Android消息机制(一):主要流程与原理解析
1.概述
- Android消息机制主要是指Handler的运行机制
- 更新UI只是Handler的其中一个特殊的使用场景
- 在Android开发中,只允许主线程更新UI(为了保证UI操作是线程安全的)
- 使用Handler机制,在子线程中完成耗时操作后可以切换到主线程更新UI
2.主要成员
Message:
- 线程间通讯的数据单元,存储信息
- Message分为3种:
普通消息(同步消息)
、屏障消息(同步屏障)
、异步消息
- 通常使用的都是普通消息,屏障消息就是为了确保异步消息的优先级,设置了屏障后,只能处理其后的异步消息,同步消息会被挡住,除非撤销屏障
MessageQueue:
- 存储消息、取走消息
- 单链表数据结构存储
- 先进先出,会根据消息执行的时间排序
Handler:
- 发送各种消息到MessageQueue,处理Looper分发过来的消息
- Handler创建时需要当前线程的Looper来构建消息循环系统(从ThreadLocal中获取当前线程的Looper)
Looper:
- 无限消息循环,从MessageQueue中取出消息,并分发给对应的Handler
- 线程默认是没有Looper的,如果需要使用Handler,就必须手动为线程创建Handler * 主线程(ActivityThread)在创建时就已经初始化了Looper
Thread、Looper和Handler之间的数量对应关系
- 1个Thread只能绑定1个Looper,但可以有多个Handler
- 1个Looper可以绑定多个Handler
- 1个Handler只能绑定一个Looper
ThreadLocal:
线程本地存储区(Thread Local Storage,简称为TLS),每个线程都有自己的私有的本地存储区域,不同线程之间彼此不能访问对方的TLS区域。Looper类利用了ThreadLocal的特性,保证每个线程只存在一个Looper对象
3.消息机制的运行流程
在子线程中执行完耗时操作后,使用Handler发送消息到MessageQueue,向消息队列中添加消息。然后Looper通过无限轮询,不断从MessageQueue中取出消息,分发给目标Handler处理,这样就切换到了Handler所在的主线程。
4.原理解析
Looper相关:
prepare():初始化一个Looper,并存储到ThreadLocal中:
public static void prepare() { prepare(true);//默认true,允许退出}private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) {//Looper类利用了ThreadLocal的特性,保证每个线程只存在一个Looper对象 throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed));}
myLooper():在子线程中获取当前线程的Looper,内部是调用了ThreadLocal的get方法:
public static @Nullable Looper myLooper() { return sThreadLocal.get();}
getMainLooper():在任意线程获取主线程的Looper:
public static Looper getMainLooper() { synchronized(Looper.class) { return sMainLooper; }}
loop():无限轮询,不断地从MessageQueue中取出消息,然后交给目标Handler去处理:
public static void loop() { final Looper me = myLooper(); if (me == null) { //当前线程必须初始化好Looper throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; ... for (;;) { Message msg = queue.next(); //next方法可能会阻塞 if (msg == null) {//消息为空,退出循环 return; } ... try { msg.target.dispatchMessage(msg);//分发Message到目标Handler if (observer != null) { observer.messageDispatched(token, msg); } dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0; } ... msg.recycleUnchecked(); }}
Handler相关:
构造方法:
public Handler() { this(null, false);}public Handler(@Nullable Callback callback) { this(callback, false);}public Handler(@NonNull Looper looper) { this(looper, null, false);}public Handler(@NonNull Looper looper, @Nullable Callback callback) { this(looper, callback, false);}public Handler(@Nullable Callback callback, boolean async) { ... mLooper = Looper.myLooper();//从当前线程的TLS中获取Looper对象 if (mLooper == null) {//Looper不能为空 throw new RuntimeException( "Can't create handler inside thread " + Thread.currentThread() + " that has not called Looper.prepare()"); } mQueue = mLooper.mQueue;//拿到Looper中的MessageQueue mCallback = callback;//回调 mAsynchronous = async;//设置是否异步处理消息}
Android中主线程在创建时就已经初始化好了Looper,Handler构造时不传Looper也能直接用:
//ActivityThread的main方法public static void main(String[] args) { ... Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } ... Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited");}public static void prepareMainLooper() { prepare(false);//主线程的Looper不允许退出 synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); }}
如果想在子线程中创建Handler,必须在Handler创建时传入一个Looper,有以下两种方法:
1.先调用 Looper.prepare() 在当前线程初始化一个 Looper:
Looper.prepare();Handler handler = new Handler();// ....// 这一步可别可少了Looper.loop();
2.通过构造方法传入Looper:
注:如果是通过这种方式,一定要确保Handler处理消息的语句执行时looper已经准备妥当,可以参考HandlerThread的实现源码
Looper looper = .....;Handler handler = new Handler(looper);
发送消息方法——send:
public final boolean sendMessage(@NonNull 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(@NonNull Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}public final boolean sendMessageAtFrontOfQueue(@NonNull 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);}
发送消息方法——post:
