Android Looper.java (안드로이드 루퍼 소스)

Programing/Android / Java

Android Looper.java (안드로이드 루퍼 소스)

빅터크룸 2014. 4. 15. 17:58

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.util.Log;
import android.util.Printer;
import android.util.PrefixPrinter;

/**
 * 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
 * {@link #prepare} in the thread that is to run the loop, and then
 * {@link #loop} to have it process messages until the loop is stopped.
 * 
 * Most interaction with a message loop is through the
 * {@link Handler} class.
 * 
 * This is a typical example of the implementation of a Looper thread,
 * using the separation of {@link #prepare} and {@link #loop} to create an
 * initial Handler to communicate with the Looper.
 *
 * <pre>
 * 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();
 * }
 * }</pre>
 */
public final class Looper {
 private static final String TAG = "Looper";

// sThreadLocal.get() will return null unless you've called prepare().
 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
 private static Looper sMainLooper; // guarded by Looper.class

final MessageQueue mQueue;
    final Thread mThread;

private Printer mLogging;

/** 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
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }

private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

/**
     * 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: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

/** Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

// Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

// This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

msg.target.dispatchMessage(msg);

if (logging != null) {
 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
 }

// Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

msg.recycle();
        }
    }

/**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {
        return sThreadLocal.get();
    }

/**
 * Control logging of messages as they are processed by this Looper. If
 * enabled, a log message will be written to <var>printer</var> 
 * at the beginning and ending of each message dispatch, identifying the
 * target Handler and message contents.
 * 
 * @param printer A Printer object that will receive log messages, or
 * null to disable message logging.
 */
 public void setMessageLogging(Printer printer) {
 mLogging = printer;
 }
 
 /**
 * Return the {@link MessageQueue} object associated with the current
 * thread. This must be called from a thread running a Looper, or a
 * NullPointerException will be thrown.
 */
 public static MessageQueue myQueue() {
 return myLooper().mQueue;
 }

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

/**
     * Returns true if the current thread is this looper's thread.
     * @hide
     */
    public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
    }

/**
 * Quits the looper.
 * 
 * Causes the {@link #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 {@link 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 {@link #quitSafely} instead to ensure
 * that all pending work is completed in an orderly manner.
 * 
 *
 * @see #quitSafely
 */
 public void quit() {
 mQueue.quit(false);
 }

/**
 * Quits the looper safely.
 * 
 * Causes the {@link #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 {@link Handler#sendMessage(Message)} method will return false.
 * 
 */
 public void quitSafely() {
 mQueue.quit(true);
 }

/**
     * 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 {@link #removeSyncBarrier} 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 {@link Message#isAsynchronous} are exempt from the barrier
     * and continue to be processed as usual.
     *
     * This call must be always matched by a call to {@link #removeSyncBarrier} with
     * the same token to ensure that the message queue resumes normal operation.
     * Otherwise the application will probably hang!
     *
     * @return A token that uniquely identifies the barrier.  This token must be
     * passed to {@link #removeSyncBarrier} to release the barrier.
     *
     * @hide
     */
    public int postSyncBarrier() {
        return mQueue.enqueueSyncBarrier(SystemClock.uptimeMillis());
    }

/**
     * Removes a synchronization barrier.
     *
     * @param token The synchronization barrier token that was returned by
     * {@link #postSyncBarrier}.
     *
     * @throws IllegalStateException if the barrier was not found.
     *
     * @hide
     */
    public void removeSyncBarrier(int token) {
        mQueue.removeSyncBarrier(token);
    }

/**
     * Return the Thread associated with this Looper.
     */
    public Thread getThread() {
        return mThread;
    }

/** @hide */
    public MessageQueue getQueue() {
        return mQueue;
    }

/**
     * Return whether this looper's thread is currently idle, waiting for new work
     * to do.  This is intrinsically racy, since its state can change before you get
     * the result back.
     * @hide
     */
    public boolean isIdling() {
        return mQueue.isIdling();
    }

public void dump(Printer pw, String prefix) {
        pw.println(prefix + toString());
        mQueue.dump(pw, prefix + "  ");
    }

public String toString() {
        return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
                + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
    }
}

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