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Android APIs
public class

Handler

extends Object
java.lang.Object
   ↳ android.os.Handler
Known Direct Subclasses

Class Overview

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(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long), and sendMessageDelayed(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(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, 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.

Summary

Nested Classes
interface Handler.Callback Callback interface you can use when instantiating a Handler to avoid having to implement your own subclass of Handler. 
Public Constructors
Handler()
Default constructor associates this handler with the Looper for the current thread.
Handler(Handler.Callback callback)
Constructor associates this handler with the Looper for the current thread and takes a callback interface in which you can handle messages.
Handler(Looper looper)
Use the provided Looper instead of the default one.
Handler(Looper looper, Handler.Callback callback)
Use the provided Looper instead of the default one and take a callback interface in which to handle messages.
Public Methods
void dispatchMessage(Message msg)
Handle system messages here.
final void dump(Printer pw, String prefix)
final Looper getLooper()
String getMessageName(Message message)
Returns a string representing the name of the specified message.
void handleMessage(Message msg)
Subclasses must implement this to receive messages.
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.
final boolean hasMessages(int what)
Check if there are any pending posts of messages with code 'what' in the message queue.
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.
final Message obtainMessage()
Returns a new Message from the global message pool.
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.
final Message obtainMessage(int what)
Same as obtainMessage(), except that it also sets the what member of the returned Message.
final Message obtainMessage(int what, Object obj)
Same as obtainMessage(), except that it also sets the what and obj members of the returned Message.
final boolean post(Runnable r)
Causes the Runnable r to be added to the message queue.
final boolean postAtFrontOfQueue(Runnable r)
Posts a message to an object that implements Runnable.
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.
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.
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.
final void removeCallbacks(Runnable r)
Remove any pending posts of Runnable r that are in the message queue.
final void removeCallbacks(Runnable r, Object token)
Remove any pending posts of Runnable r with Object token that are in the message queue.
final void removeCallbacksAndMessages(Object token)
Remove any pending posts of callbacks and sent messages whose obj is token.
final void removeMessages(int what)
Remove any pending posts of messages with code 'what' that are in the message queue.
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.
final boolean sendEmptyMessage(int what)
Sends a Message containing only the what value.
final boolean sendEmptyMessageAtTime(int what, long uptimeMillis)
Sends a Message containing only the what value, to be delivered at a specific time.
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.
final boolean sendMessage(Message msg)
Pushes a message onto the end of the message queue after all pending messages before the current time.
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.
boolean sendMessageAtTime(Message msg, long uptimeMillis)
Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis.
final 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 this object.
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

public Handler ()

Added in API level 1

Default constructor associates this handler with the 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.

public Handler (Handler.Callback callback)

Added in API level 3

Constructor associates this handler with the 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.

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

public Handler (Looper looper)

Added in API level 1

Use the provided Looper instead of the default one.

Parameters
looper The looper, must not be null.

public Handler (Looper looper, Handler.Callback callback)

Added in API level 3

Use the provided Looper instead of the default one and take a callback interface in which to handle messages.

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

Public Methods

public void dispatchMessage (Message msg)

Added in API level 1

Handle system messages here.

public final void dump (Printer pw, String prefix)

Added in API level 1

public final Looper getLooper ()

Added in API level 1

public String getMessageName (Message message)

Added in API level 14

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.

Parameters
message The message whose name is being queried

public void handleMessage (Message msg)

Added in API level 1

Subclasses must implement this to receive messages.

public final boolean hasMessages (int what, Object object)

Added in API level 1

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)

Added in API level 1

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

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

Added in API level 1

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.

public final Message obtainMessage ()

Added in API level 1

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.

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

Added in API level 1

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.

public final Message obtainMessage (int what)

Added in API level 1

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.

public final Message obtainMessage (int what, Object obj)

Added in API level 1

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.

public final boolean post (Runnable r)

Added in API level 1

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.

public final boolean postAtFrontOfQueue (Runnable r)

Added in API level 1

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.

public final boolean postAtTime (Runnable r, Object token, long uptimeMillis)

Added in API level 1

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 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 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

public final boolean postAtTime (Runnable r, long uptimeMillis)

Added in API level 1

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 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 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.

public final boolean postDelayed (Runnable r, long delayMillis)

Added in API level 1

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.

public final void removeCallbacks (Runnable r)

Added in API level 1

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

public final void removeCallbacks (Runnable r, Object token)

Added in API level 1

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.

public final void removeCallbacksAndMessages (Object token)

Added in API level 1

Remove any pending posts of callbacks and sent messages whose obj is token. If token is null, all callbacks and messages will be removed.

public final void removeMessages (int what)

Added in API level 1

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

public final void removeMessages (int what, Object object)

Added in API level 1

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.

public final boolean sendEmptyMessage (int what)

Added in API level 1

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.

public final boolean sendEmptyMessageAtTime (int what, long uptimeMillis)

Added in API level 1

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

public final boolean sendEmptyMessageDelayed (int what, long delayMillis)

Added in API level 1

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

public final boolean sendMessage (Message msg)

Added in API level 1

Pushes a message onto the end of the message queue after all pending messages before the current time. It will be received in handleMessage(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.

public final boolean sendMessageAtFrontOfQueue (Message msg)

Added in API level 1

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(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.

public boolean sendMessageAtTime (Message msg, long uptimeMillis)

Added in API level 1

Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis. The time-base is uptimeMillis(). You will receive it in handleMessage(Message), in the thread attached to this handler.

Parameters
uptimeMillis The absolute time at which the message should be delivered, using the 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.

public final boolean sendMessageDelayed (Message msg, long delayMillis)

Added in API level 1

Enqueue a message into the message queue after all pending messages before (current time + delayMillis). You will receive it in handleMessage(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.

public String toString ()

Added in API level 1

Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression: 

   getClass().getName() + '@' + Integer.toHexString(hashCode())

See Writing a useful toString method if you intend implementing your own toString method.

Returns
  • a printable representation of this object.