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