can retrieve items
     * from using {@link #getBroadcastItem}.  Note that only one broadcast can
     * be active at a time, so you must be sure to always call this from the
     * same thread (usually by scheduling with {@link Handler}) or
     * do your own synchronization.  You must call {@link #finishBroadcast}
     * when done.
     *
     * <p>A typical loop delivering a broadcast looks like this:
     *
     * <pre>
     * int i = callbacks.beginBroadcast();
     * while (i > 0) {
     *     i--;
     *     try {
     *         callbacks.getBroadcastItem(i).somethingHappened();
     *     } catch (RemoteException e) {
     *         // The RemoteCallbackList will take care of removing
     *         // the dead object for us.
     *     }
     * }
     * callbacks.finishBroadcast();</pre>
     *
     * @return Returns the number of callbacks in the broadcast, to be used
     * with {@link #getBroadcastItem} to determine the range of indices you
     * can supply.
     *
     * @see #getBroadcastItem
     * @see #finishBroadcast
     */
    public int beginBroadcast() {
        synchronized (mCallbacks) {
            if (mBroadcastCount > 0) {
                throw new IllegalStateException(
                        "beginBroadcast() called while already in a broadcast");
            }
            
            final int N = mBroadcastCount = mCallbacks.size();
            if (N <= 0) {
                return 0;
            }
            Object[] active = mActiveBroadcast;
            if (active == null || active.length < N) {
                mActiveBroadcast = active = new Object[N];
            }
            int i=0;
            for (Callback cb : mCallbacks.values()) {
                active[i++] = cb;
            }
            return i;
        }
    }

    /**
     * Retrieve an item in the active broadcast that was previously started
     * with {@link #beginBroadcast}.  This can <em>only be called after
     * the broadcast is started, and its data is no longer valid after
     * calling {@link #finishBroadcast}.
     *
     * <p>Note that it is possible for the process of one of the returned
     * callbacks to go away before you call it, so you will need to catch
     * {@link RemoteException} when calling on to the returned object.
     * The callback list itself, however, will take care of unregistering
     * these objects once it detects that it is no longer valid, so you can
     * handle such an exception by simply ignoring it.
     *
     * @param index Which of the registered callbacks you would like to
     * retrieve.  Ranges from 0 to 1-{@link #beginBroadcast}.
     *
     * @return Returns the callback interface that you can call.  This will
     * always be non-null.
     *
     * @see #beginBroadcast
     */
    public E getBroadcastItem(int index) {
        return ((Callback)mActiveBroadcast[index]).mCallback;
    }
    
    /**
     * Retrieve the cookie associated with the item
     * returned by {@link #getBroadcastItem(int)}.
     * 
     * @see #getBroadcastItem
     */
    public Object getBroadcastCookie(int index) {
        return ((Callback)mActiveBroadcast[index]).mCookie;
    }

    /**
     * Clean up the state of a broadcast previously initiated by calling
     * {@link #beginBroadcast}.  This must always be called when you are done
     * with a broadcast.
     *
     * @see #beginBroadcast
     */
    public void finishBroadcast() {
        if (mBroadcastCount < 0) {
            throw new IllegalStateException(
                    "finishBroadcast() called outside of a broadcast");
        }
        
        Object[] active = mActiveBroadcast;
        if (active != null) {
            final int N = mBroadcastCount;
            for (int i=0; i<N; i++) {
                active[i] = null;
            }
        }
        
        mBroadcastCount = -1;
    }
}

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