android 子线程和UI线程的交互主要使用Handler的方法进行通信。本文分析Handler机制

Handler 如何使用？

Handler的使用比较简单

    public class MainActivity extends Activity{private Handler handler = new Handler() {  public void handleMessage(Message msg) {   switch (msg.what) {   case 0x01://do somethings}}};@Override  protected void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.activity_main);  handler = new Handler();new Thread(new Runnable(){@Overridepublic void run(){Message message = new Message();   message.what = 0x01;   handler.sendMessage(message);}}).start();}}复制代码

如代码就是一个简单的Handler的使用Demo,有如下几个问题

1. Handler 是否可以在子线程中初始化。可以，但是如下代码执行的话会抛出该错误"Can't create handler inside thread that has not called Looper.prepare() ".说
   是不能再没有调用Looper.prepare()的线程中创建Handler。因此如果需要在线程中创建Handler首先调用一下Looper.prepare

    new Thread(new Runnable() {  @Override  public void run() {  Handler handler = new Handler();  }  }).start();复制代码

这样调用将不会抛出异常。

    new Thread(new Runnable() {  @Override  public void run() {Looper.prepare();Handler handler = new Handler();  Looper.loop();}  }).start();复制代码

Looper和Handler的联系是什么样的呢？

我们看一下Handler初始化的代码

    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;}复制代码

可以看到如果mLooper是通过Looper.myLooper获得一个Looper对象，如果Looper对象为空，则抛出上述异常。那Looper.myLooper是如何定义的呢？

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

该方法很简单就是从sThreadLocal对象中获得Looper对象。如果sThreadLocal中存在就返回Looper，如果没有就返回null。那Looper是如何存放在sThreadLocal中，
不错就是Looper.prepare。

      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));}复制代码

可以看到，首先判断sThreadLocal中是否存在Looper对象，如果已经存在，那么如果还有prepare Looper则抛出异常；否则就新建一个Looper存放到sThreadLocal中。
该代码同时说明每个线程最多一个Looper对象。

Looper采用ThreadLocal来维护各个线程的Looper对象。ThreadLocal是什么呢？官方定义是：ThreadLocal实现了线程本地存储。所有线程共享同一个ThreadLocal对象，但不同线程仅能访问与其线程相关联的值，一个线程修改ThreadLocal对象对其他线程没有影响。
我们可以将ThreadLocal理解为一块存储区，将这一大块存储区分割为多块小的存储区，每一个线程拥有一块属于自己的存储区，那么对自己的存储区操作就不会影响其他线程。对于ThreadLocal，则每一小块存储区中就保存了与特定线程关联的Looper。

主线程中使用Handler时为什么没有执行Looper.prepare()也可以使用Handler呢？其实在进程启动的时候我们已经创建了主线程也依赖的Looper，代码在ActivityThread中main方法中。

     public static void main(String[] args) {....Looper.prepareMainLooper();ActivityThread thread = new ActivityThread();thread.attach(false);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}复制代码

可以看到该方法执行的是Looper.prepareMainLooper方法,可看到归根到底还是执行prepare方法

    public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}复制代码

Message又是如何引入到Handler机制的？

众所周知，我们都知道Handler，Message，Looper是Handler机制不可或缺的要素。那么Message都是如何引入到Handler.我们看一下上述例子Message是通过handler.sendMessage(message)引入到Handler中。

      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);}return queue.enqueueMessage(msg, uptimeMillis);}复制代码

可以看到sendMessage最终调用MessageQueue中enqueueMessage

    boolean enqueueMessage(Message msg, long when) {....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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}复制代码

MessageQueue并没有使用一个集合把信息保存，它只是通过使用mMessage对象表示当前需要处理消息，然后根据时间把msg进行排序。具体方法是根据时间顺序调用msg.next。从而为每一个消息指定它
的下一个消息是什么。如果需要将msg作为队头插入到MessageQueue中可以调用sendMessageAtFrontOfQueue实现。

这样消息就进入到MessageQueue中，那如何从MessageQueue中将消息取出来呢？
大家有没有注意到Loop.prepare一般和Looper.loop对应使用。其实Looper.loop就是用来从MessageQueue中取出message。

    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 traceTag = me.mTraceTag;if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}try {msg.target.dispatchMessage(msg);} finally {if (traceTag != 0) {Trace.traceEnd(traceTag);}}....}}复制代码

上文我们知道每个Thread都有一个Looper，其实每个Looper都对应一个MessageQueue。loop方法我们获得对应looper中的MessageQueue不断取出msg，并传入到dispatchMessage.
dispatchMessage方法将取出的msg传递到定义Handler时重写的handleMessage方法。

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

Handler,Message,MessageQueue,Looper流程示意图如下：