android.os
Class Handler

java.lang.Object
  android.os.Handler

Direct Known Subclasses:

AsyncQueryHandler, AsyncQueryHandler.WorkerHandler, BrowserFrame, CallbackProxy, ContactsAsyncHelper, HttpAuthHandler, JWebCoreJavaBridge, LoadListener, NetworkStateTracker, SslErrorHandler, StreamLoader, SyncManager.SyncHandler, ViewRoot, VolumePanel, WebView.PrivateHandler

public class Handler
extends Object

A Handler allows you to send and process Message and Runnable objects associated with a thread's 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 post(java.lang.Runnable), postAtTime(Runnable, long), postDelayed(java.lang.Runnable, long), sendEmptyMessage(int), sendMessage(android.os.Message), sendMessageAtTime(android.os.Message, long), and sendMessageDelayed(android.os.Message, long) 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 Message object containing a bundle of data that will be processed by the Handler's handleMessage(android.os.Message) 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, intent 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 than be scheduled in the Handler's message queue and processed when appropriate.

Field Summary

(package private) Looper	mLooper
(package private) IMessenger	mMessenger
(package private) MessageQueue	mQueue

Constructor Summary

Handler()
Default constructor associates this handler with the queue for the current thread.

Handler(Looper looper)
Use the provided queue instead of the default one.

Method Summary

void	dispatchMessage(Message msg) Handle system messages here.
void	dump(Printer pw, String prefix)
(package private) IMessenger	getIMessenger()
Looper	getLooper()
void	handleMessage(Message msg) Subclasses must implement this to receive messages.
boolean	hasMessages(int what) Check if there are any pending posts of messages with code 'what' in the message queue.
boolean	hasMessages(int what, Object object) Check if there are any pending posts of messages with code 'what' and whose obj is 'object' in the message queue.
Message	obtainMessage() Returns a new Message from the global message pool.
Message	obtainMessage(int what) Same as obtainMessage(), except that it also sets the what member of the returned Message.
Message	obtainMessage(int what, int arg1, int arg2) Same as obtainMessage(), except that it also sets the what, arg1 and arg2 members of the returned Message.
Message	obtainMessage(int what, int arg1, int arg2, Object obj) Same as obtainMessage(), except that it also sets the what, obj, arg1,and arg2 values on the returned Message.
Message	obtainMessage(int what, Object obj) Same as obtainMessage(), except that it also sets the what and obj members of the returned Message.
boolean	post(Runnable r) Causes the Runnable r to be added to the message queue.
boolean	postAtFrontOfQueue(Runnable r) Posts a message to an object that implements Runnable.
boolean	postAtTime(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.
boolean	postAtTime(Runnable r, 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.
boolean	postDelayed(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.
void	removeCallbacks(Runnable r) Remove any pending posts of Runnable r that are in the message queue.
void	removeCallbacks(Runnable r, Object token) Remove any pending posts of Runnable r with Object token that are in the message queue.
void	removeCallbacksAndMessages(Object token) Remove any pending posts of callbacks and sent messages whose obj is token.
void	removeMessages(int what) Remove any pending posts of messages with code 'what' that are in the message queue.
void	removeMessages(int what, Object object) Remove any pending posts of messages with code 'what' and whose obj is 'object' that are in the message queue.
boolean	sendEmptyMessage(int what) Sends a Message containing only the what value.
boolean	sendEmptyMessageAtTime(int what, long uptimeMillis) Sends a Message containing only the what value, to be delivered at a specific time.
boolean	sendEmptyMessageDelayed(int what, long delayMillis) Sends a Message containing only the what value, to be delivered after the specified amount of time elapses.
boolean	sendMessage(Message msg) Pushes a message onto the end of the message queue after all pending messages before the current time.
boolean	sendMessageAtFrontOfQueue(Message msg) Enqueue a message at the front of the message queue, to be processed on the next iteration of the message loop.
boolean	sendMessageAtTime(Message msg, long uptimeMillis) Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis.
boolean	sendMessageDelayed(Message msg, long delayMillis) Enqueue a message into the message queue after all pending messages before (current time + delayMillis).
String	toString() Returns a string containing a concise, human-readable description of the receiver.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

mQueue

final MessageQueue mQueue

mLooper

final Looper mLooper

mMessenger

IMessenger mMessenger

Constructor Detail

Handler

public Handler()

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.

Handler

public Handler(Looper looper)

Use the provided queue instead of the default one.

Method Detail

handleMessage

public void handleMessage(Message msg)

Subclasses must implement this to receive messages.

dispatchMessage

public void dispatchMessage(Message msg)

Handle system messages here.

obtainMessage

public final Message obtainMessage()

Returns a new 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.

obtainMessage

public final Message obtainMessage(int what)

Same as obtainMessage(), except that it also sets the what member of the returned Message.

Parameters:

what - Value to assign to the returned Message.what field.

Returns:

A Message from the global message pool.

obtainMessage

public final Message obtainMessage(int what,
                                   Object obj)

Same as obtainMessage(), except that it also sets the what and obj members of the returned Message.

