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; }
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
【附】相关架构及资料
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