Handler源码解析
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1、概述
在分析Handler原理时少不了Looper、Message的介绍,它们共同实现Android异步消息处理。
那Handler、Looper、Message是怎样实现不同线程之间的消息处理的呢?
简单的说就是:Handler将Message发送到Looper内部的一个消息队列MessageQueue中,Looper内有个无线循环,不断的在MessageQueue中获取msg,将msg回调给Handler处理,MessageQueue为空时,线程阻塞等待。
2、源码走读
2.1、Handler消息发送
首先从Handler发送消息着手,Handler提供给外部使用的发送消息发送有以下多个:
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } public final boolean postAtTime(Runnable r, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r), uptimeMillis); } public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r, token), uptimeMillis); } public final boolean postDelayed(Runnable r, long delayMillis) { return sendMessageDelayed(getPostMessage(r), delayMillis); } public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } public final boolean sendEmptyMessage(int what) { return sendEmptyMessageDelayed(what, 0); } public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis); } public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageAtTime(msg, uptimeMillis); } 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); }方法虽然较多,不难发现它们最终都会进入54行的这个sendMessageAtTime(...)方法,该方法56行的mQueue在Hanlder构造方法中获得,这里贴出代码量较少的一个构造方法:
public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }可以看到mQueue是从looper中获取到的。回到上面的sendMessageAtTime(...)方法,方法在对mQueue判空后,调用了enqueueMessage(...)方法,并将在Looper中获取到的mQueue传入。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }这个方法很简单,先将Handler绑定在msg的target上(这里其实就是为msg加上一个标记,当消息处理时能够通过这个标记回到之前的Handler内处理),然后将msg加入到Looper内的messageQueue中。
到这里Handler的消息发送过程也就走读完了,消息已经发送到Looper的messageQueue中,接下来就看一下MessageQueue插入message的工作流程。
2.2、MessageQueue如何插入message
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; }第2行判断msg是否绑定了handler,若没有直接抛出异常。第5行判断msg是否正在插入处理,若正在处理直接抛出异常。第18行将msg标记为正在插入处理。22行:若当前队列没有消息||当前插入消息为即时消息||插入消息执行时间小于当前准备处理的消息,则将插入消息为优先级最高的消息,插入到队列最前端记录为当前准备处理消息。若22行条件不满足,则在33行循环现有队列,直到36行找到插入位置,在43-44行插入到队列中去。到这里就完成了message插入到MessageQueue的流程。下面接着看持有MessageQueue的Looper的工作流程。
2.3、Looper
Looper最重要的两个方法为prepare()和loop(),这两个方法在Activity创建时就已被调用,所以我们使用中可能并未手动调用。首先看下Looper的prepare()方法:
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)); }
这里sThreadLoacal为ThreadLoacal对象,它用来在当前线程中保存一个Looper对象,第8行先判断sThreadLoacal是否已有looper对象,若有则直接抛出异常,这也就保证了一个线程中只能有一个looper对象。第12行sThreadLoacal未保存looper时,新建一个looper并保存在sThreadLoacal。
Looper的构造方法如下:
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }在构造方法中创建了久违的MessageQueue,由于一个线程中只有一个Looper,所以一个线程中也只有一个MessageQueue。
下面看一下Looper的loop()方法:
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.isTagEnabled(traceTag)) { 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(); } }第2行myLooper()方法为从sThreadLoacal中获取looper对象,第3行若获取到的looper为空则直接抛出异常。第6行从looper中拿出messageQueue对象。13行进入进入无线循环。14行从队列中获取一条消息。15行获取到的消息为空时,也就是队列中没有消息时,跳出循环。32行将消息发送给指定的target,这里的target也就是上面绑定在message上的handler。
2.1、Handler消息接收
上面message回到了发送它的Handler的dispatchMessage(msg):
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }第3行首先判断msg是否添加了callback,若添加了则优先执行hanldleCallback()
private static void handleCallback(Message message) { message.callback.run();}即直接执行messge的回调。
若msg未添加callback,则在第6行判断Handler是否添加了callback(handler构造时可添加callback),若Handler添加了callback则将msg在第7行回调给handler的callback。
若handler也未添加callback或第7行回调失败,则执行handlerMessage(msg)方法:
public void handleMessage(Message msg) { }而handlerMessage(msg)本身为一个空实现的方法,这时就需要使用者复写该方法,在里面对msg做具体处理了,一般使用时都会根据不同的msg.what来区分不同的消息,如下:
private Handler mHandler = new Handler() { public void handleMessage(android.os.Message msg) { switch (msg.what) { case value: break; default: break; } } };到这里handler的发送消息的流程就已基本走完了。
3、总结
1、在指定线程创建handler,之后handler收到的消息都会在该线程中;
2、handler在其他线程将消息发送给当前线程的Looper的MessageQueue中去;
3、一个线程只有一个Looper,同时也就只有一个MessageQueue;
4、MessageQueue按msg的时间优先级将msg插入到合理位置;
5、looper无线循环取出msg,并将msg发回msg绑定的handler;
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