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Android example source code file (Handler.java)

This example Android source code file (Handler.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

android, callback, can't, class, find_potential_leaks, handler, imessenger, looper, message, messagequeue, messengerimpl, object, reflection, runtimeexception, string, the, util, utilities, utils

The Handler.java Android example source code

/*
 * Copyright (C) 2006 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 android.util.Log;
import android.util.Printer;

import java.lang.reflect.Modifier;

/**
 * A Handler allows you to send and process {@link Message} and Runnable
 * objects associated with a thread's {@link MessageQueue}.  Each Handler
 * instance is associated with a single thread and that thread's message
 * queue.  When you create a new Handler, it is bound to the thread /
 * message queue of the thread that is creating it -- from that point on,
 * it will deliver messages and runnables to that message queue and execute
 * them as they come out of the message queue.
 * 
 * <p>There are two main uses for a Handler: (1) to schedule messages and
 * runnables to be executed as some point in the future; and (2) to enqueue
 * an action to be performed on a different thread than your own.
 * 
 * <p>Scheduling messages is accomplished with the
 * {@link #post}, {@link #postAtTime(Runnable, long)},
 * {@link #postDelayed}, {@link #sendEmptyMessage},
 * {@link #sendMessage}, {@link #sendMessageAtTime}, and
 * {@link #sendMessageDelayed} methods.  The <em>post versions allow
 * you to enqueue Runnable objects to be called by the message queue when
 * they are received; the <em>sendMessage versions allow you to enqueue
 * a {@link Message} object containing a bundle of data that will be
 * processed by the Handler's {@link #handleMessage} method (requiring that
 * you implement a subclass of Handler).
 * 
 * <p>When posting or sending to a Handler, you can either
 * allow the item to be processed as soon as the message queue is ready
 * to do so, or specify a delay before it gets processed or absolute time for
 * it to be processed.  The latter two allow you to implement timeouts,
 * ticks, and other timing-based behavior.
 * 
 * <p>When a
 * process is created for your application, its main thread is dedicated to
 * running a message queue that takes care of managing the top-level
 * application objects (activities, broadcast receivers, etc) and any windows
 * they create.  You can create your own threads, and communicate back with
 * the main application thread through a Handler.  This is done by calling
 * the same <em>post or sendMessage methods as before, but from
 * your new thread.  The given Runnable or Message will than be scheduled
 * in the Handler's message queue and processed when appropriate.
 */
public class Handler {
    /*
     * Set this flag to true to detect anonymous, local or member classes
     * that extend this Handler class and that are not static. These kind
     * of classes can potentially create leaks.
     */
    private static final boolean FIND_POTENTIAL_LEAKS = false;
    private static final String TAG = "Handler";

    /**
     * Callback interface you can use when instantiating a Handler to avoid
     * having to implement your own subclass of Handler.
     */
    public interface Callback {
        public boolean handleMessage(Message msg);
    }
    
    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }
    
    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

    /**
     * Default constructor associates this handler with the queue for the
     * current thread.
     *
     * If there isn't one, this handler won't be able to receive messages.
     */
    public Handler() {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
    }

    /**
     * Constructor associates this handler with the queue for the
     * current thread and takes a callback interface in which you can handle
     * messages.
     */
    public Handler(Callback callback) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
    }

    /**
     * Use the provided queue instead of the default one.
     */
    public Handler(Looper looper) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = null;
    }

    /**
     * Use the provided queue instead of the default one and take a callback
     * interface in which to handle messages.
     */
    public Handler(Looper looper, Callback callback) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
    }

    /**
     * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
     * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
     *  If you don't want that facility, just call Message.obtain() instead.
     */
    public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

    /**
     * Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
     * 
     * @param what Value to assign to the returned Message.what field.
     * @return A Message from the global message pool.
     */
    public final Message obtainMessage(int what)
    {
        return Message.obtain(this, what);
    }
    
    /**
     * 
     * Same as {@link #obtainMessage()}, except that it also sets the what and obj members 
     * of the returned Message.
     * 
     * @param what Value to assign to the returned Message.what field.
     * @param obj Value to assign to the returned Message.obj field.
     * @return A Message from the global message pool.
     */
    public final Message obtainMessage(int what, Object obj)
    {
        return Message.obtain(this, what, obj);
    }

    /**
     * 
     * Same as {@link #obtainMessage()}, except that it also sets the what, arg1 and arg2 members of the returned
     * Message.
     * @param what Value to assign to the returned Message.what field.
     * @param arg1 Value to assign to the returned Message.arg1 field.
     * @param arg2 Value to assign to the returned Message.arg2 field.
     * @return A Message from the global message pool.
     */
    public final Message obtainMessage(int what, int arg1, int arg2)
    {
        return Message.obtain(this, what, arg1, arg2);
    }
    
    /**
     * 
     * Same as {@link #obtainMessage()}, except that it also sets the what, obj, arg1,and arg2 values on the 
     * returned Message.
     * @param what Value to assign to the returned Message.what field.
     * @param arg1 Value to assign to the returned Message.arg1 field.
     * @param arg2 Value to assign to the returned Message.arg2 field.
     * @param obj Value to assign to the returned Message.obj field.
     * @return A Message from the global message pool.
     */
    public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
    {
        return Message.obtain(this, what, arg1, arg2, obj);
    }

    /**
     * Causes the Runnable r to be added to the message queue.
     * The runnable will be run on the thread to which this handler is 
     * attached. 
     *  
     * @param r The Runnable that will be executed.
     * 
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
    
    /**
     * Causes the Runnable r to be added to the message queue, to be run
     * at a specific time given by <var>uptimeMillis.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * The runnable will be run on the thread to which this handler is attached.
     *
     * @param r The Runnable that will be executed.
     * @param uptimeMillis The absolute time at which the callback should run,
     *         using the {@link android.os.SystemClock#uptimeMillis} time-base.
     *  
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the Runnable will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean postAtTime(Runnable r, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
    }
    
