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Handler.javaAPI DocAndroid 5.1 API31377Thu Mar 12 22:22:10 GMT 2015android.os

Handler

public class Handler extends Object
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.

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.

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 post versions allow you to enqueue Runnable objects to be called by the message queue when they are received; the 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).

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.

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 post or sendMessage methods as before, but from your new thread. The given Runnable or Message will then be scheduled in the Handler's message queue and processed when appropriate.

Fields Summary
private static final boolean
FIND_POTENTIAL_LEAKS
private static final String
TAG
final MessageQueue
mQueue
final Looper
mLooper
final Callback
mCallback
final boolean
mAsynchronous
IMessenger
mMessenger
Constructors Summary
public Handler()
Default constructor associates this handler with the {@link Looper} for the current thread. If this thread does not have a looper, this handler won't be able to receive messages so an exception is thrown.

        this(null, false);
public Handler(Callback callback)
Constructor associates this handler with the {@link Looper} for the current thread and takes a callback interface in which you can handle messages. If this thread does not have a looper, this handler won't be able to receive messages so an exception is thrown.

param
callback The callback interface in which to handle messages, or null.

        this(callback, false);
public Handler(Looper looper)
Use the provided {@link Looper} instead of the default one.

param
looper The looper, must not be null.

        this(looper, null, false);
public Handler(Looper looper, Callback callback)
Use the provided {@link Looper} instead of the default one and take a callback interface in which to handle messages.

param
looper The looper, must not be null.
param
callback The callback interface in which to handle messages, or null.

        this(looper, callback, false);
public Handler(boolean async)
Use the {@link Looper} for the current thread and set whether the handler should be asynchronous. Handlers are synchronous by default unless this constructor is used to make one that is strictly asynchronous. Asynchronous messages represent interrupts or events that do not require global ordering with respect to synchronous messages. Asynchronous messages are not subject to the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.

param
async If true, the handler calls {@link Message#setAsynchronous(boolean)} for each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
hide

        this(null, async);
public Handler(Callback callback, boolean async)
Use the {@link Looper} for the current thread with the specified callback interface and set whether the handler should be asynchronous. Handlers are synchronous by default unless this constructor is used to make one that is strictly asynchronous. Asynchronous messages represent interrupts or events that do not require global ordering with respect to synchronous messages. Asynchronous messages are not subject to the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.

param
callback The callback interface in which to handle messages, or null.
param
async If true, the handler calls {@link Message#setAsynchronous(boolean)} for each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
hide

        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;
        mAsynchronous = async;
public Handler(Looper looper, Callback callback, boolean async)
Use the provided {@link Looper} instead of the default one and take a callback interface in which to handle messages. Also set whether the handler should be asynchronous. Handlers are synchronous by default unless this constructor is used to make one that is strictly asynchronous. Asynchronous messages represent interrupts or events that do not require global ordering with respect to synchronous messages. Asynchronous messages are not subject to the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.

param
looper The looper, must not be null.
param
callback The callback interface in which to handle messages, or null.
param
async If true, the handler calls {@link Message#setAsynchronous(boolean)} for each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
hide

        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
Methods Summary
public voiddispatchMessage(Message msg)
Handle system messages here.

        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
public final voiddump(android.util.Printer pw, java.lang.String prefix)

        pw.println(prefix + this + " @ " + SystemClock.uptimeMillis());
        if (mLooper == null) {
            pw.println(prefix + "looper uninitialized");
        } else {
            mLooper.dump(pw, prefix + "  ");
        }
private booleanenqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)

        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
final IMessengergetIMessenger()

        synchronized (mQueue) {
            if (mMessenger != null) {
                return mMessenger;
            }
            mMessenger = new MessengerImpl();
            return mMessenger;
        }
public final LoopergetLooper()

        return mLooper;
public java.lang.StringgetMessageName(Message message)
Returns a string representing the name of the specified message. The default implementation will either return the class name of the message callback if any, or the hexadecimal representation of the message "what" field.

param
message The message whose name is being queried

        if (message.callback != null) {
            return message.callback.getClass().getName();
        }
        return "0x" + Integer.toHexString(message.what);
private static MessagegetPostMessage(java.lang.Runnable r)

        Message m = Message.obtain();
        m.callback = r;
        return m;
private static MessagegetPostMessage(java.lang.Runnable r, java.lang.Object token)

        Message m = Message.obtain();
        m.obj = token;
        m.callback = r;
        return m;
private static voidhandleCallback(Message message)

        message.callback.run();
public voidhandleMessage(Message msg)
Subclasses must implement this to receive messages.
public final booleanhasCallbacks(java.lang.Runnable r)
Check if there are any pending posts of messages with callback r in the message queue.

hide

        return mQueue.hasMessages(this, r, null);
public final booleanhasMessages(int what)
Check if there are any pending posts of messages with code 'what' in the message queue.

        return mQueue.hasMessages(this, what, null);
public final booleanhasMessages(int what, java.lang.Object object)
Check if there are any pending posts of messages with code 'what' and whose obj is 'object' in the message queue.

        return mQueue.hasMessages(this, what, object);
public final MessageobtainMessage()
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.

        return Message.obtain(this);
public final MessageobtainMessage(int what)
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.

        return Message.obtain(this, what);
public final MessageobtainMessage(int what, java.lang.Object obj)
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.

        return Message.obtain(this, what, obj);
public final MessageobtainMessage(int what, int arg1, int arg2)
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.

        return Message.obtain(this, what, arg1, arg2);
public final MessageobtainMessage(int what, int arg1, int arg2, java.lang.Object obj)
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.

        return Message.obtain(this, what, arg1, arg2, obj);
public final booleanpost(java.lang.Runnable r)
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.

       return  sendMessageDelayed(getPostMessage(r), 0);
public final booleanpostAtFrontOfQueue(java.lang.Runnable r)
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. 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.

