Android Handler机制

Handler想必Android开发的程序猿们都用到过，面试的过程也经常会被问到，可有些人产生疑问：都是Google封装好的，开发中按照API规则使用也没见出现问题，干嘛要刨根问底的去研究呢？

是啊Google封装了那么多的机制，干嘛要问Handler机制呢？也许因为Android开发注重UI的刷新效率，毕竟直接展示用户给数据并与用户交互的，当然要熟知一二了。

Handler的一般用法：开启子线程完成网络、文件保存，查询数据库数据等等一系列的耗时任务，任务完成后通知主线程（main线程）进行UI刷新。

问题一：那么多子线程都通知主线程，想必主线程要按照通知的时间顺利一个个的处理Message？

解决这个问题：这里就用到消息队列--MessageQueue。

问题二：主线程是如何一个个处理消息队类中的消息的呢？

解决这个问题：使用循环机制--Looer。

问题三：主线程开启一个循环机制为什么不会导致阻塞现象呢？

解决这个问题：让主线程等待--wait()。

问题四：有新的消息发送到主线程，如何触发刷新呢？

解决这个问题：唤醒主线程--wake()。

一步步的通过代码深入研究：

首先我们要清楚一点：程序启动时系统已经为主线程开启了循环机制--Looper，监听子线程发送过来的消息。

public final class ActivityThread {
public static void main(String[] args) {

...
Looper.prepareMainLooper();

...
Looper.loop();

...
}
}

Looper.java

/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the 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 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
final 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.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}

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();
}
}

从loop()函数中看到一个没有终止条件的for循环--死循环。what？千万别怀疑Google工程师们的水平，确实是一个死循环你没看错，可是为什么没有阻塞主线程呢，原因在这行代码：Message msg = queue.next(); // might block（主体部分）

注释已经很清晰明白的告诉你这句是主要代码，下文会讲解。这里先说明一点，如果消息队列中没有消息时，这里是不会返回null Message对象的，如果真是返回了null，那就意味着线程退出，如果是主线程--程序被kill掉了，既然不会返回null，那就很好理解了，问题三也说过了那就是在这里发生了线程等待--wait()，何时被唤醒，下文会讲到。

注意这行代码：msg.target.dispatchMessage(msg);//该方法中调用了Handler的handleMessage(Message msg)函数。

Handler.java

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
/**
*我们就是重写的这个函数刷新UI的
*Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

通过Handler发送Message:

Handler.java:

boolean sendMessage(Message msg) {

return sendMessageDelayed(msg, 0);

}

boolean sendMessageDelayed(Message msg, long delayMillis) {

...

return sendMessageAtTime(msg, SystemClock.uptimeMillis+delayMillis);

}

public sendMessageAtTime(Message msg, long uptimeMillis) {

...

return enqueueMessage(queue, msg, uptimeMillis);

}

boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {

return queue.enqueueMessage(msg, uptimeMillis);

}

通过一系列的调用，最终调用的是MessageQueue的enqueueMessage(Message msg, long when)函数。

MessageQueue.java

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.next
prev.next = msg;
}

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

注意这行代码：nativeWake(mPtr)//调用本地方法，目的唤醒线程。

MessageQueue类中重要的方法next()

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 handler

boolean 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;
}
}

注意这行代码：nativePollOnce(ptr, nextPollTimeoutMillis);//条用本地函数，让线程等待--wait()