Android的消息机制

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;
final Thread mThread;

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

 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));
}

public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
}

/**
    * 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();
       }
   }

public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
 }

public static @NonNull MessageQueue myQueue() {
        return myLooper().mQueue;
}

public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
}

/**
     * 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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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.recycleUnchecked();
        }
 }

public void quit() {
      mQueue.quit(false);
  }

public void quitSafely() {
        mQueue.quit(true);
}

final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;

public interface Callback {
        public boolean handleMessage(Message msg);
}

public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

public Handler() {
       this(null, false);
}

public Handler(Callback callback) {
        this(callback, false);
}

public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
}

public Handler(Looper looper) {
        this(looper, null, false);
}

public Handler(Looper looper, Callback callback) {
        this(looper, callback, false);
}

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

 public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

 public final boolean sendMessageDelayed(Message msg, long delayMillis)
 {
    if (delayMillis < 0) {
            delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
}

public final boolean sendMessageAtFrontOfQueue(Message msg) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, 0);
}

public final boolean post(Runnable r)
{
     return  sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
}

public final boolean postAtTime(Runnable r, long uptimeMillis)
{
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

 public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
 {
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

private static Message getPostMessage(Runnable r, Object token) {
        Message m = Message.obtain();
        m.obj = token;
        m.callback = r;
        return m;
}

 public final boolean postDelayed(Runnable r, long delayMillis)
{
        return sendMessageDelayed(getPostMessage(r), delayMillis);
}

 public final boolean postAtFrontOfQueue(Runnable r)
{
        return sendMessageAtFrontOfQueue(getPostMessage(r));
}