Android消息机制分析

 转载请注明出处：http://blog.youkuaiyun.com/fishle123/article/details/47864923

对Android开发有一定了解的人都知道，Android应用程序是消息驱动的。Android应用程序的消息机制是围绕消息队列来实现的，具体的，主要通过Handler、MessageQueue、Looper三个类来实现。其中Handler用来发送和处理消息；MessageQueue表示一个消息队列，负责入队和出队消息；Looper类用于创建消息循环。本文结合源码来介绍Android的消息机制包括：消息循环的创建、消息的发送和处理。

    每个Android应用程序都一个主线程ActivityThread，每个主线程都有一个消息循环。先看一下ActivityThread的main方法：

    public static void main(String[] args) {
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        Security.addProvider(new AndroidKeyStoreProvider());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        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");
    }

    可以看到第22行调用Looper.prepareMainLooper()，再看一下prepareMainLooper具体做了哪些工作：

 public static void prepareMainLooper() {
        prepare();
        setMainLooper(myLooper());
        myLooper().mQueue.mQuitAllowed = false;
    }

    可以看到在第调用prepare，然后调用setMainLooper设置主线程的Looper。不难推测，prepare中会创建Looper，先贴上代码：

public static void prepare() {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
    }

    prepare()里面先检查是否已经创建了Looper，如果没有，则创建一个并保存到sThreadLocal中，反之，如果已经创建了一个，则会抛出异常。通过这里，我们也可以发现，prepare只能调用一次，每个线程只有一个Looper。那这里已经创建了Looper，那么第36行的Looper.loop()又是做什么的呢？继续看源码：

  public static void loop() {
        Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        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();

        while (true) {
            Message msg = queue.next(); // might block
            if (msg != null) {
                if (msg.target == null) {
                    // No target is a magic identifier for the quit message.
                    return;
                }

                long wallStart = 0;
                long threadStart = 0;

                // This must be in a local variable, in case a UI event sets the logger
                Printer logging = me.mLogging;
                if (logging != null) {
                    logging.println(">>>>> Dispatching to " + msg.target + " " +
                            msg.callback + ": " + msg.what);
                    wallStart = SystemClock.currentTimeMicro();
                    threadStart = SystemClock.currentThreadTimeMicro();
                }

                msg.target.dispatchMessage(msg);

                if (logging != null) {
                    long wallTime = SystemClock.currentTimeMicro() - wallStart;
                    long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;

                    logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
                    if (logging instanceof Profiler) {
                        ((Profiler) logging).profile(msg, wallStart, wallTime,
                                threadStart, threadTime);
                    }
                }

                // 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.recycle();
            }
        }
    }

    可以看到，在loop中会进入一个死循环，每次循环的开始调用MessageQueu.next()方法从消息队列中取出一个Message，然后调用 msg.target.dispatchMessage(msg)交给message的接受者处理。这里的msg.target就是message的接受者，其实就是一个Handler。如果消息队列为空，那么MessageQueue .next()会阻塞，直到有新的消息达到。到这里，就可以明白调用Looper.loop后，线程就进入了消息循环等待message到来并交给message对应的Handler来处理消息。正常情况下，一旦进入消息循环就不会退出，除非MessageMessage.target=null，见第16行代码。

   到此为止，我们就知道如果要给线程创建一个消息循环，可以按如下方式来定义：

 class LooperThread extends Thread {

        public void run() {
            Looper.prepare();

            ......

            Looper.loop();
        }
    }

    上面提到在Looper中会通过msg.target.dispatchMessage(msg)将msg分派到对应的Handler来出来，这里看一下dispatchMessage的实现：

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

    在dispatchMessage中先检查在Message中是否定义了消息的callback，如果有，优先调用Message的callback来处理消息；反之，如果Handler的mCallBack不为null，则调用mCallBack的handleMessage来处理，当MCallBack也为null的时候，才调用Handdler的handleMessage来处理。这里可以看出,Message有三种方式来实现它的处理逻辑：

1)在Message.callback中实现，这里的callback是一个Runnable对象；

2)在Handler的内部接口CallBack中实现；

3)重写Handler的handleMessage来实现。

    阅读Handler的源码，可以发现当调用handler.post(Runnable)的时候，就是通过第1种方式来实现：

public final boolean post(Runnable r) {
        return  sendMessageDelayed(getPostMessage(r), 0);
    }

    private final Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

    通常我们会通过第3种方式来定义自己的Handler，如在主线程：

Handler mHandler=new Handler(){

        @Override
        public void handleMessage(Message msg) {
            // TODO Auto-generated method stub
            super.handleMessage(msg);
            ......//你的处理逻辑
        }
    };

