Android——消息机制
最近在看Android源码的东西,这几天在看Android中消息机制,然后就来整理一下自己所学的东西。
我们平时都有接触到Android消息机制,最常见的用法就是在一个子线程中进行耗时的操作,然后再返回到主线程中进行UI的更新,当然还有别的用法。我们一般使用消息机制大都是从Handler出发,所以可以说Android的消息机制讲的就是Handler的机制,既然说到Handler那也就离不开Looper、MessageQueue,接下来一一进行探讨。
Handler的初始化:
首先我们需要new一个Handler,有如下几种方法:
在new Handler()时会对该线程中的Looper进行检查,如果没有Looper会报错
public Handler(Callback callback, boolean async) { 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; }这里我只截啦一部分,可以看到从Looper.myLooper()中获取到mLooper,然后进行检查,那就先来看看Looper.myLooper()内部的实现:
public static Looper myLooper() { return sThreadLocal.get(); }sThreadLocal是Looper里面的一个ThreadLocal<Looper>
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();ThreadLocal的作用是在每一个线程中维护一个副本,里面包含一个 Looper的实例,可以从ThreadLocal的get()方法中看出:
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); }get()方法中首先是拿到当前的线程,然后通过values()拿到Thrad中的 localValues,localValues是ThreadLocal中子类Values中的实例,从ThreadLocal.Values的解释可以看到:
/** * Per-thread map of ThreadLocal instances to values. */ThreadLocal.Values就像是一个Map一样对Thread中的信息进行封装,
Looper就是存储在reference所在下标的下一个下标所对应的值,这点也可以从TheadLocal.set()看出。
再回到Handler的创建,在一般的线程中我们使用Handler的过程是:
new Thread(new Runnable() { @Override public void run() { Looper.prepare(); Handler mHandler = new Handler(); mHandler.getLooper().loop(); } }).start();但是在主线程中我们不用对Looper进行操作,只用new Handler()就行,这是因为在Activity中的ActivityThread中的main方法:
public static void main(String[] args) { Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }可以看到Looper.prepareMainLooper()就是在主线程中维护一份Looper.
好啦,接下来就是消息的处理啦:
我们一般发送消息有两种方式:
1)handler.post(Runnable r);
2) handler.sendMessage(Message msg);
先看第一种 post 的源码:
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); }
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }把Runnable封装为一个Message
public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }再来看第二种方法:
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }可以看到,方法一最终也是调用啦sendMessageDelayed();
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); }这里就要讲到MessageQueue,MeaageQueue其实就是一个单链表,插入的方式是尾插法,可以从MessageQueue的enqueueMessage()方法看出:
boolean enqueueMessage(Message msg, long when) {...Message p = mMessages;//记录的是上一个插入的Message//对Message进行尾插法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; } return true; }MessageQueue中取消息是利用next()方法来完成的:
Message next() {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 (false) Log.v("MessageQueue", "Returning message: " + msg); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; }}
next方法是一个无线循环的方法,当MessageQueue中有消息就返回消息并且在该单链表中删除掉该消息,如果没有消息,next就会一直阻塞在那里
这时我们会想:到底消息是怎么被Handler处理的,Handler把消息插入到MessageQueue中,MessageQueue中又可以通过next来返回消息,那么返回的消息是怎么发回给Handler的,MessageQueue是在Looper中维护的,所以按理应该是Looper对返回的信息进行分发给Handler
Looper.loop();就是不断的从MessageQueue中回去到Message,然后分发给Handler
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;for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; }msg.target.dispatchMessage(msg);msg.recycleUnchecked(); } }这里我只截了一部分,先从looper中获取到MessageQueue,然后也是在无限循环的从queue中获取Message,然后调用handler的dispatchMessage(),msg.target就是handler,能让loop跳出无线循环的情况是,queue返回啦一个空的msg,什么情况会返回一个null的msg,需要调用looper.quit()
public void quit() { mQueue.quit(false); }
void quit(boolean safe) { if (!mQuitAllowed) { throw new IllegalStateException("Main thread not allowed to quit."); } synchronized (this) { if (mQuitting) { return; } mQuitting = true; if (safe) { removeAllFutureMessagesLocked(); } else { removeAllMessagesLocked(); } // We can assume mPtr != 0 because mQuitting was previously false. nativeWake(mPtr); } }可以看到会有一个标记 mQuitting = true, 在next方法中就会返回一个null的msg
可以在next方法中看出:
Message next() { if (mQuitting) { dispose(); return null; }}还有一种是quitSafely(),它与quit的区别在与,quit是直接退出,而quitSafely是在把所有的消息都处理完之后再退出
接下来就是看看looper分发Message的处理:
/** * Handle system messages here. */ public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }首先是对msg的类型进行处理:
还得我们在handler发送消息的时候有两种方式
1)handleCallback();就是处理post的方式发送消息
private static void handleCallback(Message message) { message.callback.run(); }就是直接通过Runnable的Run方法进行处理
2)先判断mCallback是否为空,如果不为空就用mCallback.handleMessage()进行消息的处理,
mCallback其实就是一个借口
/** * Callback interface you can use when instantiating a Handler to avoid * having to implement your own subclass of Handler. * * @param msg A {@link android.os.Message Message} object * @return True if no further handling is desired */ public interface Callback { public boolean handleMessage(Message msg); }
从注解可以很好的理解就是:这样可以不用派生Handler的子类,因为我们一般就是
private Handler mHandler = new Handler(new Handler.Callback() { @Override public boolean handleMessage(Message msg) { return false; } });所以这种就给我们有了一个不同的方式来处理message
到这里,我们对Android的消息机制有一个更深入的了解,谢谢大家的阅读。本人处于进阶征途,若有错误之处,望大家能及时提出,我们共同进步!
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