浅谈handler

handler是什么？

handler是Android提供用来更新UI的一套消息机制，也是一套消息处理的机制（发送和处理消息）

handler原理

handler负责消息发送，looper负责接收handler发送过来的消息，并把消息发送给handler，messageQueue存储消息的容器

这里先说明一下ThreadLocal，主要在线程中保存变量信息，主要有两个比较重要的方法，一个是get方法，一个是set方法

public void set(T value) { 
    Thread currentThread = Thread.currentThread(); 
    Values values = values(currentThread); 
    if (values == null) { 
        values = initializeValues(currentThread); 
    } 
    values.put(this, value); 
}

set方法设置当前线程的值，使用键值对的形式存储Thread和looper之间的关系，Thread作为key，looper作为value

public T get() { 
    // Optimized for the fast path. 
    Thread currentThread = Thread.currentThread();
    Values values = values(currentThread); 
    if (values != null) { 
        Object[] table = values.table; 
        int index = hash & values.mask; 
        if (this.reference == table[index]) { 
            return (T) table[index + 1]; 
        } 
    } else { 
        values = initializeValues(currentThread); 
    } 
    return (T) values.getAfterMiss(this); 
}

get方法就是取出当前线程对应的looper，也就是说ThreadLocal是负责thread和looper之间的关系的

下面看一下Looper.prepare()方法

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)); 
}

默认情况下ThreadLocal是没有存储的，所以要创建一个新的looper

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

从looper方法中，创建了一个MessageQueue，在looper中维护着一个消息队列

知道了looper和MessageQueue之后，究竟handler跟这两者有什么关系呢，继续看源码

public Handler(Callback callback, boolean async) { 
    if (FIND_POTENTIAL_LEAKS) { 
        final Class 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; 
}

首先调用Looper.myLooper()

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

获得当前的looper对象，通过looper拿到MessageQueue，就完成了handler和looper之间的关联下面继续看handler的消息发送

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); 
}

先获得当前的消息队列，如果队列为空就抛出异常，不为空，向消息队列中插入消息

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

插入消息之前就指定消息发送给谁（msg.target），默认情况下发送给自己的handler，然后把消息放入队列中，handler就完成了发送message到MessageQueue的过程那么消息又是如何轮询的呢？

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(); 
    } 
}

通过myLooper()方法获取当前looper，进而获得当前的消息队列，然后通过MessageQueue的next方法获取消息，消息为空时返回，不为空时，调用handler的dispatchMessage(msg)方法，然后这个过程一直循环

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

首先查看msg.callback是否为空，不为空时去调用handleCallback(msg)，这个方法在handler的构造方法中存在，可以实现消息的拦截；为空只就调用handleMessage(msg)，这个方法都是大家熟悉的，不在描述，整体的handler的原理就描述到这

总结

handler在Android中扮演的非常重要的角色，熟悉handler的原理，不仅在面试的时候有用，就连activity的生命周期也是通过handler发送消息，详细请看源码