Android开发艺术探索——第十章:Android的消息机制
一.Android的消息机制概述
前面提到,Android的消息机制主要是指Handler的运行机制以及所附带的MessageQueue和Looper的工作过程,这三者实际上是一个整体,只不过我们在开发的时候比较接触多的是Handler而已,Handler的主要作用是将一个任务切换到某个指定的线程中去执行,那么Android为什么要提供这种功能呢?这是因为android的UI规范不允许子线程更新UI,否则会抛出异常,ViewRootImpl对UI的操作做了验证,这个验证工作是由ViewRootImpl的checkThread来完成的。
void checkThread() { if (mThread != Thread.currentThread()) { throw new CalledFromWrongThreadException( "Only the original thread that created a view hierarchy can touch its views."); } }
这个异常相信很多人见过,由于这一点,并且Android又不允许在主线程有耗时的操作,所以我们必须要在子线程中完成耗时后转回到主线程更新某些东西,这里就需要用到Handler了,如果没有Handler,我们的确没有办法切换到主线程,所以,系统提供Handler,主要的原因就是解决在子线程中无法访问UI的矛盾
这里又要说了,系统为什么不允许在子线程访问UI呢?这是因为Android的UI控件不是线程安全的,如果在多线程中并发访问可能会导致UI控件处于不可预期的状态,那为什么系统不对UI控件访问加上锁机制呢?缺点有两个,首先加上锁后会让UI访问的逻辑变得复杂,其次是会降低UI的访问频率,所以最简单搞笑就是采用单线程模型来处理UI操作,对于开发者来说也不算太麻烦,只需要切换一下即可
Handler的使用方法这里我就不做介绍了,我们来说下他的工作原理,Handler的创建会采用当前线程的Lopper来构建内部的消息循环系统,如果没有,就会报错
如何解决这个问题,只需要为当前线程创建一个looper即可,或者在一个有Lopper的线程中创建Handler也行,后面会讲到
Handler创建完毕后这个时候内部的Lopper以及MeaasgeQueue也可以和Handler一起协同工作,然后通过Handler的post方法将一个Runnable投递到Handler内部的Lopper中去处理,也可以通过Handler的send方法发送一个消息,这个消息同样会在Lopper中去处理,其实post方法最终还是通过send方法来完成的,接下来我们来看下send方法的工作过程,当Handler的send被调用的时候,会他向MessageQueu的enqueueMessage方法将这个消息放入消息队列,然后Lopper发现新消息到来时,就会处理这个消息,最终消息的Runnable或者Handler的handlerMessage方法就被调用,注意Lopper是运行在创建Handler所在的线程中,这样Handler中的业务就会被切换到所在线程就执行了,如图
二.Android的消息机制分析
1.ThreadLocal的工作原理
ThreadLocal是一个线程内部的数据存储类,通过它可以在执行的线程中存储数据,数据存储后,只有在指定线程中可以获取到存储的数据,对于其他线程来说则无法获取到数据,在日常开发中用到ThreadLocal的地方很好,但是在某些特殊的场景下,通过ThreadLocal可以轻松的实现一些看起来很复杂的功能,这一点在android的源码中也有所体现,比如Lopper,ActivityThread以及AMS中都是用到了ThreadLocal,这个不好描述,一般来说,某一个数据是以线程为作用域并且不同线程具有不同的Lopper,这个时候通过ThreadLocal就可以轻松的实现Looper在线程中的存取,如果不采取ThreadLocal,那么系统就必须提供一个全局的哈希表来Handler查找指定线程的Lopper,这样一来就必须提供一个类似于LooperManager的类了,但是系统并没有这么做而是选择了ThreadLocal,这就是ThreadLocal的好处。
ThreadLocal的另一个使用场景是复杂逻辑下的对象传递,比如监听器的传承,有些时候一个线程中的任务过于复杂,这可能表现为函数调用栈比较深以及代码入口的多样性,在这种情况下,我们又需要监听器能够贯穿
整个线程的执行代码,这个时候可以怎么做呢?其实这个时候就采用ThreadLocal,采用ThreadLocal可以让监听器作为线程内的全局对象而存在,在线程内部只要通过get方法就可以获取监听器,如果不采用ThreadLocal,那么我们能想到的如下两个办法,第一个是讲监听器作为参数的形式在函数调用栈中进行传递,第二种方法就是将监听器作为静态变量供线程访问,上述的两种办法都是有局限性的,第一种办法的问题是当函数调用栈深的时候,通过函数参数来传递监听器对象这几乎是不可接受的,这会让程序设计看起来很糟糕,第二种是可以接受,但是这种状态是不具有可扩展性的,比如同时两个线程在执行,那么需要提供两个静态的监听器对象,如果10个就需要10个监听器?这显然是不可思议的,而采取ThreadLocal,每隔监听器对象都有自己的线程内部存储,根本就不存在这个方法2的问题
介绍了这么多ThreadLocal的知识,可能还是有点抽象。下面通过实际的例子来演示ThreadLocal的真正含义,首先定义一个ThreadLocal对象,这里选择Boolean类型的,如下
private ThreadLocal<Boolean> mBooleanThread = new ThreadLocal<Boolean>();
然后分别在主线程,子线程1和2中访问
mBooleanThread.set(true); Log.i(TAG, "主线程:" + mBooleanThread.get()); new Thread("Thread #1") { @Override public void run() { mBooleanThread.set(false); Log.i(TAG, "Thread #1:" + mBooleanThread.get()); } }.start(); new Thread("Thread #2") { @Override public void run() { Log.i(TAG, "Thread #2:" + mBooleanThread.get()); } }.start();
