android.os

Class MessageQueue

java.lang.Object

android.os.MessageQueue

public final class MessageQueue
extends Object

Low-level class holding the list of messages to be dispatched by a Looper. Messages are not added directly to a MessageQueue, but rather through Handler objects associated with the Looper.

You can retrieve the MessageQueue for the current thread with Looper.myQueue().

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	MessageQueue.IdleHandler Callback interface for discovering when a thread is going to block waiting for more messages.
static interface	MessageQueue.OnFileDescriptorEventListener A listener which is invoked when file descriptor related events occur.

Method Summary

Modifier and Type	Method and Description
void	addIdleHandler(MessageQueue.IdleHandler handler) Add a new MessageQueue.IdleHandler to this message queue.
void	addOnFileDescriptorEventListener(FileDescriptor fd, int events, MessageQueue.OnFileDescriptorEventListener listener) Adds a file descriptor listener to receive notification when file descriptor related events occur.
protected void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
boolean	isIdle() Returns true if the looper has no pending messages which are due to be processed.
boolean	isPolling() Returns whether this looper's thread is currently polling for more work to do.
int	postSyncBarrier() Posts a synchronization barrier to the Looper's message queue.
void	removeIdleHandler(MessageQueue.IdleHandler handler) Remove an MessageQueue.IdleHandler from the queue that was previously added with addIdleHandler(android.os.MessageQueue.IdleHandler).
void	removeOnFileDescriptorEventListener(FileDescriptor fd) Removes a file descriptor listener.
void	removeSyncBarrier(int token) Removes a synchronization barrier.

Methods inherited from class java.lang.Object

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

Method Detail

finalize

protected void finalize()
                 throws Throwable

Description copied from class: Object

Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.

Overrides:

finalize in class Object

Throws:

Throwable

isIdle

public boolean isIdle()

Returns true if the looper has no pending messages which are due to be processed.

This method is safe to call from any thread.

Returns:

True if the looper is idle.

addIdleHandler

public void addIdleHandler(MessageQueue.IdleHandler handler)

Add a new MessageQueue.IdleHandler to this message queue. This may be removed automatically for you by returning false from IdleHandler.queueIdle() when it is invoked, or explicitly removing it with removeIdleHandler(android.os.MessageQueue.IdleHandler).

This method is safe to call from any thread.

Parameters:

handler - The IdleHandler to be added.

removeIdleHandler

public void removeIdleHandler(MessageQueue.IdleHandler handler)

Remove an MessageQueue.IdleHandler from the queue that was previously added with addIdleHandler(android.os.MessageQueue.IdleHandler). If the given object is not currently in the idle list, nothing is done.

This method is safe to call from any thread.

Parameters:

handler - The IdleHandler to be removed.

isPolling

public boolean isPolling()

Returns whether this looper's thread is currently polling for more work to do. This is a good signal that the loop is still alive rather than being stuck handling a callback. Note that this method is intrinsically racy, since the state of the loop can change before you get the result back.

This method is safe to call from any thread.

Returns:

True if the looper is currently polling for events.

addOnFileDescriptorEventListener

public void addOnFileDescriptorEventListener(FileDescriptor fd,
                                             int events,
                                             MessageQueue.OnFileDescriptorEventListener listener)

Adds a file descriptor listener to receive notification when file descriptor related events occur.

If the file descriptor has already been registered, the specified events and listener will replace any that were previously associated with it. It is not possible to set more than one listener per file descriptor.

It is important to always unregister the listener when the file descriptor is no longer of use.

Parameters:

fd - The file descriptor for which a listener will be registered.

events - The set of events to receive: a combination of the MessageQueue.OnFileDescriptorEventListener.EVENT_INPUT, MessageQueue.OnFileDescriptorEventListener.EVENT_OUTPUT, and MessageQueue.OnFileDescriptorEventListener.EVENT_ERROR event masks. If the requested set of events is zero, then the listener is unregistered.

listener - The listener to invoke when file descriptor events occur.

See Also:

MessageQueue.OnFileDescriptorEventListener, removeOnFileDescriptorEventListener(java.io.FileDescriptor)

removeOnFileDescriptorEventListener

public void removeOnFileDescriptorEventListener(FileDescriptor fd)

Removes a file descriptor listener.

This method does nothing if no listener has been registered for the specified file descriptor.

Parameters:

fd - The file descriptor whose listener will be unregistered.

See Also:

MessageQueue.OnFileDescriptorEventListener, addOnFileDescriptorEventListener(java.io.FileDescriptor, int, android.os.MessageQueue.OnFileDescriptorEventListener)

postSyncBarrier

public int postSyncBarrier()

Posts a synchronization barrier to the Looper's message queue. Message processing occurs as usual until the message queue encounters the synchronization barrier that has been posted. When the barrier is encountered, later synchronous messages in the queue are stalled (prevented from being executed) until the barrier is released by calling removeSyncBarrier(int) and specifying the token that identifies the synchronization barrier. This method is used to immediately postpone execution of all subsequently posted synchronous messages until a condition is met that releases the barrier. Asynchronous messages (see Message.isAsynchronous() are exempt from the barrier and continue to be processed as usual. This call must be always matched by a call to removeSyncBarrier(int) with the same token to ensure that the message queue resumes normal operation. Otherwise the application will probably hang!

Returns:

A token that uniquely identifies the barrier. This token must be passed to removeSyncBarrier(int) to release the barrier.

removeSyncBarrier

public void removeSyncBarrier(int token)

Removes a synchronization barrier.

Parameters:

token - The synchronization barrier token that was returned by postSyncBarrier().

Throws:

IllegalStateException - if the barrier was not found.