handler机制的源码分析

looper.prepare();

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));

}

ThreadLocal类似Map,调用sThreadLocal.set(Object obj)方法,以当前线程对象为键，obj为值存入sThreadLocal中。而这里是new了一个Looper对象作为值存入。   

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

从上面的构造函数可以看出，生成Looper对象的同时，又生成了一个与之对应的mQueue,得到了当前线程mThread。

sThreadLocal是静态变量，所以无论有多少个线程对象和looper，都能存入到同一个sThreadLocal中。一个键对应一个值，所以一个线程对应唯一的一个looper。

sThreadLocal.get() ，以当前线程为键，取出之前存入的对应的looper。

handler = new Handler();

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

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

执行myLooper() 方法实际上就是返回了之前在looper.prepare()中存入sThreadLocal的looper，这里把这个looper赋值给了Handler的成员变量mLooper。

上面在存入looper的时候是new了一个Looper，并且还生成了一个与之对应的mQueue。mQueue = mLooper.mQueue这句代码就是把mQueue赋值给Handler的成员变量mQueue。

至此，mLooper mQueue handler mThread就联系起来了。

Looper.loop();  静态方法

    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.recycleUnchecked();
        }
    }

final Looper me = myLooper();获取到当前线程对应的Looper对象

final MessageQueue queue = me.mQueue;获取到与Looper对象对应的MessageQueue对象

for(;;)相当于while(sure)，在遇到break之前是死循环。代码块的作用是循环从消息队列取消息。

Message msg = queue.next(); next()方法返回值类型是Message，所以这里的意思是从消息队列中取出数据。如果没有消息，就会阻塞，直到又有消息。

msg.target.dispatchMessage(msg);

Message msg = handler.obtainMessage();

  public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

 public static Message obtain(Handler h) {
        Message m = obtain();
        m.target = h;

return m;
    }

 public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

实际上，m.target = handler     msg.target.dispatchMessage(msg)相当于handler.dispatchMessage(msg)

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }