最近复习到了Handler一直只知道怎么用，却没有仔细分析过源码，这下就来看看

为什么要使用handler呢？

因为在安卓应用中ui操作是线程安全的，只能ui线程去操作ui组件，但是在现实开发中，可能有多个线程去并发操作ui，所以将会导致线程不安全，所以就用到了handler

Looper1.主要负责创建message Queue和自身的创建    2.    消息的循环

public static void prepare() {prepare(true);
}private static void prepare(boolean quitAllowed) {
//判断当前的对象是否为空 ，即prepare不能调用两次if (sThreadLocal.get() != null) {throw new RuntimeException("Only one Looper may be created per thread");}创建looper对象放入线程中sThreadLocal.set(new Looper(quitAllowed));
}

private Looper(boolean quitAllowed) {//looper对象创建了messagequeuemQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();
}

2.looper.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 blockif (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 loggerfinal Printer logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;final long traceTag = me.mTraceTag;if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();final long end;try {
//吧msg交给msg.target实际就是绑定的handler去处理这个方法中调用了handlemessage所以在创建handler
的时候要重写handlemessage方法msg.target.dispatchMessage(msg);end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}if (slowDispatchThresholdMs > 0) {final long time = end - start;if (time > slowDispatchThresholdMs) {Slog.w(TAG, "Dispatch took " + time + "ms on "+ Thread.currentThread().getName() + ", h=" +msg.target + " cb=" + msg.callback + " msg=" + msg.what);}}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();}
}

Looper的作用就是创建message队列，然后吧队列中的消息派发给handler，然后handler去处理消息

然后我们在去看下Handler的源码

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;
}

可以看出handler创建的时候就会绑定他的looper并找见他的队列

handler有两种发送消息的方法 send和post

public final boolean sendMessage(Message msg)
{return sendMessageDelayed(msg, 0);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{if (delayMillis < 0) {delayMillis = 0;}return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

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);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;if (mAsynchronous) {msg.setAsynchronous(true);}
//看出来最后调用了队列的方法，也就是handler发出的消息，最后会保存到对应的队列中return queue.enqueueMessage(msg, uptimeMillis);
}

所以就是一个应用的创建，系统会自动给你生成一个主线程，并在这个主线程中生成一个Looper对象来让你和子线程之间进行消息的传递，Looper会创建一个消息队列，当我们在子线程中给主线程发消息的时候就需要用到handler，handler创建的时候就会去找到这个对应的looper。looper不断的执行loop方法去发消息给队列中 ，没当handler去发送一个消息的时候，就吧消息加入到队列中 ，然后队列的loop方法就会接收到这个消息，然后消息去调用dispatchmsg方法，然后dispatchmsg方法就会调用handler中的handlemessage方法，我们就能接收到这个消息