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looper
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));
}
//我们看到在new Looper(quitAllowed)中，创建了一个消息队列MessageQueen
private 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,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);
        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,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。

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