移步Android Handler机制详解

1 Looper这个类的结构

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2 Looper(boolean)构造函数

private Looper(boolean quitAllowed) {        // 创建MessageQueue对象        mQueue = new MessageQueue(quitAllowed);        // 记录当前线程        mThread = Thread.currentThread();    }

创建了一个MessageQueue，并且把当前线程赋值给本地变量的mThread。
1 这样就实现了Looper和MessageQueue的关联
2 这样就实现了Thread和Looper的关联

Loop类就一个构造函数，构造函数也是private，只有在private static void prepare(boolean quitAllowed)调用**，而这个静态方法也是private的

Looper这个类的对象不能直接创建，必须通过Looper来的两个静态方法prepare()/prepareMainLooper()来间接创建

3 Looper.prepare();

代码在Looper.java的82行

/** Initialize the current thread as a looper.      * This gives you a chance to create handlers that then reference      * this looper, before actually starting the loop. Be sure to call      * {@link #loop()} after calling this method, and end it by calling      * {@link #quit()}.      */    public static void prepare() {        prepare(true);    }

初始化当前线程和Looper

4 prepare(boolean)方法

代码在Looper.java的86行

入参boolean表示Looper是否允许退出，true就表示允许退出，对于false则表示Looper不允许退出。

private static void prepare(boolean quitAllowed) {         //每个线程只允许执行一次该方法，第二次执行的线程的TLS已有数据，则会抛出异常。        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }        //创建Looper对象，并且保存到当前线程的TLS区域。        sThreadLocal.set(new Looper(quitAllowed));    }

代码在Looper.java的86行

 static final ThreadLocal sThreadLocal = new ThreadLocal();

5 prepareMainLooper()方法

代码在Looper.java的99行

/**     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */    public static void prepareMainLooper() {         // 设置不允许退出的Looper        prepare(false);        synchronized (Looper.class) {            //将当前的Looper保存为Looper。每个线程只允许执行一次            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

5.1 注释

初始化当前当前线程的looper。并且标记为一个程序的主Looper。
由Android环境来创建应用程序的主Looper。因此这个方法不能由咱们来调用。

5.2 方法解析

首先通过方法我们看到调用了prepare(false);注意这里的入参是false

其次做了sMainLooper的非空判断，如果是有值的，直接抛异常，因为这个sMainLooper必须是空，因为主线程有且只能调用一次。prepareMainLooper()，如果sMainLooper有值，怎说说明prepareMainLooper()已经被调用了，而sMainLooper的赋值是由myLooper来执行，

5.3 sMainLooper

代码在Looper.java的 69行

 private static Looper sMainLooper;  // guarded by Looper.class

Looper对象啊。但是它是静态的
在Java7之前，静态变量存在永久代(PermGen)。
在Java7之后，将变量的存储转移到了堆。
个sMainLooper就是主线程的Looper。所以只有通过prepareMainLooper()就可以给主线程Looper赋值了

6 myLooper()方法

代码在Looper.java的173行

/**     * Return the Looper object associated with the current thread.  Returns     * null if the calling thread is not associated with a Looper.     */    public static @Nullable Looper myLooper() {        return sThreadLocal.get();    }

这里的sThreadLocal.get()是和prepare(boolean)方法里面的sThreadLocal.set(new Looper(quitAllowed));一一对应的。
在prepareMainLooper里面调用myLooper()，那么myLooper()方法的返回有没有可能为null?
第一步就是调用prepare(false);，所以说myLooper()这个方法的返回值是一定有值的。

7 Looper.loop();

代码在Looper.java的 122行

/**     * Run the message queue in this thread. Be sure to call     * {@link #quit()} to end the loop.     */    public static void loop() {         // 获取TLS存储的Looper对象        final Looper me = myLooper();        //没有Looper 对象，直接抛异常        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }        //获取当前Looper对应的消息队列        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();        // 确保权限检查基于本地进程，而不是基于最初调用进程        final long ident = Binder.clearCallingIdentity();        // 进入 loop的主循环方法       // 一个死循环，不停的处理消息队列中的消息，消息的获取是通过MessageQueue的next()方法实现        for (;;) {             // 可能会阻塞            Message msg = queue.next(); // might block             // 如果没有消息，则退出循环            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            // 默认为null，可通过setMessageLogging()方法来指定输出，用于debug功能            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }           // 用于分发消息，调用Message的target变量(也就是Handler了)的dispatchMessage方法来处理消息            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.            // 确保分发过程中identity不会损坏            final long newIdent = Binder.clearCallingIdentity();            if (ident != newIdent) {                // 打印identiy改变的log，在分发消息过程中是不希望身份被改变                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);            }            // 将Message放入消息池            msg.recycleUnchecked();        }    }

loop进入循环模式，不断重复下面的操作，直到没有消息时退出循环

读取MessageQueue的下一条Message

把Message分发给相应的target

再把分发后的Message回到消息池，以便重复利用

8 Looper的退出循环方法

Looper里面退出循环有两个方法分别是quit()和quitSafely()方法

8.1 Looper.quit()方法

代码在Looper.java的 227行

/**     * Quits the looper.     *      * Causes the {@link #loop} method to terminate without processing any     * more messages in the message queue.     * 
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.     * For example, the {@link Handler#sendMessage(Message)} method will return false.     * 
     * Using this method may be unsafe because some messages may not be delivered     * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure     * that all pending work is completed in an orderly manner.     * 
     *     * @see #quitSafely     */    public void quit() {        mQueue.quit(false);    }

退出循环
将终止(loop()方法)而不处理消息队列中的任何更多消息。在调用quit()后，任何尝试去发送消息都是失败的。
例如Handler.sendMessage(Message)方法将返回false。因为循环终止之后一些message可能会被无法传递，所以这个方法是不安全的。

8.2 Looper.quitSafely()方法

代码在Looper.java的 227行

/**     * Quits the looper safely.     *      * Causes the {@link #loop} method to terminate as soon as all remaining messages     * in the message queue that are already due to be delivered have been handled.     * However pending delayed messages with due times in the future will not be     * delivered before the loop terminates.     * 
     * Any attempt to post messages to the queue after the looper is asked to quit will fail.     * For example, the {@link Handler#sendMessage(Message)} method will return false.     * 
     */    public void quitSafely() {        mQueue.quit(true);    }

安全退出循环
调用quitSafely()方法会使循环结束，只要消息队列中已经被传递的所有消息都将被处理。然而，在循环结束之前，将来不会提交处理延迟消息。
调用退出后，所有尝试去发送消息都将失败。就像调用Handler.sendMessage(Message)将返回false。

9 主线程的Looper的初始化

那么主线程的Looper是什么时候初始化的那？是在系统启动的时候，初始化的。
代码在ActivityThread.java 5401行

public static void main(String[] args) {        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");        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());        AndroidKeyStoreProvider.install();        // Make sure TrustedCertificateStore looks in the right place for CA certificates        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());        TrustedCertificateStore.setDefaultUserDirectory(configDir);        Process.setArgV0("");        /*** 重点 */        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"));        }        // End of event ActivityThreadMain.        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);        /*** 重点 */        Looper.loop();       throw new RuntimeException("Main thread loop unexpectedly exited");    }

ActivityThread不是一个thread，ActivityThread是一个类

首先调用了Looper的静态方法prepareMainLooper()给主线程绑定一个Looper，同时设置Looper对应的MessageQueue对象的mQuitAllowed为false，则该messageQueue是不能退出的。
其次调用Looper.loop();开启循环

参考

Android Handler机制4之Looper与Handler简介

Android Handler机制2--Looper