Parameters:

what - Value to assign to the returned Message.what field.

obj - Value to assign to the returned Message.obj field.

Returns:

A Message from the global message pool.

obtainMessage

public final Message obtainMessage(int what,
                                   int arg1,
                                   int arg2)

Same as obtainMessage(), except that it also sets the what, arg1 and arg2 members of the returned Message.

Parameters:

what - Value to assign to the returned Message.what field.

arg1 - Value to assign to the returned Message.arg1 field.

arg2 - Value to assign to the returned Message.arg2 field.

Returns:

A Message from the global message pool.

obtainMessage

public final Message obtainMessage(int what,
                                   int arg1,
                                   int arg2,
                                   Object obj)

Same as obtainMessage(), except that it also sets the what, obj, arg1,and arg2 values on the returned Message.

Parameters:

what - Value to assign to the returned Message.what field.

arg1 - Value to assign to the returned Message.arg1 field.

arg2 - Value to assign to the returned Message.arg2 field.

obj - Value to assign to the returned Message.obj field.

Returns:

A Message from the global message pool.

post

public final boolean post(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.

Parameters:

r - The Runnable that will be executed.

Returns:

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.

postAtTime

public final boolean postAtTime(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 SystemClock.uptimeMillis(). The runnable will be run on the thread to which this handler is attached.

Parameters:

r - The Runnable that will be executed.

uptimeMillis - The absolute time at which the callback should run, using the SystemClock.uptimeMillis() time-base.

Returns:

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.

postAtTime

public final boolean postAtTime(Runnable r,
                                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 SystemClock.uptimeMillis(). The runnable will be run on the thread to which this handler is attached.

Parameters:

r - The Runnable that will be executed.

uptimeMillis - The absolute time at which the callback should run, using the SystemClock.uptimeMillis() time-base.

Returns:

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 Also:

SystemClock.uptimeMillis()

postDelayed

public final boolean postDelayed(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.

Parameters:

r - The Runnable that will be executed.

delayMillis - The delay (in milliseconds) until the Runnable will be executed.

Returns:

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.

postAtFrontOfQueue

public final boolean postAtFrontOfQueue(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.

Parameters:

r - The Runnable that will be executed.

Returns:

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.

removeCallbacks

public final void removeCallbacks(Runnable r)

Remove any pending posts of Runnable r that are in the message queue.

removeCallbacks

public final void removeCallbacks(Runnable r,
                                  Object token)

Remove any pending posts of Runnable r with Object token that are in the message queue.

sendMessage

public final boolean sendMessage(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 handleMessage(android.os.Message), in the thread attached to this handler.

Returns:

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.

sendEmptyMessage

public final boolean sendEmptyMessage(int what)

Sends a Message containing only the what value.

Returns:

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.

sendEmptyMessageDelayed

public final boolean sendEmptyMessageDelayed(int what,
                                             long delayMillis)

Sends a Message containing only the what value, to be delivered after the specified amount of time elapses.

Returns:

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.

See Also:

sendMessageDelayed(android.os.Message, long)

sendEmptyMessageAtTime

public final boolean sendEmptyMessageAtTime(int what,
                                            long uptimeMillis)

Sends a Message containing only the what value, to be delivered at a specific time.

Returns:

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.

See Also:

sendMessageAtTime(android.os.Message, long)

sendMessageDelayed

public final boolean sendMessageDelayed(Message msg,
                                        long delayMillis)

Enqueue a message into the message queue after all pending messages before (current time + delayMillis). You will receive it in handleMessage(android.os.Message), in the thread attached to this handler.

Returns:

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.

sendMessageAtTime

public boolean sendMessageAtTime(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 SystemClock.uptimeMillis(). You will receive it in handleMessage(android.os.Message), in the thread attached to this handler.

Parameters:

uptimeMillis - The absolute time at which the message should be delivered, using the SystemClock.uptimeMillis() time-base.

Returns:

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.

sendMessageAtFrontOfQueue

public final boolean sendMessageAtFrontOfQueue(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 handleMessage(android.os.Message), 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.

Returns:

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.

removeMessages

public final void removeMessages(int what)

Remove any pending posts of messages with code 'what' that are in the message queue.

removeMessages

public final void removeMessages(int what,
                                 Object object)

Remove any pending posts of messages with code 'what' and whose obj is 'object' that are in the message queue.

removeCallbacksAndMessages

public final void removeCallbacksAndMessages(Object token)

Remove any pending posts of callbacks and sent messages whose obj is token.

hasMessages

public final boolean hasMessages(int what)

Check if there are any pending posts of messages with code 'what' in the message queue.

hasMessages

public final boolean hasMessages(int what,
                                 Object object)

Check if there are any pending posts of messages with code 'what' and whose obj is 'object' in the message queue.

getLooper

public final Looper getLooper()

dump

public final void dump(Printer pw,
                       String prefix)

toString

public String toString()

Description copied from class: Object

Returns a string containing a concise, human-readable description of the receiver.

Overrides:

toString in class Object

Returns:

String a printable representation for the receiver.

getIMessenger

final IMessenger getIMessenger()