深入理解 Handler 消息机制

public Handler() {
    this(null, false);
}
public Handler(Callback callback) {
    this(callback, false);
}
public Handler(Callback callback, boolean async) {
    // 如果是匿名类、内部类、本地类，且没有使用 static 修饰符，提示可能导致内存泄漏
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }
    // 从当前线程的 ThreadLocal获取 Looper
    mLooper = Looper.myLooper();
    if (mLooper == null) {  // 创建 Handler 之前一定要先创建 Looper。主线程已经自动为我们创建。
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue; // Looper 持有一个 MessageQueue
    mCallback = callback; // handleMessage 回调
    mAsynchronous = async; // 是否异步处理
}
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public interface Callback {
    public boolean handleMessage(Message msg);
}
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mLooper = Looper.myLooper(); // 获取当前线程的 Looper
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public Handler(Looper looper) {
    this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}
public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}
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sendMessage(Message msg)
sendEmptyMessage(int what)
sendEmptyMessageDelayed(int what, long delayMillis)
sendEmptyMessageAtTime(int what, long uptimeMillis)
sendMessageAtTime(Message msg, long uptimeMillis)
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post(Runnable r)
postAtTime(Runnable r, long uptimeMillis)
postAtTime(Runnable r, Object token, long uptimeMillis)
postDelayed(Runnable r, long delayMillis)
postDelayed(Runnable r, Object token, long delayMillis)
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private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}
private static Message getPostMessage(Runnable r, Object token) {
    Message m = Message.obtain();
    m.obj = token;
    m.callback = r;
    return m;
}
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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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis); // 调用 Messagequeue 的 enqueueMessage() 方法
}
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boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) { // msg 必须有 target
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) { // msg 不能正在被使用
        throw new IllegalStateException(msg + " This message is already in use.");
    }
    synchronized (this) {
        if (mQuitting) { // 正在退出，回收消息并直接返回
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }
        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.
            // 插入消息队列头部，需要唤醒队列
            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) { // 按消息的触发时间顺序插入队列
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }
        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}
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Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
        // 阻塞方法，主要是通过 native 层的 epoll 监听文件描述符的写入事件来实现的。
        // 如果 nextPollTimeoutMillis = -1，一直阻塞不会超时。
        // 如果 nextPollTimeoutMillis = 0，不会阻塞，立即返回。
        // 如果 nextPollTimeoutMillis > 0，最长阻塞nextPollTimeoutMillis毫秒(超时)，如果期间有程序唤醒会立即返回。
        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.
                // msg.target == null表示此消息为消息屏障（通过postSyncBarrier方法发送来的）
                // 如果发现了一个消息屏障，会循环找出第一个异步消息（如果有异步消息的话），
                // 所有同步消息都将忽略（平常发送的一般都是同步消息）
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // 消息触发时间未到，设置下一次轮询的超时时间
                    // 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.
                    // 得到 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(); // 标记 FLAG_IN_USE
                    return msg;
                }
            } else {
                // No more messages.
                // 没有消息，会一直阻塞，直到被唤醒
                nextPollTimeoutMillis = -1;
            }
            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }
            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            // Idle handle 仅当队列为空或者队列中的第一个消息将要执行时才会运行
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                // 没有 idle handler 需要运行，继续循环
                mBlocked = true;
                continue;
            }
            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }
        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        // 只有第一次循环时才会执行下面的代码块
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler
            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }
            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }
        // Reset the idle handler count to 0 so we do not run them again.
        // 将 pendingIdleHandlerCount 置零保证不再运行
        pendingIdleHandlerCount = 0;
        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}
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public static void main(String[] args) {
...
 Looper.prepareMainLooper(); // 创建主线程 Looper
...
}
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public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}
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public static void prepare() {
        prepare(true);
}
private static void prepare(boolean quitAllowed) {
    // 每个线程只能执行一次 prepare()，否则会直接抛出异常
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    // 将 Looper 存入 ThreadLocal
    sThreadLocal.set(new Looper(quitAllowed));
}
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mLooper = Looper.myLooper();
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public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}
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private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed); // 创建 MessageQueue
    mThread = Thread.currentThread(); // 当前线程
}
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public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}
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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();
    }
}
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public static void loop() {
    final Looper me = myLooper(); // 从 ThreadLocal 中获取当前线程的 Looper
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue; // 获取当前线程的消息队列
   ...  // 省略部分代码
    for (;;) { // 循环取出消息，没有消息的时候可能会阻塞
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        ...  // 省略部分代码
        try {
            msg.target.dispatchMessage(msg); // 通过 Handler 分发 Message
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        ...  // 省略部分代码
        msg.recycleUnchecked(); // 将消息放入消息池，以便重复利用
    }
}
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public void dispatchMessage(Message msg) {
    if (msg.callback != null) { // callback 是 Runnable 类型，通过 post 方法发送
        handleCallback(msg);
    } else {
        if (mCallback != null) { // Handler 的 mCallback参数 不为空时，进入此分支
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg); // Handler 子类实现的  handleMessage 逻辑
    }
}
private static void handleCallback(Message message) {
    message.callback.run();
}
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int what ：消息标识
int arg1 : 可携带的 int 值
int arg2 : 可携带的 int 值
Object obj : 可携带内容
long when : 超时时间
Handler target : 处理消息的 Handler
Runnable callback : 通过 post() 发送的消息会有此参数
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public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}
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void recycleUnchecked() {
    // Mark the message as in use while it remains in the recycled object pool.
    // Clear out all other details.
    flags = FLAG_IN_USE;
    what = 0;
    arg1 = 0;
    arg2 = 0;
    obj = null;
    replyTo = null;
    sendingUid = -1;
    when = 0;
    target = null;
    callback = null;
    data = null;
    synchronized (sPoolSync) {
        if (sPoolSize < MAX_POOL_SIZE) {
            next = sPool;
            sPool = this;
            sPoolSize++;
        }
    }
}
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public static void main(){
    System.out.println("Hello World");
}
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