public final boolean post(@NonNull Runnable r) { return sendMessageDelayed(getPostMessage(r), 0);} public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r), uptimeMillis);}public final boolean postAtTime( @NonNull Runnable r, @Nullable Object token, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);}public final boolean postDelayed(@NonNull Runnable r, long delayMillis) { return sendMessageDelayed(getPostMessage(r), delayMillis);}/** @hide */public final boolean postDelayed(Runnable r, int what, long delayMillis) { return sendMessageDelayed(getPostMessage(r).setWhat(what), delayMillis);}public final boolean postDelayed( @NonNull Runnable r, @Nullable Object token, long delayMillis) { return sendMessageDelayed(getPostMessage(r, token), delayMillis);}public final boolean postAtFrontOfQueue(@NonNull Runnable r) { return sendMessageAtFrontOfQueue(getPostMessage(r));}
可以看出,不管是send还是post,最终都是调用了sendMessageAtTime方法:调用MessageQueue的enqueueMessage方法将消息入队:
public boolean sendMessageAtTime(@NonNull 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);}
send和post的联系和区别:
- 两者本质上是没有区别的,都是发送一个消息到消息队列中,只不过post使用方式更简单
- post会通过getPostMessage方法会将Runnable赋给callback,接下来还是和sendMessage一致的操作,进入sendMessageDelayed:
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m;}
dispatchMessage():处理MessageQueue分发过来的消息:
public void dispatchMessage(Message msg) { //如果通过post发送消息,则callback不为空,直接进入handleCallback if (msg.callback != null) { handleCallback(msg); } else { //如果通过send发送消息 if (mCallback != null) {//创建Handler时传进来了Callback if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg);//创建Handler时没传Callback(常用) }}//调用Runnable的run方法,并不会启动新线程,所以post的runnable里面可以直接更新UIprivate static void handleCallback(Message message) { message.callback.run();}
Handler与Looper如何关联?
- 无参构造方法,会从当前线程的ThreadLocal中去取Looper
- 通过构造方法传入Looper
Looper与Thread如何关联?
ThreadLocal(Looper.prepare)
MessageQueue相关:
enqueueMessage():往消息队列中添加一个消息:
boolean enqueueMessage(Message msg, long when) { //... //mMessages是消息队列的表头 msg.when = when; Message p = mMessages; boolean needWake; //如果消息链表为空||当前的消息需要立即执行||当前的消息比消息链表头部的消息执行要早 if (p == null || when == 0 || when < p.when) { // 上面的条件都满足就将消息插入到链表的表头 msg.next = p; mMessages = msg; needWake = mBlocked; } else { Message prev; //根据消息的延迟时间排序,越早触发的消息在链表中的位置越靠前 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; } //唤醒消息队列去取出消息,分发消息 if (needWake) { nativeWake(mPtr); } } return true; }
关于Message.when:
- Message.when是一个时间,用于表示 Message期望被分发的时间,该值是
SystemClock.uptimeMillis()
与delayMillis
之和。 -
SystemClock.uptimeMillis()
是一个表示当前时间的一个相对时间,它代表:自系统启动开始从0开始的到调用该方法时相差的毫秒数 -
Message.when
用时间差来表示先后关系
next():在Looper中通过loop()
方法,不断地从MessageQueue获取消息。当队列中消息为空时,则会进行阻塞操作。
Message next() { ... int pendingIdleHandlerCount = -1; int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } //会阻塞,除非被唤醒(如果有消息被插入到消息队列或者超时时间到) nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; //遇到屏障:msg.target == null if (msg != null && msg.target == null) { do { prevMsg = msg; msg = msg.next; //遍历消息队列找到最近的一条异步消息 } while (msg != null && !msg.isAsynchronous()); } if (msg != null) {//没有遇到屏障||遇到屏障后找到异步消息 if (now < msg.when) { //当消息触发时间大于当前时间,则设置下一次轮询的超时时长 nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else {// 获取一条消息,并返回 mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; //设置消息的使用状态,即flags |= FLAG_IN_USE msg.markInUse(); return msg; //成功地获取MessageQueue中的下一条即将要执行的消息 } } else { //没有消息 nextPollTimeoutMillis = -1; } //消息正在退出,返回null if (mQuitting) { dispose(); return null; } ... }}
next()在没有消息的时候会阻塞,如何恢复?
当其他线程调用enqueueMessage再次插入消息时,就会被唤醒
参考资料:
https://blog.csdn.net/carson_ho/article/details/80175876
https://www.jianshu.com/p/f10cff5b4c25
https://www.jianshu.com/p/f70ee1765a61
https://www.jianshu.com/p/43d6cd7b06f1
http://www.dss886.com/2016/08/17/01/
https://blog.csdn.net/start_m...
https://blog.csdn.net/start_m...
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