    这里看一下Handler的构造函数（还有其他几种构造函数）：

public Handler() {
        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());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                    "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
    }

这里可以看到，创建Handler的时候，会在保存当前线程的Looper及Looper对应的MessageQueue。这样发送消息时Handler就能将Message发送到当前线程的消息循环中。当然，我们也可以指定Looper，这样就可以给另一个线程发送消息，如在另一个线程中给主线程发送消息。这里也可以看出，定义Handler之前一定要先调用Looper.prepare()来创建一个Looper,主线程中会在ActivityThread的main方法中创建，不需要我们手动调用。但是在子线程中，一定要先调用Looper.prepare()才能创建 Handler,否则会抛出一个异常，如第78行所示。

Handler常见的用法之一就是在子线程中更新UI，我们可以通过handler.post(Runnable)让runnable在主线程中执行。其中Activity的runOnUIThread就是利用handler来实现的，view.post(Runnable)也是相似的实现。

public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

当调用handler.sendMessage()发送消息时，最终会调用Handler.sendMessageAtTime,源码如下：

 public boolean sendMessageAtTime(Message msg, long uptimeMillis)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;
            sent = queue.enqueueMessage(msg, uptimeMillis);
        }
        else {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }

    在这里可以看出，发送消息的时候，将Message添加到队列中，并且会将Message的接受者设置为发送Message的handler，如代码第6行所示。前面有提到，在loop中会调用MessageQueue.next()来取消息。这里先看一下消息的入队操作：

final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg
                    + " This message is already in use.");
        }
        if (msg.target == null && !mQuitAllowed) {
            throw new RuntimeException("Main thread not allowed to quit");
        }
        final boolean needWake;
        synchronized (this) {
            if (mQuiting) {
                RuntimeException e = new RuntimeException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            } else if (msg.target == null) {
                mQuiting = true;
            }

            msg.when = when;
            //Log.d("MessageQueue", "Enqueing: " + msg);
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked; // new head, might need to wake up
            } else {
                Message prev = null;
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
                msg.next = prev.next;
                prev.next = msg;
                needWake = false; // still waiting on head, no need to wake up
            }
        }
        if (needWake) {
            nativeWake(mPtr);
        }
        return true;
    }

    看代码的第29到35行代码，这里会根据消息的处理时间Message.when来确定消息插入到队列中的具体位置，从而保证队首的消息是下一个要处理的。在这里也可以发现，主线程的消息循环是不能退出的，如第6行所示。

前面有提到MessageQueue.next()可能会阻塞。

    这里一起看一下消息的出队操作：

final Message next() {
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;

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

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                final Message msg = mMessages;
                if (msg != null) {
                    final long when = msg.when;
                    if (now >= when) {
                        mBlocked = false;
                        mMessages = msg.next;
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    } else {
                        nextPollTimeoutMillis = (int) Math.min(when - now, Integer.MAX_VALUE);
                    }
                } else {
                    nextPollTimeoutMillis = -1;
                }

                // If first time, then get the number of idlers to run.
                if (pendingIdleHandlerCount < 0) {
                    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("MessageQueue", "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;
        }
    }

    在第3行，定义变量nextPollTimeoutMills,用来描述当消息队列中没有新的消息时，当前线程需要睡眠多长时间。当nextPollTimeoutMills等于-1时，表示无限制等待，直到有新的消息到达。在没有消息到达的时候会调用nativePollOnce来检查是否有新消息需要处理，nativePollOnce是一个native函数，主要是通过Linux中的poll函数来实现的。

    当队列不为空时，在第17行先判断当前时间是否大于等于message的处理时间，如果是，则返回该message，反之，计算需要等待的时间，并更新nextPollTimeoutMills。

    如果队列为空，则nextPollTimeoutMills的值设置为-1。在进入睡眠之前，先调用注册到消息队列中的空闲消息处理器IdleHandler对象的queueIdle函数，以便有机会在线程空闲时执行一些操作。可以通过MessageQueue的addIdleHandler和removeIdleHandler来注册和注销一个空闲消息处理器。

   到此为止，已经分析完消息循环的创建，发送消息，以及消息的处理。