Added in API level 1

Looper


public final class Looper
extends Object

java.lang.Object
   ↳ android.os.Looper


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(Looper.myLooper()) {
              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()

This method was deprecated in API level 30. The main looper for your application is created by the Android environment, so you should never need to call this function yourself.

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 representation of the object.

Inherited methods

Public methods

dump

Added in API level 1
public void dump (Printer pw, 
                String prefix)

Dumps the state of the looper for debugging purposes.

Parameters
pw Printer: A printer to receive the contents of the dump. This value cannot be null.

prefix String: A prefix to prepend to each line which is printed. This value cannot be null.

getMainLooper

Added in API level 1
public static Looper getMainLooper ()

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

Returns
Looper

getQueue

Added in API level 23
public MessageQueue getQueue ()

Gets this looper's message queue.

Returns
MessageQueue The looper's message queue. This value cannot be null.

getThread

Added in API level 3
public Thread getThread ()

Gets the Thread associated with this Looper.

Returns
Thread The looper's thread. This value cannot be null.

isCurrentThread

Added in API level 23
public boolean isCurrentThread ()

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

Returns
boolean

loop

Added in API level 1
public static void loop ()

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

myLooper

Added in API level 1
public static Looper myLooper ()

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

Returns
Looper

myQueue

Added in API level 1
public static MessageQueue myQueue ()

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 This value cannot be null.

prepare

Added in API level 1
public static void prepare ()

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().

prepareMainLooper

Added in API level 1
Deprecated in API level 30
public static void prepareMainLooper ()

This method was deprecated in API level 30.
The main looper for your application is created by the Android environment, so you should never need to call this function yourself.

Initialize the current thread as a looper, marking it as an application's main looper. See also: prepare()

quit

Added in API level 1
public void quit ()

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 Handler#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:

quitSafely

Added in API level 18
public void quitSafely ()

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 Handler#sendMessage(Message) method will return false.

setMessageLogging

Added in API level 1
public void setMessageLogging (Printer printer)

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.

toString

Added in API level 1
public String toString ()

Returns a string representation of the object.

Returns
String a string representation of the object.