Handler、Message、MessageQueue随笔
2016-03-17 10:58
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Handler、Message
1、基本用法:
创建Handler重写handlerMessage(Message msg)处理消息Handler handler = new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); // TODO 处理消息 } }; Handler handler = new Handler(new Handler.Callback(){ @Override public boolean handleMessage(Message msg) { // TODO 处理消息 return false; } });
handler.sendMessage(msg); handler.post(new Runnable(){...});
2、主线程默认已经创建Looper,无须重复创建
ActivityThread.javapublic static void main(String[] args) { Looper.prepareMainLooper(); ... Looper.loop(); ... throw new RuntimeException("Main thread loop unexpectedly exited"); }
Handler解析
Handler负责把消息放入线程的消息队列中以及分发消息。1、创建Handler
创建Handler之前需要创建Looper,否则抛出throw new RuntimeException(“Can’t create handler inside thread that has not called Looper.prepare()”);
/* * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ public Handler(Callback callback, boolean async) { mLooper = Looper.myLooper();// 使用当前线程所在的Looper 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; //标志Message是否为异步Message.setAsynchronous }
/* * @hide */ public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; // 为handler指定Looper mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }
2、发送消息到消息队列MesageQueue
Handler中提供了很多个发送消息的方法,其中除了sendMessageAtFrontOfQueue()方法之外,其它的发送消息方法最终都会辗转调用到sendMessageAtTime()方法中(1)boolean sendEmptyMessage(int);
// 发送一条消息到队列,成功返回true,失败返回false,通常是因为Looper已经退出
(2)sendEmptyMessageAtTime(int, long)指定时间发送消息 相对时间
SystemClock.uptimeMillis() + 3000; // 从开机到现在的毫秒数(手机睡眠的时间不包括在内);
(3)通过sendMessageAtFrontOfQueue()方法来发送消息的,它也会调用enqueueMessage()来让消息入队,只不过when为0,这时会把mMessages(MQ头部消息)赋值为新入队的这条消息,然后将这条消息的next指定为刚才的mMessages,这样也就完成了添加消息到队列头部的操作。
(3)
Post Runnable 到消息队列。将Runnable转换为Message
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; // msg的callback指向Runnale }
3、消息队列中移除消息
一般在Activity页面结束时调用handler.removeCallbacksAndMessages(null);
移除队列中所用的callbacks和messages。详见MessageQueue. removeCallbacksAndMessages();
4、消息事件处理
Handler 里面的mLooper所在的线程决定了 handleMessage 方法所在的线程message的处理比较简单,先判断Message中有没有指定的callback对象(Runnable),有的话就调用callback的run方法,没有则调用我们自己创建Handler对象时实现的handleMessage(Message msg)方法,就这样实现了消息的分发。
public void dispatchMessage(Message msg) { if (msg.callback != null) { // 处理Runnable消息调用runnable.run(); handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
5、子线程中创建Handler
在一个子线程中创建Handler时,必须初始化该线程的Looper对象,因为普通的Thread默认是没有消息队列的。class MyThread extends Thread { public Handler mHandler; public void run() { Looper.prepare(); mHandler = new Handler() { public void handleMessage(Message msg) { /* 处理接收到的消息 */ } }; Looper.loop(); } }
6、Handler的特点
1.handler可以在任意线程发送消息,这些消息会被添加到关联的MQ上。2.handler是在它关联的looper线程中处理消息的。
3.一个线程可以有多个Handler,但是只能有一个Looper和一个MessageQueue!
Q?
同一个线程中的所有消息是否共享MQ?
MQ如何分发到不同的Handler处理消息?