这段代码中,主线程设置了ThreadLocal为true。而在子线程1中设置了false然后分别获取他们的值。这个时候主线程为true,子线程1中应该为false,而子线程2中由于没有设置,所以应该是null
虽然在不同线程中访问的是同一个ThreadLocal对象,但是他们通过ThreadLocal获取到的值确实不一样的,这就是ThreadLocal的奇妙之处,结合这个例子然后再看一遍前面对ThreadLocal这个场景的使用和分析,我们应该就能比较好的理解ThreadLocal的使用方法了,ThreadLocal之所以有这么奇妙的效果,是因为不同线程访问同一个ThreadLocal的get,ThreadLocal内部会从各自的线程中取出一个数组,然后从数组中根据当前的ThreadLocal索引查出对应的calue值,很显然,不同线程中的数组是不同的,这就是为什么通过ThreadLocal可以在不同的线程中维护一套数据的副本并且互不干扰。
首先看ThreadLocal的set方法
public void set(T value) { Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values == null) { values = initializeValues(currentThread); } values.put(this, value); }
在上面的set方法中会通过values方法来获取当前线程的ThreadLocal数组,其实获取的方法也很简单,在Thread内部有一个成员专门用于存储线程的ThreadLocal数据:ThreadLocal.Values localValues,因此获取当前线程的ThreadLocal数据就变成异常简单了,如果loaclValues内部有一个数组:private Object[]tavle,ThreadLocal的值就存在这个table数组中,下面看一下localValues是如何使用put方法将ThreadLocal的值存储到table数组中的:
void put(ThreadLocal<?> key, Object value) { cleanUp(); // Keep track of first tombstone. That's where we want to go back // and add an entry if necessary. int firstTombstone = -1; for (int index = key.hash & mask;; index = next(index)) { Object k = table[index]; if (k == key.reference) { // Replace existing entry. table[index + 1] = value; return; } if (k == null) { if (firstTombstone == -1) { // Fill in null slot. table[index] = key.reference; table[index + 1] = value; size++; return; } // Go back and replace first tombstone. table[firstTombstone] = key.reference; table[firstTombstone + 1] = value; tombstones--; size++; return; } // Remember first tombstone. if (firstTombstone == -1 && k == TOMBSTONE) { firstTombstone = index; } } }
上面的代码实现了一个数据的存储过程,这里不去分析具体算法,但是我们可以看出一个存储规则,那就是ThreadLocal的值在table数组中的存储位置总是为ThreadLocal的refeence字段所标识的对象的下一个位置,比如ThreadLocal的reference对象在table数组中的索引为index,那么ThreadLocal的值在table数组中的索引就是index + 1 ,最终ThreadLocal的值将会被存储在table数组中,table[index + 1] = vales
我们再来看下get方法
public T get() { // Optimized for the fast path. Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values != null) { Object[] table = values.table; int index = hash & values.mask; if (this.reference == table[index]) { return (T) table[index + 1]; } } else { values = initializeValues(currentThread); } return (T) values.getAfterMiss(this); }
可以发现ThreadLocal的get方法逻辑还算是比较清晰,他同样是取当前线程的localValues对象,如果这个对象为null那么就返回初始值,初始值由ThreadLocal的initialValue方法来描述,默认情况下为null,当然也可以重写这个方法,他的默认实现是:
protected T initialValue() { return null; }
如果localValues对象不为null,那就取出他的table数组并找出ThreadLocal的rederence对象在table数组中的位置,然后table数组的下一个位置所存储的数据就是ThreadLocal的值
从ThreadLocal的set和get方法可以看出,他们所操作的独享都是当前线程的localValues对象和table数组,因此在不同线程中访问同一个ThreadLocal的set和get方法,他们对ThreadLocal所做的读写操作仅限于各自内部,这就是为什么ThreadLocal可以在多个线程找那个互不干扰的存储和修改数据,理解了ThreadLocal我们就可以理解Lopper的工作原理了
2.消息队列的工作原理
