handler源码分析之Looper

类描述

class LooperThread extends Thread {
        public Handler mHandler;
  
        public void run() {
            Looper.prepare();
  
            mHandler = new Handler() {
                public void handleMessage(Message msg) {
                    // process incoming messages here
                }
            };
  
            Looper.loop();
        }
   }

Looper线程实现的经典例子，使用prepare和looper方法分开创建一个初始handler来与Looper来通信。

此类包含基于MessageQueue设置和管理事件循环所需的代码。 影响队列状态的API应在MessageQueue或Handler上定义，而不是在Looper本身上定义。 例如，空闲处理程序和同步屏障在队列中定义，而准备线程，循环和退出在循环中定义。

// sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

如果不调用prepare方法，sThreadLocal.get()方法将返回空，也就是在myLooper()静态方法返回为空，my Looper是在Handler构造时候调用，如果为null，将会抛出异常.

• prepare() 方法

/** 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);
    }

在当前线程初始化一个looper，在创建handler时启动looper。确保在调用prepare方法后，调用loop方法，并使用quit方法退出。

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));
    }

每个线程只能绑定一个Looper，否则会抛异常。
创建一个新的looper实例，保存在sThreadLocal

• 构造

private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

在创建Looper实例的时候，创建一个Message Queue和当前线程，
也是就Looper绑定了当前线程和一个Message Queue。quitAllowed默认为true。

• prepareMainLooper()

/**
     * 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() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

调用prepare()，quitAllowed参数为false，校验sMainLooper是否已经存在，如果已经存在，会抛异常，最后调用 sMainLooper = myLooper()，到目前为止，还没有体现出来Main这个意思。

看看方法描述怎么将：将当前线程初始化一个Looper，并标记成mainLooper，main Looper是在android应用环境创建的，所以不需要调用这个方法。

还是没看出来，先放这里。

• getMainLooper()

/**
     * Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

返回应用程序的mainLooper，位于应用程序的主线程主。
又是这个sMainLooper。不行了 我必须看看它到底怎么就办成main的。妈的看来不是在looper类中显示出来的。还得先放一放。

• myLooper()

/**
     * 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();
    }

返回与当前线程相关联的Looper，如果线程未与Looper绑定，返回null。sThreadLocal.get()这玩意一会在搞它。

-myQueue

/**
     * Return the {@link MessageQueue} object associated with the current
     * thread.  This must be called from a thread running a Looper, or a
     * NullPointerException will be thrown.
     */
    public static @NonNull MessageQueue myQueue() {
        return myLooper().mQueue;
    }

这个mQueue就是初始化Looper时绑定的mQueue。必须从Looper的线程调用，否则会出险空指针一场。

• isCurrentThread

/**
     * Returns true if the current thread is this looper's thread.
     */
    public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
    }

判断当前的线程是否是looper的线程

• setMessageLogging

/**
     * Control logging of messages as they are processed by this Looper.  If
     * enabled, a log message will be written to <var>printer</var>
     * at the beginning and ending of each message dispatch, identifying the
     * target Handler and message contents.
     *
     * @param printer A Printer object that will receive log messages, or
     * null to disable message logging.
     */
    public void setMessageLogging(@Nullable Printer printer) {
        mLogging = printer;
    }

控制由这个Looper处理的消息记录，如果启用将在每个消息分派的开始和结束时，将日志消息写入 printer 中，用以识别目标的Handler和Message。

• quit

/**
     * Quits the looper.
     * <p>
     * Causes the {@link #loop} method to terminate without processing any
     * more messages in the message queue.
     * </p><p>
     * 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.
     * </p><p class="note">
     * 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.
     * </p>
     *
     * @see #quitSafely
     */
    public void quit() {
        mQueue.quit(false);
    }

退出Looper 真正的操作是调用 Message Queue中的quit方法。
描述：终止loop方法，不会在处理消息队列(MessageQueue)中其它消息。
在退出Looper之后，任何尝试将Message加入到消息队列都将失败，消息队列不会接受任何消息，handler.sendMessage将返回false。
使用这个方法可能是不安全的，因为有一些消息可能不会在循环终止之前派发。可以使用quitSafely（）方法来确保有所的待处理的Handler都以有序的方式完成。

• quitSafely()

/**
     * Quits the looper safely.
     * <p>
     * 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.
     * </p><p>
     * 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.
     * </p>
     */
    public void quitSafely() {
        mQueue.quit(true);
    }

和quit方法比起来就差了一个参数，true表示安全退出，
一旦消息队列的素有消息都处理完毕，就会立即退出loop方法。
但是延迟发送的消息会在looper终止之前传递。
退出looper之后任何，不允许任何Message发送到Message Queue

• loop()最主要的方法了

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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.recycleUnchecked();
        }
    }

上面的一些方法只要知道是干啥的 又是作用就行，最主要的方法就是这个loop方法。
调用loop方法首先去调用myLooper()获取当前线程绑定的Looper，
没有肯定不行，抛异常，之前也都说过，一个线程绑定一个Looper，一个Message Queue（prepare-->new Looper()），然后获取到绑定的Message Queue，接下来是一个死循环,从消息队列中取出Message，这里如果没有消息，线程就会阻塞在这里，如果Message 返回 null，结束方法，如果有Message，继续往下走，
mLogging控制由这个Looper处理的消息记录，如果启用将在每个消息分派的开始和结束时，将日志消息写入 printer 中。最后把Message释放，并放入到回收池中。

总结

Looper类的作用：
Looper绑定一个线程和消息队列（new Looper()体现），loop方法开启死循环，不停的从MessageQueue中获取Message，并把消息根据Target分发出去。使用getMainLooper可以获取到主线程Looper在MainThread#Main中初始化调用。

未知问题解决

• prepareMainLooper
  prepareMainLooper静态方法，实在ActivityThread#main方法中调用，Looper也是在这里调用的。在app创建的时候开始创建这些东西。

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);
        Looper.loop();

为什么开启死循环looper不会卡死
参考文章：
https://www.zhihu.com/question/34652589