Android:消息机制

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我们都知道在主线程（即UI线程）要发送消息的话，只需要创建一个Handler即可，但你试下在子线程里面创建一个Handler，然后发送消息，你会发现程序报如下异常了，“can‘t create handler inside thread that has not called Looper.prepare();”

new Thread(new Runnable() {
			
			@Override
			public void run() {
				Handler mHandler = new Handler();
				mHandler.sendEmptyMessage(0);
			}
		}).start();

我们来看看Looper的源码，其路径为\frameworks\base\core\java\android\os\Looper.java

/** 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));
    }

我们再来看看Looper的构造函数：

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

接下来看看Handler，我们知道Handler是用来发送消息以及处理消息的，先看看Handler是如何发送消息的，如何把该消息发到该线程对应Looper对象的MessageQueue队列上的?

一般我们都是直接new一个Handler，先看看Handler的源码：其路径为\frameworks\base\core\java\android\os\Handler.java，其构造函数：

public Handler() {
		        this(null, false);
		    }
		 
		 public Handler(Callback callback) {
		        this(callback, false);
		    }
		 
		 public Handler(Looper looper, Callback callback) {
		        this(looper, callback, 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 Looper myLooper() {
        return sThreadLocal.get();
    }

接下来看看如何发送消息的，API提供了很多send或post方法来发消息，不管用的哪个，最后都会调用enqueueMessage方法，看看该方法的具体实现：

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

/**
     * 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.recycleUnchecked();
        }
    }

/**
     * 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);
        }
    }

private Handler mHandler = new Handler()
	{
		public void handleMessage(android.os.Message msg)
		{
			switch (msg.what)
			{
			case value:
				
				break;

			default:
				break;
			}
		};
	};

Android:消息机制

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原文地址：http://blog.csdn.net/hxf779159361/article/details/51329090