了解Handler,Looper, MessageQueue，Message的工作流程

Handler的作用

异步通信，消息传递

Handler的基本用法

Handler的用法，示例1、（子线程向主线程发送消息）

public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {Toast.makeText(HandlerActivity.this, "接收到了消息", Toast.LENGTH_SHORT).show();}
};
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_handler);}public void onClickView(View v) {switch (v.getId()) {case R.id.button: {Thread thread = new Thread() {@Overridepublic void run() {Message msg = Message.obtain();handler.sendMessage(msg);}};thread.start();break;}}
}
}

这个界面只有一个Button, 点击事件是：启动一个线程，在这个线程里使用handler，发送一个消息；这个消息不带任何数据。
Message: 这个可以使用new来生成，但是最好还是使用Message.obtain()来获取一个实例。这样便于消息池的管理。
handler初始化的时候，我们复写了handleMessage方法，这个方法用来接收，子线程中发过来的消息。

示例2、主线程向子线程发送消息

public class HandlerActivity extends AppCompatActivity {
Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {Log.d("child", "----" + Thread.currentThread().getName());}
};
Handler childHandler = null;@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_handler);Thread child = new Thread("child thread") {@Overridepublic void run() {Looper.prepare();childHandler = new Handler(Looper.myLooper()) {@Overridepublic void handleMessage(Message msg) {Log.d("child", "----" + Thread.currentThread().getName() + ",  " + msg.obj);}};Looper.loop();}};child.start();
}public void onClickView(View v) {switch (v.getId()) {case R.id.button: {Thread thread = new Thread() {@Overridepublic void run() {Message msg = Message.obtain();handler.sendMessage(msg);}};thread.start();break;}case R.id.button1: {if (childHandler != null) {Message msg = Message.obtain(childHandler);msg.obj = "123---" + SystemClock.uptimeMillis();childHandler.sendMessage(msg);} else {Log.d("-----", "onClickView: child Handler is null");}break;}}
}
}

主线程向子线程发送消息时，我们需要使用的是handler,但是这个handler是需要在子线程中实例化，否则子线程无法接收到消息。
在child thread中准备的工作：
* 1. Looper.prepare();
* 2. 实例化childHandler的时候出入了一个Looper实例。
* 3. Looper.loop()调用
假如我们直接在子线程中直接实例化一个handler，而不传入Looper实例。程序会直接抛出异常：
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
这个异常是在hanlder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;
}

出现的，从这里可知，在线程中实例化一个handler需要一个Looper实例。

如果其他线程的实例出来的Looper会怎么样？

首先看Looper源码中是怎么写的。Looper的构造方法是私有的，这样我们只能通过Looper的静态方法来实例化一个Looper.在Looper类中还有一个ThreadLocal sThreadLocal 变量，而在上面的代码中使用了Looper.myLooper()方法来给handler一个Looper实例。

* Looper.myLooper()

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
这个方法返回了一个存放在ThreadLocal中的Looper实例。

为什么在调用Looper.myLooper()方法之前需要先调用 Looper.prepare()呢？

Looper.prepare源码：

public static void prepare() {prepare(true);
}
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));
}

方法很简单，就是检查sThreadLocal里面是否有有值，然后将Looper的实例存放到ThreadLocal，如果不为空，直接就是 RuntimeException 异常。这也保证了一个线程中只能有一个Looper实例。因此，Looper.prepare方法只能调用一次。

那么ThreadLocal的作用是什么呢？

在这个程序中，可以大概理解成线程间数据的隔离。意思就是我存放在ThreadLocal中的数据，只能在我本线程中可以获得到值，在其他线程中获取不到（其他线程中获取的是null）。
这样就能得出，其他线程中的Looper，在本线程中通过Looper.myLooper()获取不到数据。

那么为什么在主线程中不需要传入Looper实例呢？

通过查找Android源码，可以知道在ActivityThread中main方法里面，UI线程已经初始化了Looper.prepareMainLooper();这样就在UI线程中有Looper实例了。当然在main方法的下面，也调用了Looper.loop()方法。

Looper.loop()方法是做什么的

   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 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 {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();**}**
}

这个方法里面，我们需要注意到：
1. Looper me这个不允许为空
2. for (;;)循环
3. Message msg = queue.next()
4. msg.target.dispatchMessage(msg);

1. 在looper.loop方法里面，检查了本线程中的Looper实例，也对应了在调用looper.loop方法之前必须先调用looper.prepare方法。