A:是
handler.target
Message 解析
Message本身是一个Parcelable对象Message 可以传递的参数有:
arg1 arg2 整数类型,是setData的低成本替代品。传递简单类型Object 类型 obj(Parcelable)
what 用户自定义的消息代码,这样接受者可以了解这个消息的信息。每个handler各自包含自己的消息代码,所以不用担心自定义的消息跟其他handler有冲突。
其他的可以通过Bundle进行传递
Message可以通过new Message构造来创建一个新的Message,但是这种方式很不好,不建议使用。最好使用Message.obtain()来获取Message实例,它创建了消息池来处理的。
1、 创建Message
Message msg = new Message(); (不要这样写) Message msg = handler.obtainMessage(); Message msg = Message.obtain(); // 这两种方式的区别 public static Message obtain(Handler h) { Message m = obtain(); m.target = h; // 指定Message的句柄(消息处理者)Handler return m; }
/** * Return a new Message instance from the global pool. Allows us to * avoid allocating new objects in many cases. */ // 从缓存池中构建一个Message,如果sPool为空说明没有缓存的Message,则新建一个 public static Message obtain() { synchronized (sPoolSync) { if (sPool != null) { Message m = sPool; sPool = m.next; m.next = null; m.flags = 0; // clear in-use flag sPoolSize--; return m; } } return new Message(); }
2、Message的回收缓存
/** * Recycles a Message that may be in-use. * Used internally by the MessageQueue and Looper when disposing of queued Messages. */ void recycleUnchecked() { // Mark the message as in use while it remains in the recycled object pool. // Clear out all other details. flags = FLAG_IN_USE; // 标示为in_use_flag what = 0; arg1 = 0; arg2 = 0; obj = null; replyTo = null; sendingUid = -1; when = 0; target = null; callback = null; data = null; synchronized (sPoolSync) { // 最多可以缓存50个Message if (sPoolSize < MAX_POOL_SIZE**(50)**) { // 将缓存的sPool指向当前msg,next指向原有的sPool next = sPool; sPool = this; sPoolSize++; } } }
在一个无限for循环中遍历消息队列,然后调用Handler进行消息分发处理,分发之后调用recycleUnchecked ()把Message对象回收到Message Pool中(最大值为50个,若消息池中已经有50个Message,则不做缓存)
例:
MQ—-> m1,m2,m3; // 未处理的message
m1.recycleUnchecked();
m2.recycleUnchecked();
m3.recycleUnchecked();
Message.obtain();
3、msg. markInUse();
// 标记msg为FLAG_IN_USE,加入MQ时做检查(此状态的msg不能多次加入MQ),调用Message.obtain()清除flags;4、将一个消息设置为异步
Message.setAsynchronous(Boolean async) // 详见“MessageQueue同步分割栏”Looper解析
1、Looper.prepare()
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)); }
创建一个Looper对象,它的内部维护了一个消息队列MQ。注意,一个Thread只能有一个Looper对象,不能多次调用Looper.prepare(),否则将抛出异常。
2、Looper.loop()
public static void loop() { final Looper me = myLooper(); // 获取当前线程所在 Looper,不能为空 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 (;;) { // 循环从MQ中取出消息,没有消息则阻塞 Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // 交给相关Handler处理消息 **msg.target.dispatchMessage(msg);** // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); msg.recycleUnchecked(); // 消息回收 } }
3、Looper.quit() quitSafely();
调用mQueue.quit();当我们调用Looper的quit方法时,实际上执行了MessageQueue中的removeAllMessagesLocked方法,该方法的作用是把MessageQueue消息池中所有的消息全部清空,无论是延迟消息(延迟消息是指通过sendMessageDelayed或通过postDelayed等方法发送的需要延迟执行的消息)还是非延迟消息。
当我们调用Looper的quitSafely方法时,实际上执行了MessageQueue中的removeAllFutureMessagesLocked方法,通过名字就可以看出,该方法只会清空MessageQueue消息池中所有的延迟消息,并将消息池中所有的非延迟消息派发出去让Handler去处理,quitSafely相比于quit方法安全之处在于清空消息之前会派发所有的非延迟消息。
无论是调用了quit方法还是quitSafely方法只会,Looper就不再接收新的消息。即在调用了Looper的quit或quitSafely方法之后,消息循环就终结了,这时候再通过Handler调用sendMessage或post等方法发送消息时均返回false,表示消息没有成功放入消息队列MessageQueue中,因为消息队列已经退出了。
需要注意的是Looper的quit方法从API Level 1就存在了,但是Looper的quitSafely方法从API Level 18才添加进来。
4、其他方法
getMainLooper() 获取主线程LoopermyLooper() 获取当前线程Looper
getQueue() 获取MQ
getThread() 获取当前Looper所在线程
MessageQueue
MessageQueue是一个按照when大小排列的链表结构。1、enqueueMessage(Message msg, long when)
同一个没有被处理的message不能多次加入队列// 添加消息到消息队列, 最终的mMessages是按照when的由小到大排列 boolean enqueueMessage(Message msg, long when) { // 检查msg合法性,必须包含handler且非FLAG_IN_USE状态 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) { //如果已经调用MessageQueue.quit, 那么不再接收新的Message msg.recycle(); return false; } msg.markInUse();// msg in_use_flag msg.when = when; Message p = mMessages; boolean needWake; // 队列为空或msg.when == 0或 msg.when < mMessages.when时 // 将msg直接插入到队列头部 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; // 根据when的大小顺序,插入到合适的位置 for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } // 如果在插入位置以前,发现异步消息,则不需要唤醒 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); //唤醒nativeMessageQueue } } return true; }
nativeWake,和natePollonce的作用:
