Android中的消息机制(MessageQueue, Looper, Handler)总结

2017-01-14 10:05:17来源:http://www.jianshu.com/p/1f1163f54aa7作者:小白杨_LH人点击

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背景

    最近在使用Handler,想搞清楚他的原理,在网上看了好几篇文章都看的云里雾里的,直到看到了任玉刚老师的文章我才有了“啊,原来是这样!”的感觉。他的博客一再提分享精神,对我感触很大。所以决定把自己对Handler的想法给大家分享一下, 哪里有不对的地方联系我, 我们一起探讨一起进步。


前言

    我们大家都知道Android为了满足线程间的通信为我们提供了Looper和Handler。相信大家也都知道Handler怎么使用,可是Handler为什么要这么用呢?什么原理呢?今天不讨论他怎么用,我们来分析一下他的工作流程。如果坚持读到最后,相信你一定会有所收获的(ps:哪位小伙伴不了解用法可以留言,空闲下来我会写一篇用法供你了解)


正文
消息机制的工作流程:

  我们先来看一下他的流程图



消息机制


    最开始先由执行任务的线程通过Handler发送消息即去向MessageQueu(消息队列)去插入一条消息,然后Looper从MessageQueue中不断地循环来取出消息,经由Looper处理最终将他交给了Hanlder,这样最终就由Handler所在的线程来处理这条消息了。想必大家还是不太清楚,让我们接下来详细的分析一下每个步骤。


MessageQueue(消息队列)

    我们先来看一下他的源码


public final class MessageQueue {
.....
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 {
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;
}
.....
Message next() {
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;
}
if (mQuitting) {
dispose();
return null;
}
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);
}
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);
}
}
}
pendingIdleHandlerCount = 0;
nextPollTimeoutMillis = 0;
}
}
}

    代码比较长,大家不用深究,看代码我们不难发现实际上他就是一个链表。通过enqueueMessage()方法来插入消息,通过next()来返回并移除消息,这就是MessageQueue的工作原理,让我们先记住这两个方法的名字。


Looper

    我们还是先来看一下代码在做解释


public final class Looper {
......
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));
}
}

    先来看一下他的prepare()方法,首先他先做了一个判断,如果已经存在了Looper他就会抛出


throw new RuntimeException("Only one Looper may be created per thread");

    这也就是为什么一个线程只能存在一个Looper的原因。让我们继续向下看


public final class Looper {
......
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;
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // 注意这里!!!
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg); //注意这里!!!
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
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();
}
}

}

    从代码我们可以看出内部就是一个无限循环,不断地调用MessageQueue的next()方法来取出消息,没错就是之前让你记住MessageQueue中的两个方法之一。最后执行这行代码


msg.target.dispatchMessage(msg); //注意这里!!!

这里的msg.target就是我们所说的Handler(),没错,就是在这里将我们的消息最终交给了Handler。也许你会说:你说是就是啊我们怎么知道你有没有骗我。


public final class Message implements Parcelable {
........
Handler target;
.......
}

    这回你信了吧。这里Looper你应该明白了,经过Looper的处理最终将消息交给了我们的Handler,让我们继续看Handler。


Handler

    我们先来看他的sendMessage().


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);//注意这里!!!
}

    经过sendMessage的层层调用,最后我们发现他调用了enqueueMessage(),也就是最开始我们记住的两个方法之一,就是向MessageQueue中插入消息。这也就是我们的发送阶段。我们还记得Looper最后调用了Handler的dispatchMessage(),他内部是什么机制呢,让我们来看一下。


public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);//注意这里!!!
}
}

  到了这里我相信大家一定不会陌生了吧,看到了我们最熟悉的handleMessage()了。这里就到了Handler对消息的处理阶段了, 现在我们就可以在Handler所在的线程做自己想做的处理了。


尾语

  到这里消息机制的流程我们已经梳理完了,各部分的原理相信你也有了一定的了解,相信你现在回过头去看前面的流程图已经有了不一样的感觉了。本人能力有限,只想分享一下自己的见解,希望我们一起进步,有什么疏漏一定要联系我哦!!!很乐意与你交流探讨!!!


注!!!

   总结不易,请尊重劳动成果,转载请标明出处,谢谢!!!




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