android消息机制(handler运行机制)解析
在android日常开发中,我们不可避免的会使用到有关handler的知识。可以利用handler来进行消息的传递。由于android不允许ui线程访问网络,非ui线程又不能更新ui。这种情况handler是怎么处理的。还有想延迟加载某个方法,大部分兄弟都会用到handler.postDelayed(runnable,delayMillis)方法来延迟加载。。也有些人会误认为这里是新开了一个线程来处理。
首先我们先来了解两个类MessageQueue和Looper, messageQueue可以叫他消息队列,里面采用了单链表的数据结构来存储消息。而Looper就是用来取数据的。Looper通过Looper.prepare()为当前线程创建了一个Looper,然后通过Looper.loop()来开启消息循环,Looper中有个特殊的类叫做ThreadLocal用来存储不同线程中创建的Looper。
static final ThreadLocal sThreadLocal = new ThreadLocal();
Looper通过prepare()方法来初始化
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)); }
prepare默认设置true可以退出,主线程(ui)这里设置的肯定是不可退出。
再来看看Looper()函数:
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
创建了一个messageQueue。然后我们来查看一个Looper()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.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(); } }
首先看前面Looper me = myLooper();myLooper()方法其实就是通过前面讲的ThreadLocal获取当前线程的Looper,如果没有就报错“No Looper; Looper.prepare() wasn’t called on this thread.”这句话相信很多在子线程创建handler的同学都遇到过。final MessageQueue queue = me.mQueue;获取looper的messageQueue接下来就是一个死循环一直通过messageQueue的next方法来获取数据。
然后来讲一下messageQueue,messageQueue主要有两个方法,enqueueMessage()和next()方法,前者是插入数据,后者是获取采用了无限循环的方式来返回消息,如果没有消息就堵塞在这里。
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; } }
最后来看下handler的运行机制,就是靠messageQueue和looper支撑的。
handler的构造函数:
public Handler() { this(null, false); }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; }
获取当前线程的looper和messageQueue。
再 看下handler的Message(msg)方法:
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); }
可以看出最后调用的是enqueueMessage方法:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
设置msg的target为本身然后通过queue的enqueueMessage方法插入到消息队列中。
然后回到looper的loop()方法中可以看到通过messageQueue的next方法获取到msg后调用了msg.target的dispatchMessage()方法。最后看一下handler中dispatchMessage方法的实现:
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
首先判断msg的callback是否为null,其实这个callback就是我们handler.post方法中传入的runnable对象。如果不为null,也就是我们是通过post方法传入了runnable,就是调用handleCallback方法
private static void handleCallback(Message message) { message.callback.run(); }
这方法也就是使用了run方法。
接下来判断mCallback 是否为null,不为null调用mCallback.handleMessage。这个callback是我们new Handler的时候传入的。这样就不需要重新handeMessage方法。最后才开始调用我们熟悉的handlMessage方法。
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