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.

        return sendMessageAtFrontOfQueue(getPostMessage(r));
public final booleanpostAtTime(java.lang.Runnable r, long uptimeMillis)
Causes the Runnable r to be added to the message queue, to be run at a specific time given by uptimeMillis. The time-base is {@link android.os.SystemClock#uptimeMillis}. Time spent in deep sleep will add an additional delay to execution. 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.

        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
public final booleanpostAtTime(java.lang.Runnable r, java.lang.Object token, long uptimeMillis)
Causes the Runnable r to be added to the message queue, to be run at a specific time given by uptimeMillis. The time-base is {@link android.os.SystemClock#uptimeMillis}. Time spent in deep sleep will add an additional delay to execution. 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

        return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
public final booleanpostDelayed(java.lang.Runnable r, long delayMillis)
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. The time-base is {@link android.os.SystemClock#uptimeMillis}. Time spent in deep sleep will add an additional delay to execution.

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.

        return sendMessageDelayed(getPostMessage(r), delayMillis);
public final voidremoveCallbacks(java.lang.Runnable r)
Remove any pending posts of Runnable r that are in the message queue.

        mQueue.removeMessages(this, r, null);
public final voidremoveCallbacks(java.lang.Runnable r, java.lang.Object token)
Remove any pending posts of Runnable r with Object token that are in the message queue. If token is null, all callbacks will be removed.

        mQueue.removeMessages(this, r, token);
public final voidremoveCallbacksAndMessages(java.lang.Object token)
Remove any pending posts of callbacks and sent messages whose obj is token. If token is null, all callbacks and messages will be removed.

        mQueue.removeCallbacksAndMessages(this, token);
public final voidremoveMessages(int what)
Remove any pending posts of messages with code 'what' that are in the message queue.

        mQueue.removeMessages(this, what, null);
public final voidremoveMessages(int what, java.lang.Object object)
Remove any pending posts of messages with code 'what' and whose obj is 'object' that are in the message queue. If object is null, all messages will be removed.

        mQueue.removeMessages(this, what, object);
public final booleanrunWithScissors(java.lang.Runnable r, long timeout)
Runs the specified task synchronously.

If the current thread is the same as the handler thread, then the runnable runs immediately without being enqueued. Otherwise, posts the runnable to the handler and waits for it to complete before returning.

This method is dangerous! Improper use can result in deadlocks. Never call this method while any locks are held or use it in a possibly re-entrant manner.

This method is occasionally useful in situations where a background thread must synchronously await completion of a task that must run on the handler's thread. However, this problem is often a symptom of bad design. Consider improving the design (if possible) before resorting to this method.

One example of where you might want to use this method is when you just set up a Handler thread and need to perform some initialization steps on it before continuing execution.

If timeout occurs then this method returns false but the runnable will remain posted on the handler and may already be in progress or complete at a later time.

When using this method, be sure to use {@link Looper#quitSafely} when quitting the looper. Otherwise {@link #runWithScissors} may hang indefinitely. (TODO: We should fix this by making MessageQueue aware of blocking runnables.)

param
r The Runnable that will be executed synchronously.
param
timeout The timeout in milliseconds, or 0 to wait indefinitely.
return
Returns true if the Runnable was successfully executed. Returns false on failure, usually because the looper processing the message queue is exiting.
hide
This method is prone to abuse and should probably not be in the API. If we ever do make it part of the API, we might want to rename it to something less funny like runUnsafe().

        if (r == null) {
            throw new IllegalArgumentException("runnable must not be null");
        }
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout must be non-negative");
        }

        if (Looper.myLooper() == mLooper) {
            r.run();
            return true;
        }

        BlockingRunnable br = new BlockingRunnable(r);
        return br.postAndWait(this, timeout);
public final booleansendEmptyMessage(int what)
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.

        return sendEmptyMessageDelayed(what, 0);
public final booleansendEmptyMessageAtTime(int what, long uptimeMillis)
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.

        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
public final booleansendEmptyMessageDelayed(int what, long delayMillis)
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.

        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
public final booleansendMessage(Message msg)
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.

        return sendMessageDelayed(msg, 0);
public final booleansendMessageAtFrontOfQueue(Message msg)
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. 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.

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.

        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, 0);
public booleansendMessageAtTime(Message msg, long uptimeMillis)
Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis. The time-base is {@link android.os.SystemClock#uptimeMillis}. Time spent in deep sleep will add an additional delay to execution. 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.

        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
public final booleansendMessageDelayed(Message msg, long delayMillis)
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.

        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
public java.lang.StringtoString()

        return "Handler (" + getClass().getName() + ") {"
        + Integer.toHexString(System.identityHashCode(this))
        + "}";