nativePollOnce(mPtr, nextPollTimeoutMillis);暂时无视mPtr参数,阻塞等待nextPollTimeoutMillis毫秒的时间返回,与Object.wait(long timeout)相似
nativeWake(mPtr);暂时无视mPtr参数,唤醒等待的nativePollOnce函数返回的线程,从这个角度解释nativePollOnce函数应该是最多等待nextPollTimeoutMillis毫秒
2、removeMessage(int what);
//删除所有what 和obj = object 的msg void removeMessages(Handler h, int what, Object object) { if (h == null) { return; } synchronized (this) { Message p = mMessages; // Remove all messages at front. // 循环移除MQ 队列头部所有符合要求的Message while (p != null && p.target == h && p.what == what && (object == null || p.obj == object)) { Message n = p.next; mMessages = n; p.recycleUnchecked(); p = n; } // Remove all messages after front. // 循环移除MQ中间所有符合要求的Message while (p != null) { Message n = p.next; if (n != null) { if (n.target == h && n.what == what && (object == null || n.obj == object)) { // 移除message并交换位置 Message nn = n.next; n.recycleUnchecked(); p.next = nn; continue; } } p = n; } } }
3、Message 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 空闲handler数量 int nextPollTimeoutMillis = 0; // MQ阻塞时间 for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); //MessageQueue阻塞nextPollTimeoutMillis 指定时间 synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); // 开机相对时间(不包含休眠时间) Message prevMsg = null; Message msg = mMessages; // 遇到同步分隔栏,忽略该消息,取下一个 异步消息 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) { // 遇到延迟消息,则阻塞一段时间 nextPollTimeoutMillis // 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. // 得到Message,从MQ里移除此消息 mBlocked = false; if (prevMsg != null) { // prevMsg != null,说明是同步分隔栏消息, prevMsg.next = msg.next; // 保留MQ头部为同步分隔栏消息(为了取出下一个异步消息),替换next消息 } else { mMessages = msg.next; // 不包含同步分隔栏消息,替换当前head为next消息 } 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)) { // 线程空闲,计算IdleHandler的数量 pendingIdleHandlerCount = mIdleHandlers.size(); } // 没有IdleHandler 阻塞队列 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. // 处理IdleHandler部分 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); } // 是否需要移除IdleHandler 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; } }
4、MessageQueue.IdleHandler
messageQueue.addIdleHandler(new MessageQueue.IdleHandler() { /** * 返回值boolean 意思是needKeep * true,表示要保留保留, 代表不移除这个idleHandler,可以反复执行 * false代表执行完毕之后就移除这个idleHandler, 也就是只执行一次 */ @Override public boolean queueIdle() { Log.e(TAG, "-------------->queueIdle 主线程空闲了"); return true; } });
5、同步分割栏
也是一个targer = null 的Message“同步分割栏”是起什么作用的呢?它就像一个卡子,卡在消息链表中的某个位置,当消息循环不断从消息链表中摘取消息并进行处理时,一旦遇到这种“同步分割栏”,那么即使在分割栏之后还有若干已经到时的普通Message,也不会摘取这些消息了。请注意,此时只是不会摘取“普通Message”了,如果队列中还设置有“异步Message”,那么还是会摘取已到时的“异步Message”的。
在Android的消息机制里,“普通Message”和“异步Message”也就是这点儿区别啦,也就是说,如果消息列表中根本没有设置“同步分割栏”的话,那么“普通Message”和“异步Message”的处理就没什么大的不同了。
int postSyncBarrier(long when) // 往MQ里加入一个同步分割栏,按照when的大小插入到合适位置
removeSyncBarrier(int token) // 从MQ中移除同步分割栏
6、清空MQ
void quit(boolean safe) { if (!mQuitAllowed) { //UI线程的Looper消息队列不可退出 mQuitAllowed = false throw new IllegalStateException("Main thread not allowed to quit."); } synchronized (this) { if (mQuitting) { return; } mQuitting = true; if (safe) { // 移除MQ所有的延迟消息 removeAllFutureMessagesLocked(); } else { // 移除MQ中的所有Message removeAllMessagesLocked(); } // We can assume mPtr != 0 because mQuitting was previously false. nativeWake(mPtr); // 唤醒MQ } }
// 移除MQ中的所有Message private void removeAllMessagesLocked() { Message p = mMessages; while (p != null) { Message n = p.next; p.recycleUnchecked(); p = n; } mMessages = null; }
// 移除MQ所有的延迟消息 n.when > now private void removeAllFutureMessagesLocked() { final long now = SystemClock.uptimeMillis(); Message p = mMessages; if (p != null) { if (p.when > now) { // 如果队列头部消息为延迟消息,则清空MQ removeAllMessagesLocked(); } else { Message n; for (;;) { n = p.next; if (n == null) { return; } if (n.when > now) { // 遍历找到延迟消息,退出循环 break; } p = n; } p.next = null; // 清空所有的延迟消息 do { p = n; n = p.next; p.recycleUnchecked(); } while (n != null); } } }
Handler-memory-leak
http://www.androiddesignpatterns.com/2013/01/inner-class-handler-memory-leak.html参考:
http://bbs.9ria.com/thread-247435-1-1.htmlhttp://www.mamicode.com/info-detail-984722.html
/article/6981722.html
http://my.oschina.net/youranhongcha/blog/492591?fromerr=d6t15a3t#OSC_h3_10
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