消息队列在Android中指的是MessageQueue,MessageQueue主要包含两个操作,插入和读取,读取操作本身会伴随着删除操作,插入和读取对应的方法分别为enqueueMessage和next,其中enqueueMessage的作用是往消息队列中插入一条消息,而next的作用是从消息队列中取出一条消息并将其从消息队列中一处,尽管MessageQueue叫消息队列,但是他的内部实现并不是用的队列,实际上它是通过一个单链表的数据结构来维护消息队列,单链表的插入和删除上比较有优势,下面主要看一下他的enqueueMessage和next方法的实现:
boolean enqueueMessage(Message msg, long when) { if (msg.isInUse()) { throw new AndroidRuntimeException(msg + " This message is already in use."); } if (msg.target == null) { throw new AndroidRuntimeException("Message must have a target."); } synchronized (this) { if (mQuitting) { RuntimeException e = new RuntimeException( msg.target + " sending message to a Handler on a dead thread"); Log.w("MessageQueue", e.getMessage(), e); return false; } 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; }
从enqueueMessage的实现中可以看出,他的主要操作就是单链接的插入操作,这里就不一一再过多解释了,下面看一下next方法的实现,next的主要逻辑:
Message next() { int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } // We can assume mPtr != 0 because the loop is obviously still running. // The looper will not call this method after the loop quits. nativePollOnce(mPtr, 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 (false) Log.v("MessageQueue", "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("MessageQueue", "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; } }
可以发现next方法是一个无限循环的方法,如果消息队列中没有消息,那么next方法会一直阻塞在这里,当有新消息到来时,next方法会返回这条消息并将其从单链表中移除
3.Lopper的工作原理
Looper在消息机制中扮演着消息循环的角色,具体来说就是她会不停的从MessageQueue中查看是否有新消息,如果有新消息就会立即处理,否则就会一直阻塞在那里,我们先来看下他的构造方法,在构造方法里他会创建一个MessageQueue,然后将当前线程的对象保存起来:
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
我们都知道,Handler的工作需要Looper,没有Looper的线程就会报错,那么如何为一个线程创建Looper,其实很简单,就是通过Looper.prepare()就可以为他创建了,然后通过Looper.loop来开启循环
new Thread("Thread #2") { @Override public void run() { Looper.prepare(); Handler mHandler = new Handler(); Looper.loop(); } }.start();
Looper除了prepare方法外,还提供了parpareMainLooper方法,这个方法主要是给主线程也就是ActivityThread创建Looper使用的,由于主线程的Looper比较特殊,所以Looper提供了一个getMainLooper方法,通过他可以在任何地方获取到主线程的Looper,Looper也是可以退出的,提供了quir和quitSafely来退出一个Looper,二者的区别是:quit会直接退出,但是quitSafely是退出一个Looper,然后把消息队列中已有消息处理完毕后才安全退出,Looper退出后,通过Handler发送的消息会失败,这个时候Handler的send方法会返回false,在子线程中,如果手动为其创建了Looper,那么所有的事情完成以后应该调用quit方法来终止循环,否则子线程会一直处于等待状态,而如归Looper退出以后,这个县城就会立刻终止,因此建议不需要的时候终止Looper
Looper最重要的一个方法是loop,只有调用loop,这个消息循环才会真正的起作用
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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg); 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.recycle(); } }
Looper的loop方法在工作过程也比较好理解,loop方法是死循环,唯一跳出循环的方法是MessageQueue的next方法返回null,当Looper被quit方法调用时,Looper就会调用MessageQueue的quit和quitSafely方法来通知队列退出,当消息队列被标记为退出状态的时候,他的next方法就会返回null,也就是说,Looper必须退出,否则loop方法就会无限循环下去,loop方法会调用MessageQueue的nect来获取最新消息,而next是一个阻塞操作,当没有消息事,next就会阻塞,这也就导致loop方法一直阻塞在哪里,如果MessageQueue的next返回最新消息,Looper就会处理这条消息:msg.target.dispatchMessage(msg),这里的msg.target是发送这条消息的Handler对象,这样Handler的dispacthManage方法是在创建Handler时所使用的Lopper执行,这样就成功的将代码逻辑切换到指定的线程中去执行。
