Android APIs
public final class

Looper

extends Object
java.lang.Object
   ↳ android.os.Looper

Class Overview

Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.

Most interaction with a message loop is through the Handler class.

This is a typical example of the implementation of a Looper thread, using the separation of prepare() and loop() to create an initial Handler to communicate with the Looper.

  class LooperThread extends Thread {
      public Handler mHandler;

      public void run() {
          Looper.prepare();

          mHandler = new Handler() {
              public void handleMessage(Message msg) {
                  // process incoming messages here
              }
          };

          Looper.loop();
      }
  }

Summary

Public Methods
void dump(Printer pw, String prefix)
Dumps the state of the looper for debugging purposes.
static Looper getMainLooper()
Returns the application's main looper, which lives in the main thread of the application.
MessageQueue getQueue()
Gets this looper's message queue.
Thread getThread()
Gets the Thread associated with this Looper.
boolean isCurrentThread()
Returns true if the current thread is this looper's thread.
static void loop()
Run the message queue in this thread.
static Looper myLooper()
Return the Looper object associated with the current thread.
static MessageQueue myQueue()
Return the MessageQueue object associated with the current thread.
static void prepare()
Initialize the current thread as a looper.
static void prepareMainLooper()
Initialize the current thread as a looper, marking it as an application's main looper.
void quit()
Quits the looper.
void quitSafely()
Quits the looper safely.
void setMessageLogging(Printer printer)
Control logging of messages as they are processed by this Looper.
String toString()
Returns a string containing a concise, human-readable description of this object.
[Expand]
Inherited Methods
From class java.lang.Object

Public Methods

public void dump (Printer pw, String prefix)

Added in API level 1

Dumps the state of the looper for debugging purposes.

Parameters
pw Printer: A printer to receive the contents of the dump.
prefix String: A prefix to prepend to each line which is printed.

public static Looper getMainLooper ()

Added in API level 1

Returns the application's main looper, which lives in the main thread of the application.

Returns
Looper

public MessageQueue getQueue ()

Added in API level 23

Gets this looper's message queue.

Returns
MessageQueue The looper's message queue.

public Thread getThread ()

Added in API level 3

Gets the Thread associated with this Looper.

Returns
Thread The looper's thread.

public boolean isCurrentThread ()

Added in API level 23

Returns true if the current thread is this looper's thread.

Returns
boolean

public static void loop ()

Added in API level 1

Run the message queue in this thread. Be sure to call quit() to end the loop.

public static Looper myLooper ()

Added in API level 1

Return the Looper object associated with the current thread. Returns null if the calling thread is not associated with a Looper.

Returns
Looper

public static MessageQueue myQueue ()

Added in API level 1

Return the MessageQueue object associated with the current thread. This must be called from a thread running a Looper, or a NullPointerException will be thrown.

Returns
MessageQueue

public static void prepare ()

Added in API level 1

Initialize the current thread as a looper. This gives you a chance to create handlers that then reference this looper, before actually starting the loop. Be sure to call loop() after calling this method, and end it by calling quit().

public static void prepareMainLooper ()

Added in API level 1

Initialize the current thread as a looper, marking it as an application's main looper. The main looper for your application is created by the Android environment, so you should never need to call this function yourself. See also: prepare()

public void quit ()

Added in API level 1

Quits the looper.

Causes the loop() method to terminate without processing any more messages in the message queue.

Any attempt to post messages to the queue after the looper is asked to quit will fail. For example, the sendMessage(Message) method will return false.

Using this method may be unsafe because some messages may not be delivered before the looper terminates. Consider using quitSafely() instead to ensure that all pending work is completed in an orderly manner.

See also:

public void quitSafely ()

Added in API level 18

Quits the looper safely.

Causes the loop() method to terminate as soon as all remaining messages in the message queue that are already due to be delivered have been handled. However pending delayed messages with due times in the future will not be delivered before the loop terminates.

Any attempt to post messages to the queue after the looper is asked to quit will fail. For example, the sendMessage(Message) method will return false.

public void setMessageLogging (Printer printer)

Added in API level 1

Control logging of messages as they are processed by this Looper. If enabled, a log message will be written to printer at the beginning and ending of each message dispatch, identifying the target Handler and message contents.

Parameters
printer Printer: A Printer object that will receive log messages, or null to disable message logging.

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
String a printable representation of this object.