public class

Handler

extends Object
java.lang.Object
   ↳ mindroid.os.Handler

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 queue for the current thread.
Handler(Handler.Callback callback)
Constructor associates this handler with the queue for the current thread and takes a callback interface in which you can handle messages.
Handler(Looper looper)
Use the provided queue instead of the default one.
Handler(Looper looper, Handler.Callback callback)
Use the provided queue 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 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 runnable)
Causes the Runnable r to be added to the message queue.
final boolean postAtTime(Runnable runnable, 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 runnable, 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 runnable, long delayMillis)
Causes the Runnable r to be added to the message queue, to be run after the specified amount of time elapses.
final boolean removeCallbacks(Runnable runnable)
Remove any pending posts of Runnable r that are in the message queue.
final boolean removeCallbacks(Runnable runnable, Object token)
Remove any pending posts of Runnable r with Object token that are in the message queue.
final boolean removeCallbacksAndMessages(Object token)
Remove any pending posts of callbacks and sent messages whose obj is token.
final boolean removeMessages(int what)
Remove any pending posts of messages with code 'what' that are in the message queue.
final boolean 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 message)
Pushes a message onto the end of the message queue after all pending messages before the current time.
boolean sendMessageAtTime(Message message, long uptimeMillis)
Enqueue a message into the message queue after all pending messages before the absolute time (in milliseconds) uptimeMillis.
final boolean sendMessageDelayed(Message message, long delayMillis)
Enqueue a message into the message queue after all pending messages before (current time + delayMillis).
String toString()
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

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.

public Handler (Handler.Callback callback)

Constructor associates this handler with the queue for the current thread and takes a callback interface in which you can handle messages.

public Handler (Looper looper)

Use the provided queue instead of the default one.

public Handler (Looper looper, Handler.Callback callback)

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

Public Methods

public void dispatchMessage (Message msg)

Handle system messages here.

public final Looper getLooper ()

public String getMessageName (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.

Parameters
message The message whose name is being queried

public void handleMessage (Message msg)

Subclasses must implement this to receive messages.

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.

public final boolean hasMessages (int what)

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)

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 ()

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)

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)

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)

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 runnable)

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

Parameters
runnable 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 runnable, 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 uptimeMillis() . The runnable will be run on the thread to which this handler is attached.

Parameters
runnable 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 runnable, 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
runnable 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 boolean removeCallbacks (Runnable runnable)

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

public final boolean removeCallbacks (Runnable runnable, 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.

public final boolean removeCallbacksAndMessages (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.

public final boolean removeMessages (int what)

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

public final boolean 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. If object is null, all messages will be removed.

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.

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.

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.

public final boolean sendMessage (Message message)

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 boolean sendMessageAtTime (Message message, long uptimeMillis)

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 message, long delayMillis)

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 ()