4.Handler的工作原理
Handler的工作主要是包含消息的发送和接收过程,消息的发送是通过post的一系列和send来实现的,我们来看下:
/** * Pushes a message onto the end of the message queue after all pending messages * before the current time. It will be received in {@link #handleMessage}, * in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } /** * Sends a Message containing only the what value. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendEmptyMessage(int what) { return sendEmptyMessageDelayed(what, 0); } /** * Sends a Message containing only the what value, to be delivered * after the specified amount of time elapses. * @see #sendMessageDelayed(android.os.Message, long) * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis); } /** * Sends a Message containing only the what value, to be delivered * at a specific time. * @see #sendMessageAtTime(android.os.Message, long) * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageAtTime(msg, uptimeMillis); } /** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } /** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) uptimeMillis. * The time-base is {@link android.os.SystemClock#uptimeMillis}. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ 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); } /** * Enqueue a message at the front of the message queue, to be processed on * the next iteration of the message loop. You will receive it in * {@link #handleMessage}, in the thread attached to this handler. * This method is only for use in very special circumstances -- it * can easily starve the message queue, cause ordering problems, or have * other unexpected side-effects. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendMessageAtFrontOfQueue(Message msg) { 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, 0); } private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
可以发现,Handler发消息仅仅是向消息队列里插入一条消息,MessageQueue的next方法就会返回这条消息给Looper,最终Handler的dispatchMessage方法就会被调用,这个时候Handler就进入了处理消息的阶段,dispatchMessage的实现如下:
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
首先,他会检查Message的callback是否为null,不为null就通过handlerCallback来处理消息,Message的callback是一个Runnable对象,实际上就是Handler的post方法所传递Runnable参数,handlerCallback的逻辑也很简单。
private static void handleCallback(Message message) { message.callback.run(); }
其次是检查mCallback是否为null,不为null就调用mCallback的handlerMessage方法来处理消息,Callback是个接口
public interface Callback { public boolean handleMessage(Message msg); }
通过Callback可以采用如下的方式来创建Handler对象,Handler handler = new Handler(callback),那么callback的含义在什么呢?源码里面的注释已经说明,可以用来创建一个Handler的实例单并不需要派生Handler的子类,在日常开发中,创建handler最常见的方式就是派生一个handler子类并重写handlerMessage来处理具体的消息,而Callback给我们提供了另外一种使用Handler的方式,当我们不想派生子类的时候,就可以通过Callback来实现,最后通过调用Handler的handlerMessage方法来处理消息,Handler处理消息的过程
Handler有一个特殊的构造方法,那就是通过一个特定的Looper来构造Handler,他的实现如下:
public Handler(Looper looper) { this(looper, null, false); }
