Handler消息机制详解

Handler机制是Android开发中最常见的机制，可以说贯穿整个Android，在探究Handler机制原理之前，我们先来捋一下用法

1.handler.post(Runnable)

2.handler.postdelayed(Runnable,int)

3.sendMessage(Message)

4.sendDelayMessage(Message,int)

从形式上就可以看出，第一种用法和第二种用法其实是一样的，只不过一个立即发送一个延迟发送而已，同理三四也是一样

我们先探究handler.post，post具体用法是

handler = new Handler();

hander.post(new runnable(){run(//具体逻辑){}});

首先我们找到Handler源码

    public Handler() {        this(null, false);    }

    public Handler(Callback callback, boolean async) {        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 = callback;        mAsynchronous = async;    }

代码不多，一个一个解析，当我们handler = new Handler();时，实际上传入了两个参数进入Handler，一个是null，另一个是false

callback是一个接口，里面只有一个方法

    public interface Callback {        public boolean handleMessage(Message msg);    }

这里传入的是null，暂且放下不谈。这个函数主要过程是初始化Handler里的变量mLooper，mQueue，mCallback以及mAsynchronous

首先mLooper = Looper.myLooper();从这里拿到一个looper，为什么这里会拿到一个looper对象？从这里就要引出ActivityThread.main方法

一个app启动开始，最先调用的方法是ActivityThread.main方法，如同java里的main方法一样，这个方法是程序的入口，那么这个方法到底干了什么

    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");    }

多余的东西我们不看，我们只看Looper.prepareMainLooper();以及Looper.loop();

    /**     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */    public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

这里先调用了prepare（false）

    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));    }

这里先通过sThreadLocal.get()去查找looper，如果发现线程中以及有looper了就抛出异常，Only one Looper may be created per thread每一个线程中只能有一个loop

这里我们是程序第一次启动，当然没有looper，于是接下来调用new Looper创建一个looper

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

looper里包含了一个mQueue和mThread，将当前线程传入，这里我们是主线程传入

然后将looper放入sThreadLocal中，ThreadLocal是一个数据内部储存类，它的神奇之处就在于，使用ThreadLocal储存的对象在不同线程访问会得出不同的对象

打个比方，我们用static进行变量储存的时候，就像我们玩游戏时的公会银行，谁都可以取，谁也可以存，不管谁取出来都是一样的东西，因此在我们不同线程取出来static变量其实是一样的。而使用ThreadLocal进行储存的话，就像我们现实中的银行，一人一个账户，我不可以取你存进去的东西。这里有什么好处呢，这里就可以保证了我(Thread)取出来的东西(looper)一定是我的。其实说了这么说，ThreadLocal里面的实现也很简单，只不过将当前的Thread作为key传进去，本质上还是键值对的方式进行存储。

好，说了这么多让我们回到主线，prepare方法我们已经执行完了，在里面我们新建了looper并存储在ThreadLocal中

程序继续走下去，走到sMainLooper = myLooper();

    public static Looper myLooper() {        return sThreadLocal.get();    }

这里设置了主线程的looper。

综上分析，mLooper = Looper.myLooper();拿到的是主线程的looper。这里就要注意一点，如果我们是在子线程中调用Handler，我们必须给它传入一个looper，因为在子线程中是没有looper的，那我们在子线程中就应该通过Looper.prepare（）拿到一个新的looper，将这个looper传入Handler的构造函数就好了

OK，我们回到handler的构造函数中，我们已经拿到了一个looper对象，接下来我们就可以从Looper中取出mQueue，然后的初始化就没有什么好分析了。

我们来看看handler.post函数为什么明明在子线程中，而它的run方法却是在主线程中

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

这里我们进入getPostMessage看看

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

Message.obtain返回一个新的Message对象

    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();    }

然后将Runnable塞进m.callback里

然后我们接着调用sendMessageDelayed(getPostMessage(r), 0);

    public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }

    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);    }

最后将mQueue，msg，uptimeMillis一起传入

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {        msg.target = this;        if (mAsynchronous) {            msg.setAsynchronous(true);        }        return queue.enqueueMessage(msg, uptimeMillis);    }

这里的target是一个Handler变量，这里它传入了主线程的Handler

接着走下去

    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("MessageQueue", 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;    }

这里我们可以看到一个死循环，我们不必拘泥于代码本身，这段代码的意思将manager插入mQueue

代码走到这里仿佛走死了，说好的调用切换线程呢？

不急，还记得我们ActivityThread.main函数吗？里面我们还有一个方法没讲呢，那就是looper.loop（）;

这就是Handler之所以能切换线程的关键所在，让我们走进方法

    /**     * 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.");        }        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 (;;) {            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            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            msg.target.dispatchMessage(msg);            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();        }    }

进入queue.next()方法

    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();            }            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 (false) Log.v("MessageQueue", "Returning message: " + msg);                        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("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;        }    }

这里的next方法是一个无线循环的方法，如果消息队形中没有消息，那么next方法会一直阻塞在这里，当有新消息时，next方法会返回这条消息并将消息删除

其实当初看书看到这里还是有疑问的，作者本身爱较真，奈何能力实在有限，等到很长时间才反应过来。作者是这么理解的，我的方法虽然在子线程调用，但是我将子线程中执行的方法打包成一个Message，然后发送到一个共同的Messagequeue，提醒主线程告诉它我已经发消息了，主线程收到我的提醒以后就去Messagequeue中拿出Message，把Message中的方法调用出来执行，这才是Handler消息机制的原理。很多Handler详解都没有告诉作者原来还有提醒这一步，导致作者原来对Handler机制十分模糊。

提醒主线程这一步是在native层实现的，翻了翻源码，发现只有在enqueueMessage方法中if (needWake) nativeWake(mPtr);比较可疑，只能靠一些边角东西推理了

private long mPtr; // used by native code

而在next方法中有这么一个方法

nativePollOnce(ptr, nextPollTimeoutMillis);

作者猜测这个方法应该是阻塞这个方法并接收nativeWake的了。

扯了这么多，还是回到Looper.loop方法中，我们已经取出Message了，接下来就调用msg.target.dispatchMessage(msg);

        /**     * 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);        }    }

这就是消息处理的方法，不同的消息发送处理方式是有先后之分的，一开始我们调用的方法中是将runnable放入Message中，所以msg.callback != null

这个方法实现就更简单了

    private static void handleCallback(Message message) {        message.callback.run();    }

直接就调用runnable.run方法，简单粗暴我喜欢，到这里通过handler.post(Runnable)的流程已经梳理完了

接下来我们来看看sendMessage(Message)，其实两种方式原理都是一样的，只不过一开始走的路不太一样

如果我们要用sendMessage方法，我们需要重写Handler.handleMessage方法，让我们进入源码

    public final boolean sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }

    public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }

    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);    }

剩下的源码都和post是一模一样的，只有结尾是不一样的，那我再贴一遍好了

        /**     * 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);        }    }

因为我们没有传入Runnable，所以msg.callback == null，而mcallback是Handler初始化时传入的参数，我们调用的不是这个构造方法，所以mcallback也是空，最后调用我们重写的handleMessage方法。

其实Handler使用方法还不止四种，作者只是举出了几个常见的，例如还有view.post方法，或者new一个实现了callback接口的对象，将callback传入Handler一样能异步调用，这里就不再多说了。当然，最常见的还是直接在子线程中调用runOnUIThread方法，该方法里的逻辑代码全部运行在主线程中。

最后再说一点其他的，毕竟文章写了是详解

1.假如我们需要消息回传怎么办呢，我不满足子线程光发送消息给主线程，我还想主线程发消息给子线程？

我们回看当初子线程给主线程发送消息的机制，其实是子线程拿到了主线程的looper，从Looper中取出了mqueque，然后两个线程通过mqueue进行通信

所以如果主线程要想发消息给子线程，就一定要拿到子线程的looper。子线程的looper怎么创建在上文已经说了，最后还要调用Looper.loop方法，不然光主线程做好发消息的准备了，子线程还没做好接收消息的准备。

2.Handler中还有其他构造方法，比如说传入looper对象进行构造，当主线程给子线程Thread进行通信时，调用Thread.loop可能拿不到子线程的loop，因为这个时候子线程的loop可能还没创建，这时候又该怎么办？

换个构造方法！！！！好吧，你不想换怎么办，那就使用HandlerThread，HandlerThread是一个扩展了Thread的类，专门用于Handler通信，在HandlerThread的run方法里自带创建looper和loop.loop

public class HandlerThread extends Thread

HandlerThread中有一个getloop方法，我们来看一下

    public Looper getLooper() {        if (!isAlive()) {            return null;        }                // If the thread has been started, wait until the looper has been created.        synchronized (this) {            while (isAlive() && mLooper == null) {                try {                    wait();                } catch (InterruptedException e) {                }            }        }        return mLooper;    }

如果Thread还没有开始，那么就会调用wait方法，那么何时被唤醒呢

    @Override    public void run() {        mTid = Process.myTid();        Looper.prepare();        synchronized (this) {            mLooper = Looper.myLooper();            notifyAll();        }        Process.setThreadPriority(mPriority);        onLooperPrepared();        Looper.loop();        mTid = -1;    }

在HandlerThread的notifyAll中，调用这个方法后线程就不再处于wait状态，于是主线程的Handler就可以获取到子线程的loop

3.非UI线程真的不能更新ui吗，平常我们都是用Handler来更新UI的，可是真的如此吗？

并不是，我们之所以不让子线程更新UI，是因为有一个并发的问题，Android为了解决这个问题，就在View类里更新UI的方法进行了判断，判断该线程是不是主线程，可是View的实现类ViewRootImp是在OnResume中初始化的额，于是，当我们在oncrate中创建线程并更新UI的时候就有可能在OnResume方法之前完成，因此凡事都不是绝对的

详解到这里就结束了，要是解决了你的疑惑，别忘了顶一个，这是对作者的肯定