    /**
     * Causes the Runnable r to be added to the message queue, to be run
     * at a specific time given by <var>uptimeMillis.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * The runnable will be run on the thread to which this handler is attached.
     *
     * @param r The Runnable that will be executed.
     * @param uptimeMillis The absolute time at which the callback should run,
     *         using the {@link android.os.SystemClock#uptimeMillis} time-base.
     * 
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the Runnable will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     *         
     * @see android.os.SystemClock#uptimeMillis
     */
    public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
    }
    
    /**
     * Causes the Runnable r to be added to the message queue, to be run
     * after the specified amount of time elapses.
     * The runnable will be run on the thread to which this handler
     * is attached.
     *  
     * @param r The Runnable that will be executed.
     * @param delayMillis The delay (in milliseconds) until the Runnable
     *        will be executed.
     *        
     * @return Returns true if the Runnable was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the Runnable will be processed --
     *         if the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean postDelayed(Runnable r, long delayMillis)
    {
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }
    
    /**
     * Posts a message to an object that implements Runnable.
     * Causes the Runnable r to executed on the next iteration through the
     * message queue. The runnable will be run on the thread to which this
     * handler is attached.
     * <b>This method is only for use in very special circumstances -- it
     * can easily starve the message queue, cause ordering problems, or have
     * other unexpected side-effects.</b>
     *  
     * @param r The Runnable that will be executed.
     * 
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean postAtFrontOfQueue(Runnable r)
    {
        return sendMessageAtFrontOfQueue(getPostMessage(r));
    }

    /**
     * Remove any pending posts of Runnable r that are in the message queue.
     */
    public final void removeCallbacks(Runnable r)
    {
        mQueue.removeMessages(this, r, null);
    }

    /**
     * Remove any pending posts of Runnable <var>r with Object
     * <var>token that are in the message queue.
     */
    public final void removeCallbacks(Runnable r, Object token)
    {
        mQueue.removeMessages(this, r, token);
    }

    /**
     * Pushes a message onto the end of the message queue after all pending messages
     * before the current time. It will be received in {@link #handleMessage},
     * in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

    /**
     * Sends a Message containing only the what value.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }

    /**
     * Sends a Message containing only the what value, to be delivered
     * after the specified amount of time elapses.
     * @see #sendMessageDelayed(android.os.Message, long) 
     * 
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
    }

    /**
     * Sends a Message containing only the what value, to be delivered 
     * at a specific time.
     * @see #sendMessageAtTime(android.os.Message, long)
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */

    public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) <var>uptimeMillis.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public boolean sendMessageAtTime(Message msg, long uptimeMillis)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;
            sent = queue.enqueueMessage(msg, uptimeMillis);
        }
        else {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }

    /**
     * Enqueue a message at the front of the message queue, to be processed on
     * the next iteration of the message loop.  You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     * <b>This method is only for use in very special circumstances -- it
     * can easily starve the message queue, cause ordering problems, or have
     * other unexpected side-effects.</b>
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendMessageAtFrontOfQueue(Message msg)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;
            sent = queue.enqueueMessage(msg, 0);
        }
        else {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }

    /**
     * Remove any pending posts of messages with code 'what' that are in the
     * message queue.
     */
    public final void removeMessages(int what) {
        mQueue.removeMessages(this, what, null, true);
    }

    /**
     * Remove any pending posts of messages with code 'what' and whose obj is
     * 'object' that are in the message queue.
     */
    public final void removeMessages(int what, Object object) {
        mQueue.removeMessages(this, what, object, true);
    }

    /**
     * Remove any pending posts of callbacks and sent messages whose
     * <var>obj is token.
     */
    public final void removeCallbacksAndMessages(Object token) {
        mQueue.removeCallbacksAndMessages(this, token);
    }

    /**
     * Check if there are any pending posts of messages with code 'what' in
     * the message queue.
     */
    public final boolean hasMessages(int what) {
        return mQueue.removeMessages(this, what, null, false);
    }

    /**
     * Check if there are any pending posts of messages with code 'what' and
     * whose obj is 'object' in the message queue.
     */
    public final boolean hasMessages(int what, Object object) {
        return mQueue.removeMessages(this, what, object, false);
    }

    // if we can get rid of this method, the handler need not remember its loop
    // we could instead export a getMessageQueue() method... 
    public final Looper getLooper() {
        return mLooper;
    }

    public final void dump(Printer pw, String prefix) {
        pw.println(prefix + this + " @ " + SystemClock.uptimeMillis());
        if (mLooper == null) {
            pw.println(prefix + "looper uninitialized");
        } else {
            mLooper.dump(pw, prefix + "  ");
        }
    }

    @Override
    public String toString() {
        return "Handler{"
        + Integer.toHexString(System.identityHashCode(this))
        + "}";
    }

    final IMessenger getIMessenger() {
        synchronized (mQueue) {
            if (mMessenger != null) {
                return mMessenger;
            }
            mMessenger = new MessengerImpl();
            return mMessenger;
        }
    }
    
    private final class MessengerImpl extends IMessenger.Stub {
        public void send(Message msg) {
            Handler.this.sendMessage(msg);
        }
    }
    
    private final Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

    private final Message getPostMessage(Runnable r, Object token) {
        Message m = Message.obtain();
        m.obj = token;
        m.callback = r;
        return m;
    }

    private final void handleCallback(Message message) {
        message.callback.run();
    }

    final MessageQueue mQueue;
    final Looper mLooper;
    final Callback mCallback;
    IMessenger mMessenger;
}

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