android异步消息机制，源码层面彻底解析（一）

Handler、Message、Loopler、MessageQueen

首先看一下我们平常使用Handler的一个最常见用法。

Handler handler =new Handler(){        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);            //这里进行一些UI操作等处理        }         new Thread(new Runnable() {            @Override            public void run() {                Message message = Message.obtain();                ........                handler.sendMessage(message);            }        });    };

看一下handler的构造函数的源码

public Handler() {   this(null, false);}//他会调用本类中的如下构造函数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;    }

看到当mLooper == null时会抛一个“Can’t create handler inside thread that has not called Looper.prepare()”这个异常，所以我们在创建handler实例前首先需要调用Looper.prepare()

public static void prepare() {        prepare(true);}//将looper保存到ThreadLocal中，这里可以把ThreadLocal理解为一个以当前线程为键的Map，所以一个线程中只会有一个looperprivate 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));}//我们看到在new Looper(quitAllowed)中，创建了一个消息队列MessageQueenprivate Looper(boolean quitAllowed) {    mQueue = new MessageQueue(quitAllowed);    mThread = Thread.currentThread();}

接下来我们看handler.sendMessage(message)这个方法，从字面意思就是将信息发送出去。一般sendMessage累的方法最终都会调用sendMessageAtTime(Message msg, long uptimeMillis)这个方法

    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);    }

我们看到最终会执行enqueueMessage(queue, msg, uptimeMillis)这个方法

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

最终又会调用MessageQueen中的queue.enqueueMessage(msg, uptimeMillis)这个方法，这里的queue就是looper构造方法中创建的那个消息队列

//MessageQueen的enqueueMessage方法    boolean enqueueMessage(Message msg, long when) {        if (msg.target == null) {            throw new IllegalArgumentException("Message must have a target.");        }        if (msg.isInUse()) {            throw new IllegalStateException(msg + " This message is already in use.");        }        synchronized (this) {            if (mQuitting) {                IllegalStateException e = new IllegalStateException(                        msg.target + " sending message to a Handler on a dead thread");                Log.w(TAG, e.getMessage(), e);                msg.recycle();                return false;            }            msg.markInUse();            msg.when = when;            Message p = mMessages;            boolean needWake;            if (p == null || when == 0 || when < p.when) {                // New head, wake up the event queue if blocked.                msg.next = p;                mMessages = msg;                needWake = mBlocked;            } else {                // Inserted within the middle of the queue.  Usually we don't have to wake                // up the event queue unless there is a barrier at the head of the queue                // and the message is the earliest asynchronous message in the queue.                needWake = mBlocked && p.target == null && msg.isAsynchronous();                Message prev;                for (;;) {                    prev = p;                    p = p.next;                    if (p == null || when < p.when) {                        break;                    }                    if (needWake && p.isAsynchronous()) {                        needWake = false;                    }                }                msg.next = p; // invariant: p == prev.next                prev.next = msg;            }            // We can assume mPtr != 0 because mQuitting is false.            if (needWake) {                nativeWake(mPtr);            }        }        return true;    }

MessageQueen虽然名字是一个队列，但实质上他是一个单向链表，这个结构能快速进行插入和删除操作。从上面源码可以看出来，主要是按照发送消息的时间顺序将msg插入到消息队列中。接下来我们就需要从消息队列中取出msg了。这时候就需要调用Looper.loop()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 (;;) {            //不断从消息队列中取出msg            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);            }            //将msg交由handler处理            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();        }    }

可以看到Looper.loop()方法通过在一个死循环中调用Message msg = queue.next()将消息不断的从消息队列中取出来。queue.next()方法的作用就是从消息队列中取msg，唯一跳出循环的方式是MessageQueen的next方法返回了null。现在msg已经取出来，下一步就是怎样将他传递给handler了对吧。所以在死循环中还有一个方法msg.target.dispatchMessage(msg) ，而msg.target就是handler，在上面handler的enqueueMessage()方法中传入的msg.target = this，this就是handler本身，接下来就看看handler的dispatchMessage()方法

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

如果我们采用无参的构造函数创建handler，msg.callback与mCallback均为空，所以我们会调用handleMessage(msg)，这样文章开头的那个实例整个流程就走完了，handleMessage(msg)会在handler实例所在的线程中执行。

//当我们通过这种方式创建handler时，dispatchMessage中的mCallback就不为null public Handler(Callback callback) {        this(callback, false); }//Callback是一个接口，里面正好也有我们需要的handleMessage(Message msg)，dispatchMessage中的 if (mCallback != null) 语句内的内容，就是我们需要重写的handleMessage(Message msg)方法 public interface Callback {     public boolean handleMessage(Message msg); }

//当我们调用handler.post()方法执行异步任务时    public final boolean post(Runnable r)    {       return  sendMessageDelayed(getPostMessage(r), 0);    }//getPostMessage(r)这个方法中我们看到给m.callback赋值了，就是我们传入的runnable接口    private static Message getPostMessage(Runnable r) {        Message m = Message.obtain();        m.callback = r;        return m;    }//最后在handleCallback方法中我们执行了它的run方法,这也就解释了为什么在子线程中可以用handler.post(Runnable r)更新UI    private static void handleCallback(Message message) {        message.callback.run();    }

总结

• 梳理整个执行过程
  1.调用Looper.prepare()方法，这是创建handler所必须的。在主线程中由于ActivityThread已经通过Looper.prepareMainLooper（）方法创建过looper，所以在主线程中创建handler以前无需创建looper，并通过Looper.loop()来开启主线程的消息循环。
  2.通过调用handler.sendMessage(message)方法最终会执行enqueueMessage(queue, msg, uptimeMillis)，enqueueMessage又会调用MessageQueen的queue.enqueueMessage(msg, uptimeMillis)，这样消息就会被添加到消息队列中。
  3.调用Looper.loop()方法在死循环中执行Message msg = queue.next()，不断的将msg从消息队列中取出来，同时执行msg.target.dispatchMessage(msg)，将消息传递给handler，由handler来处理，如我们调用的handleMessage就是处理消息的方式之一。
• 异步处理机制流程图
  

从子线程进行UI 操作的几种方式

Android 提供了几种途径来从其他线程访问 UI 线程。以下列出了几种有用的方法：
• Activity.runOnUiThread(Runnable)
• View.post(Runnable) 这里的view就是我们需要改变的ui控件
• View.postDelayed(Runnable, long)
• Handler.post(Runnable, long)

但是，随着操作日趋复杂，这类代码也会变得复杂且难以维护。 要通过工作线程处理更复杂的交互，可以考虑在工作线程中使用 Handler 处理来自 UI 线程的消息。当然，最好的解决方案或许是扩展 AsyncTask 类，此类简化了与 UI 进行交互所需执行的工作线程任务。