Android Handler 源码解析

前言

我相信，用过Android的人基本都会使用Handler，或者多多少少会听到这个东西，在安卓里面，这东西太重要了，如果你还不会基本用法，那应该是需要反省一下。当然，用过它的人也不必沾沾自喜，我们真的很了解Handler吗，还是说只会使用？你有看过他的每一行代码？仔细思考过吗？对于我来说，确实没有，所以我带着问题，想全面了解 Handler。

是什么东西？

handler，英文有（信息）处理机的意思，这里我们暂且可以认为他有处理消息的功能，当然，这也是他最主要的功能，他还有其他的功能，我们可以根据源码一谈究竟。用最通俗的总结一下，他是用来接收从各个线程发过来的消息，然后集中处理的东西。你可以在主线程（MainThread）处理消息，在子线程（WorkerThread）发送消息，当然，这个关系也可以转换，下面我手写一小段代码展示一下（只为了演示，实际中不会这么写）

// 写在Activity中作为实例变量
private Handler mHandler = new Handler() {
      @Override
      public void handleMesssage(Message msg) {
             ui.setText("msg received");
      }      
}
// 启动一个线程发一个消息
new Thread(() -> mHandler.sendEmptyMessage());

上面简单的代码演示了如何从一个子线程发送消息到主线程，然后更新UI，至于Android不能再主线程更新UI，我想这个道理大家多少都知道原因，以后有机会再好好分析。

前世今生

很干净，只有自己

      public class Handler {}

源码分析

属性

这里个都是类变量，MAIN_THREAD_HANDLER 顾名思义，主线程的 Handler

    /*
     * Set this flag to true to detect anonymous, local or member classes
     * that extend this Handler class and that are not static. These kind
     * of classes can potentially create leaks.
     */
    private static final boolean FIND_POTENTIAL_LEAKS = false;
    private static final String TAG = "Handler";
    private static Handler MAIN_THREAD_HANDLER = null;

接下来这几个很重要，一个是Looper对象，MessageQueue，Callback，mAsynchronous，还有IMessager，集体有什么用，后面用到了再慢慢说

    final Looper mLooper;
    final MessageQueue mQueue;
    final Callback mCallback;
    final boolean mAsynchronous;
    IMessenger mMessenger;

这个东西大家用的可能不多，是一个内部接口，主要是说你可以不用写一个Handler子类就可以做到消息处理的功能。比如说上面的代码，我就是实现了自己的子类才能做到消息处理，你也可以实现这个Callback，然后 new Handler（Callback）就可以了。

    /**
     * Callback interface you can use when instantiating a Handler to avoid
     * having to implement your own subclass of Handler.
     */
    public interface Callback {
        /**
         * @param msg A {@link android.os.Message Message} object
         * @return True if no further handling is desired
         */
        public boolean handleMessage(Message msg);
    }

函数

构造函数
4个参数，都有默认值，分别是 Looper.myLooper() 获取到的，如果你在主线程初始化，那默认就是MainLooper，否则就是其他线程自己的Looper，这里有一个要注意的地方，就是说，当前线程必须执行过 Looper.prepare()，一般你在子线程都需要先执行这个方法，但在主线程是不需要的，因为早在应用启动的时候，ActivityThread 的 main 方法以及帮你执行了，这是我们程序比较早的一个入口。然后绑定MessageQueue，默认是Looper的Queue，还有Callback，默认为空，还有异步mAsynchronous ，默认为false。

    public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
    
    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    // 静态方法获取Looper，使用到了ThreadLocal
    // 这个强大的变量，从当前线程取值，
    // 前提是你之前有放入过东西，不然怎么取
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

    // 静态方法，往ThreadLocal存在Looper实例
    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));
    }

然后是我们通常需要重写的函数，里面是消息处理的逻辑

    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

这个函数原则上你也可以重写，但是不推荐，我们可以看到，它首先判断msg是否带有callback，如果不为空，则执行处理逻辑，否则判断mCallback是否为空，这两个Callback室友区别的，后面这个就是上面那个接口的实现类，而msg的callback是背后自己封装的，稍后我们在看这个细节。如果没有传进去callback，那么则调用handleMessage（），就到了我们重写的那个地方了，这些是在主线程执行的。

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

发送消息的函数，默认是直接发送，注意，虽然是 EmptyMessage，但还是在内部封装了一个 Message，以后获取Message，不要再傻傻的new Message了，通过Message.obtain 或者 Handler.obtain。这一步可以在子线程调用

    /**
     * Sends a Message containing only the what value.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public final boolean sendEmptyMessage(int what) {
        return sendEmptyMessageDelayed(what, 0);
    }

    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
    }
    // 最后还是调用了atTime
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

获取MessageQueue，将Message入队，注意，这里msg.target = this，将handler绑定到message上，到时候就能找到消息对应的handler实例了，强调一下，一个线程对应一个queue，一个queue可以有多个handler，所以有需要区分多个handler，就是这里设置target，绑定实例，然后入队，注意，异步也是在这里配置

    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);
    }

MessageQueue 的源码，使用synchronised保证顺序性，至于MessageQueue的种种细节，下次在分析

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                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.
                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) {
                        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;
    }

    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();
            }
            // poll 消息
            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.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                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;
                }

                // 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.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    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.
            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.
            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;
        }
    }

获取消息，调用MessageQueue的next，然后看到了msg.target.dispatchMessage(msg)这个是在主线程的

    /**
     * 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.");
        }
        // 获取 Queue 通过 MessageQueue
        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 (;;) {
            // 获取消息，调用MessageQueue的next
            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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            final long end;
            try {
                msg.target.dispatchMessage(msg);
                end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            if (slowDispatchThresholdMs > 0) {
                final long time = end - start;
                if (time > slowDispatchThresholdMs) {
                    Slog.w(TAG, "Dispatch took " + time + "ms on "
                            + Thread.currentThread().getName() + ", h=" +
                            msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                }
            }

            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();
        }
    }

当然，handler还有其他用法，比如post，但需要注意，这个虽然提交了一个Runnable，但跟随者消息链，你会发现，它的执行是直接在主线程调用run方法，也就是说运行在主线程的，所以使用时候需要注意了，别用错了

    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }
    // callback message 的callback，与之前Callback有很大的区别
    private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

小结

• 使用方法，继承Handler，或者实现Callback，又或者post
• 各部分运行在什么地方，主线程和子线程要区分
• MessageQueue，Looper，Handler 之间的关系，在什么地方初始化

欢迎讨论