下面再来看下Handler的默认构造方法public Handler,这个构造方法是调用下面的构造方法,很明显,如果当前线程没有Looper的话,就会抛出Cant create handler inside thread that has not called Looper.prepare 这个异常,这个解释在没有Looper的子线程创建handler会引发程序异常的原因
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; }
三.主线程的消息循环
Android的主线程就是ActivityThread,主线程的入口为main,在main方法中系统通过Looper.prepareMainLooper来创建主线程的Looper和MessageQueue,并且通过loop来循环
public static void main(String[] args) { SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); Security.addProvider(new AndroidKeyStoreProvider()); Process.setArgV0("" ); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } AsyncTask.init(); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }}
主线程开始循环之后,ActivityThread还需要一个Handler来和消息队列交互,也就是H
private class H extends Handler { public static final int LAUNCH_ACTIVITY = 100; public static final int PAUSE_ACTIVITY = 101; public static final int PAUSE_ACTIVITY_FINISHING= 102; public static final int STOP_ACTIVITY_SHOW = 103; public static final int STOP_ACTIVITY_HIDE = 104; public static final int SHOW_WINDOW = 105; public static final int HIDE_WINDOW = 106; public static final int RESUME_ACTIVITY = 107; public static final int SEND_RESULT = 108; public static final int DESTROY_ACTIVITY = 109; public static final int BIND_APPLICATION = 110; public static final int EXIT_APPLICATION = 111; public static final int NEW_INTENT = 112; public static final int RECEIVER = 113; public static final int CREATE_SERVICE = 114; public static final int SERVICE_ARGS = 115; public static final int STOP_SERVICE = 116; public static final int REQUEST_THUMBNAIL = 117; public static final int CONFIGURATION_CHANGED = 118; public static final int CLEAN_UP_CONTEXT = 119; public static final int GC_WHEN_IDLE = 120; public static final int BIND_SERVICE = 121; public static final int UNBIND_SERVICE = 122; public static final int DUMP_SERVICE = 123; public static final int LOW_MEMORY = 124; public static final int ACTIVITY_CONFIGURATION_CHANGED = 125; public static final int RELAUNCH_ACTIVITY = 126; public static final int PROFILER_CONTROL = 127; public static final int CREATE_BACKUP_AGENT = 128; public static final int DESTROY_BACKUP_AGENT = 129; public static final int SUICIDE = 130; public static final int REMOVE_PROVIDER = 131; public static final int ENABLE_JIT = 132; public static final int DISPATCH_PACKAGE_BROADCAST = 133; public static final int SCHEDULE_CRASH = 134; public static final int DUMP_HEAP = 135; public static final int DUMP_ACTIVITY = 136; public static final int SLEEPING = 137; public static final int SET_CORE_SETTINGS = 138; public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
ActivityThread通过ApplicationThread和AMS进程进程间通信,AMS以
进程间通信的方法完成ActivityThread的请求后回调后者的Binder方法,然后通过H发送消息,H收到消息后将ApplicationThread中的逻辑切换到ActivityThread去执行,这就是切换到主线程去执行,这个过程就是主线程的消息循环模型。
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