Handler

public Handler(@Nullable Callback callback, boolean async) {
    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

• 创建handler会获取looper对象

public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}
public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

• looper从ThreadlocalMap中获取，ThreadLocalMap通过Thread的一个成员变量threadLocals赋值，内部维护了一个Entry数组，存储了对应的ThreadLocal和Looper对象，根据Threadlocal获取对应的Looper对象。
• ThreadLocal可以当做是ThreadlocalMap的包装类，提供get()和set()

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));
}
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

public static void main(String[] args) {
    Looper.prepareMainLooper();
    ActivityThread thread = new ActivityThread();
    thread.attach(false, startSeq);

    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");
}

• 在Looper.prepare()中创建Looper对象，并从当前Thread中将ThreadLocal和创建的Looper对象存储进ThreadLocalMap对象。在主线程中不需要执行prepare()方法是因为在ActivityThread的main()中已经执行了

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

• 在创建Looper对象时，会创建MessageQueue对象。所以一个线程对应一个Looper，对应一个MessageQueue

handler.sendMessage()
handler.sendMessageAtTime()
handler.sendMessageDelayed()

• 几种发送消息的方法，最后都会走到sendMessageAtTime(Message msg, long delayMillis)方法中，只是delayMeillis参数修改

private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
        long uptimeMillis) {
    msg.target = this;
    msg.workSourceUid = ThreadLocalWorkSource.getUid();

    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

• 发送消息时会将msg.targe赋值为当前的handler，后续分发消息时也是通过该标志，找到对应的handler

boolean enqueueMessage(Message msg, long when) {
    synchronized (this) {
        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        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 {
            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是一个链表结构，根据delay的时间来修改next指向。以上是往消息队列中添加消息的流程

for (;;) {
    Message msg = queue.next();
}

• 通过Looper.loop(）方法取出消息，该方法是一个死循环，通过next()方法获取下一个需要处理的消息

for (;;) {
    if (nextPollTimeoutMillis != 0) {
        Binder.flushPendingCommands();
    }

    nativePollOnce(ptr, nextPollTimeoutMillis);  //nextPollTimeoutMillis 0立即返回  -1 一直阻塞  >0 阻塞时长

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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
        msg.markInUse();
        return msg;
    }
} else {
    // No more messages.
    nextPollTimeoutMillis = -1;
}

• 获取消息，通过nativePollOnce设置阻塞状态，假如下一消息时间未到则阻塞等待的时长，假如没有没有消息则传入-1，一直阻塞。唤醒会通过添加消息enqueueMessage()#nativeWake()，或者quit()#nativeWake()

msg.target.dispatchMessage(msg);
public void dispatchMessage(@NonNull Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

• 取出消息后通过handler#dispatchMessage()进行分发，并根据handler创建的方式回调到对应的位置

public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}
public final boolean post(@NonNull Runnable r) {
   return  sendMessageDelayed(getPostMessage(r), 0);
}

• runOnUiThread使用的也是handler的方法，在非UI线程时，取主线程的handler。在子线程中发送消息，最后会将消息发送到handler对应的messageQueue中，再通过Loop()方法去除消息处理，而Looper.loop()方法是在主线程中启动的，所以线程切换到主线程

public final boolean postDelayed(
        @NonNull Runnable r, @Nullable Object token, long delayMillis) {
    return sendMessageDelayed(getPostMessage(r, token), delayMillis);
}

• 至于其他的postDelayed()方法原理都类似