Android handler机制

Handler机制是线程间通信的实现；

Handler类持有MessageQueue 引用，Looper引用。主要是用来处理Message的发送，入列和出列；

持有Looper引用的目的是获取MessageQueue引用，Looper通过ThreadLocal才属于它的线程（主线程或子线程），由于主线程之前就持有自己的Looper对象，所以实例化Handler的时候不用调用Looper.prepare()方法进行Looper的设定，直接在主线程中实例化Handler对象就可以发送消息；

Looper类持有一个MessageQueue引用，用来对queue中的Message进行循环处理；

如果Handler是在子线程中实例化的，那么它持有的Looper就是属于该子线程的，在子线程中进行消息的循环需要先设定Looper：

public void run(){
Looper.prepare();//会实例化一个looper对象，并属于当前线程
handler = new Handler(){
public void handleMessage(Message msg){}
}
Looper.loop();//获取当前线程的looper进行循环
}

Handler在哪个线程中实例化，那么它就处理该线程中的消息；可是如果多个线程同时通过这个handler发送消息呢，由于Looper是线程隔离的，但是messageQueue却要暴露给其他的线程，这样才能允许其他的线程发送入列消息，所以MessagQueue入列时需要同步化：

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;
}
<span style="white-space:pre">	</span>    msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;

<span style="white-space:pre">	</span>   //如果链表没有节点，或时间为0，或小于头节点，则
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;

<span style="white-space:pre">		</span>    //如果到了链表的最后或时间小于某一节点，则插入到该节点之前
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}

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

Message类持有一个链表，它会缓存用过的Messages，loop循环就是不断的从链表头中取出消息，如果取出的是null则中断loop：

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;
}

<span style="white-space:pre">		</span>// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}