2. for循环是一个死循环，这个需要一直取出MessageQueue队列中的数据，也就是刚才所列的第三个 queue.next方法。这个方法会阻塞，直到有消息从消息队列中取出来。
   next方法源码：

   Message next() {
   // Return here if the message loop has already quit and been disposed.
   // This can happen if the application tries to restart a looper after quit
   // which is not supported.
   final long ptr = mPtr;
   if (ptr == 0) {return null;
   }
   int pendingIdleHandlerCount = -1; // -1 only during first iteration
   int nextPollTimeoutMillis = 0;
   for (;;) {if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}nativePollOnce(ptr, nextPollTimeoutMillis);synchronized (this) {// Try to retrieve the next message.  Return if found.final long now = SystemClock.uptimeMillis();Message prevMsg = null;Message msg = mMessages;if (msg != null && msg.target == null) {// Stalled by a barrier.  Find the next asynchronous message in the queue.do {prevMsg = msg;msg = msg.next;} while (msg != null && !msg.isAsynchronous());}if (msg != null) {if (now < msg.when) {// Next message is not ready.  Set a timeout to wake up when it is ready.nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);} else {// Got a message.mBlocked = false;if (prevMsg != null) {prevMsg.next = msg.next;} else {mMessages = msg.next;}msg.next = null;if (DEBUG) Log.v(TAG, "Returning message: " + msg);msg.markInUse();return msg;}} else {// No more messages.nextPollTimeoutMillis = -1;}// Process the quit message now that all pending messages have been handled.if (mQuitting) {dispose();return null;}// If first time idle, then get the number of idlers to run.// Idle handles only run if the queue is empty or if the first message// in the queue (possibly a barrier) is due to be handled in the future.if (pendingIdleHandlerCount < 0&& (mMessages == null || now < mMessages.when)) {pendingIdleHandlerCount = mIdleHandlers.size();}if (pendingIdleHandlerCount <= 0) {// No idle handlers to run.  Loop and wait some more.mBlocked = true;continue;}if (mPendingIdleHandlers == null) {mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];}mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);}// Run the idle handlers.// We only ever reach this code block during the first iteration.for (int i = 0; i < pendingIdleHandlerCount; i++) {final IdleHandler idler = mPendingIdleHandlers[i];mPendingIdleHandlers[i] = null; // release the reference to the handlerboolean keep = false;try {keep = idler.queueIdle();} catch (Throwable t) {Log.wtf(TAG, "IdleHandler threw exception", t);}if (!keep) {synchronized (this) {mIdleHandlers.remove(idler);}}}// Reset the idle handler count to 0 so we do not run them again.pendingIdleHandlerCount = 0;// While calling an idle handler, a new message could have been delivered// so go back and look again for a pending message without waiting.nextPollTimeoutMillis = 0;
   }
   

   }

3. queue.next方法，返回一个Message，而这个message会被传入到msg.target的dispatchMessage方法中。msg.target是一个handler，就是发送消息的实例。
   dispatchMessage(Message)源码：
   public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

   Handler初始化的时候，我们并没有传入msg.callback和mCallback这两个回调。所以这个方法最终执行的是handleMessage方法。
   上面我们分析了ThreadLocal,Looper,Message,MessageQueue,下面开始分析handler发送消息的方式。

Handler发送消息

Handler 通过sendMessage方法发送消息。这个方法最终调用的是

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

参数：MessageQueue queue则是Handler中mQueue变量，这个变量在Handler(Callback callback, boolean async)初始化完成。它是Looper中一个final MessageQueue的变量，在初始化Looper的时候，就开始初始化MessageQueue。这也是一个线程中只有一个MessageQueue的原因.
Message msg 是封装的消息。
long uptimeMillis 延迟发送时间。
之前分析looper.loop方法的时候，说了msg.target.dispatchMessage, 这个target就是在这个方法里面赋值的。
从这个方法里面，可以知道handler的sendMessage，只是把消息(Message实例)添加到了queue队列中。

    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.getMessage(), 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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;
}

Handler, Looper, Message, MessageQueue之间的关系

通过handler发送Message，将Message压入MessageQueue队列中；而Looper.loop方法又在不停的循环这个消息队列，取出压入MessageQueue的Message， 然后dispatchMessage分发，最终会调用handler.handleMessage方法来处理发送过来的Message.
到此一个消息发送并处理的流程结束。
博客里面有写的不对的地方，欢迎大家指出，一起讨论技术。