一、数据成员

每个Looper拥有一个消息队列，归属于一个线程。

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // 由Looper.class控制

final MessageQueue mQueue;
final Thread mThread;

二、初始化

每个线程创建一个Looper，如果线程尝试创建第二个Looper就会出现异常。

public static void prepare() {
    prepare(true);
}

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));
}

先创建一个消息队列和Looper所运行的线程。方法是私有的，只能由prepare()调用。

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

Android环境会通过这个方法自动创建主线程Looper，千万不要在主线程中再次调用这个方法。

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

三、启动Looper

把这个线程Looper的queue里面的消息送去处理

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;
    // 确保线程就是本地线程，并实时跟踪线程身份
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();
    // 循环遍历，从消息队列去消息
    for (;;) {
        Message msg = queue.next(); // MessageQueue可能会阻塞
        if (msg == null) {
            // 消息队列关闭，Looper退出
            return;
        }
        // msg.handler.dispatchMessage(msg)，消息发送到Handler回调
        msg.target.dispatchMessage(msg);
        // 确保消息在分发的时候线程没有改变
        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);
        }
        // 消息体回收
        msg.recycleUnchecked();
    }
}

四、消息队列退出

消息队列关闭

public void quit() {
    mQueue.quit(false);
}

public void quitSafely() {
    mQueue.quit(true);
}

五、一般方法

返回Looper所属的线程和消息队列

public @NonNull Thread getThread() {
    return mThread;
}

public @NonNull MessageQueue getQueue() {
    return mQueue;
}

public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

返回所属线程的Looper。如果子线程没有调用Looper.prepare()，sThreadLocal.get()为空。

public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

获取线程Looper的消息队列

public static @NonNull MessageQueue myQueue() {
    return myLooper().mQueue;
}

返回当前的线程是否就是Looper的线程

public boolean isCurrentThread() {
    return Thread.currentThread() == mThread;
}