public
class
Handler
extends Object
| java.lang.Object | |
| ↳ | android.os.Handler |
 Known Direct Subclasses
|
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.
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 |
|
Handler(Handler.Callback callback)
Constructor associates this handler with the |
|
Handler(Looper looper)
Use the provided |
|
Handler(Looper looper, Handler.Callback callback)
Use the provided |
|
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)
Check if there are any pending posts of messages with code 'what' in the message queue. |
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
Message
|
obtainMessage(int what, Object obj)
Same as |
final
Message
|
obtainMessage()
Returns a new |
final
Message
|
obtainMessage(int what, int arg1, int arg2)
Same as |
final
Message
|
obtainMessage(int what, int arg1, int arg2, Object obj)
Same as |
final
Message
|
obtainMessage(int what)
Same as |
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, 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, 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
|
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 representation of the object. |
Inherited methods | |
|---|---|
From
class
java.lang.Object
| |
Handler ()
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.
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.
If this thread does not have a looper, this handler won't be able to receive messages
so an exception is thrown.
| Parameters | |
|---|---|
callback |
Handler.Callback:
The callback interface in which to handle messages, or null.
|
Handler (Looper looper)
Use the provided Looper instead of the default one.
| Parameters | |
|---|---|
looper |
Looper:
The looper, must not be null.
|
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.
| Parameters | |
|---|---|
looper |
Looper:
The looper, must not be null. |
callback |
Handler.Callback:
The callback interface in which to handle messages, or null.
|
void dispatchMessage (Message msg)
Handle system messages here.
| Parameters | |
|---|---|
msg |
Message
|
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 |
Message:
The message whose name is being queried
|
| Returns | |
|---|---|
String |
|
void handleMessage (Message msg)
Subclasses must implement this to receive messages.
| Parameters | |
|---|---|
msg |
Message
|
boolean hasMessages (int what)
Check if there are any pending posts of messages with code 'what' in the message queue.
| Parameters | |
|---|---|
what |
int
|
| Returns | |
|---|---|
boolean |
|
Developers
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.
| Parameters | |
|---|---|
what |
int
|
object |
Object
|
| Returns | |
|---|---|
boolean |
|
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 |
int:
Value to assign to the returned Message.what field. |
obj |
Object:
Value to assign to the returned Message.obj field. |
| Returns | |
|---|---|
Message |
A Message from the global message pool. |
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.
| Returns | |
|---|---|
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.
| Parameters | |
|---|---|
what |
int:
Value to assign to the returned Message.what field. |
arg1 |
int:
Value to assign to the returned Message.arg1 field. |
arg2 |
int:
Value to assign to the returned Message.arg2 field. |
| Returns | |
|---|---|
Message |
A Message from the global message pool. |
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 |
int:
Value to assign to the returned Message.what field. |
arg1 |
int:
Value to assign to the returned Message.arg1 field. |
arg2 |
int:
Value to assign to the returned Message.arg2 field. |
obj |
Object:
Value to assign to the returned Message.obj field. |
| Returns | |
|---|---|
Message |
A 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.
| Parameters | |
|---|---|
what |
int:
Value to assign to the returned Message.what field. |
| Returns | |
|---|---|
Message |
A Message from the global message pool. |
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 |
Runnable:
The Runnable that will be executed. |
| Returns | |
|---|---|
boolean |
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. |
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 |
Runnable:
The Runnable that will be executed. |
| Returns | |
|---|---|
boolean |
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. |
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 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.
| Parameters | |
|---|---|
r |
Runnable:
The Runnable that will be executed. |
uptimeMillis |
long:
The absolute time at which the callback should run,
using the uptimeMillis() time-base. |
| Returns | |
|---|---|
boolean |
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. |
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 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.
| Parameters | |
|---|---|
r |
Runnable:
The Runnable that will be executed. |
token |
Object
|
uptimeMillis |
long:
The absolute time at which the callback should run,
using the uptimeMillis() time-base. |
| Returns | |
|---|---|
boolean |
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:
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.
The time-base is uptimeMillis().
Time spent in deep sleep will add an additional delay to execution.
| Parameters | |
|---|---|
r |
Runnable:
The Runnable that will be executed. |
delayMillis |
long:
The delay (in milliseconds) until the Runnable
will be executed. |
| Returns | |
|---|---|
boolean |
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. |
void removeCallbacks (Runnable r)
Remove any pending posts of Runnable r that are in the message queue.
| Parameters | |
|---|---|
r |
Runnable
|
void removeCallbacks (Runnable r, 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.
| Parameters | |
|---|---|
r |
Runnable
|
token |
Object
|
void 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.
| Parameters | |
|---|---|
token |
Object
|
void removeMessages (int what)
Remove any pending posts of messages with code 'what' that are in the message queue.
| Parameters | |
|---|---|
what |
int
|
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. If object is null, all messages will be removed.
| Parameters | |
|---|---|
what |
int
|
object |
Object
|
boolean sendEmptyMessage (int what)
Sends a Message containing only the what value.
| Parameters | |
|---|---|
what |
int
|
| Returns | |
|---|---|
boolean |
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. |
boolean sendEmptyMessageAtTime (int what,
long uptimeMillis)
Sends a Message containing only the what value, to be delivered at a specific time.
| Parameters | |
|---|---|
what |
int
|
uptimeMillis |
long
|
| Returns | |
|---|---|
boolean |
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. |
boolean sendEmptyMessageDelayed (int what,
long delayMillis)
Sends a Message containing only the what value, to be delivered after the specified amount of time elapses.
| Parameters | |
|---|---|
what |
int
|
delayMillis |
long
|
| Returns | |
|---|---|
boolean |
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. |
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(Message),
in the thread attached to this handler.
| Parameters | |
|---|---|
msg |
Message
|
| Returns | |
|---|---|
boolean |
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. |
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(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.
| Parameters | |
|---|---|
msg |
Message
|
| Returns | |
|---|---|
boolean |
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. |
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 uptimeMillis().
Time spent in deep sleep will add an additional delay to execution.
You will receive it in handleMessage(Message), in the thread attached
to this handler.
| Parameters | |
|---|---|
msg |
Message
|
uptimeMillis |
long:
The absolute time at which the message should be
delivered, using the
uptimeMillis() time-base. |
| Returns | |
|---|---|
boolean |
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. |
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(Message), in the thread attached to this handler.
| Parameters | |
|---|---|
msg |
Message
|
delayMillis |
long
|
| Returns | |
|---|---|
boolean |
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. |
String toString ()
Returns a string representation of the object. In general, the
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
| Returns | |
|---|---|
String |
a string representation of the object. |