05_消息机制的原理

跟消息机制相关的API一共有这么几个：①Looper轮询器②MessageQueue消息队列③Handler④Message 消息

这个消息实际上是放在消息队列里。写Handler的时候并没有写跟MessageQueue相关的代码。

消息是放在消息队列里面的，消息队列什么时候把这个消息取出来？实际上就用到了这么一个东西叫做Looper.Looper也叫做消息泵，能把这个消息不断地从消息队列里给它取出来。Looper消息泵和MessageQueue消息队列是消息机制里面不需要咱们手动去写的这个代码。在主线程不需要手动去写。在子线程你也想用消息队列，那你就要去写相应的代码。

主线程为什么不需要手动去写？因为框架已经帮咱们写好了。

    /**
     * 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();
        }
    }

 实际上应用一跑起来的时候它首先就会给当前的线程调一下这个prepareMainLooper，准备一个主要的消息轮询器。sThreadLocal是线程级的单例。就是我这一个线程用这个sThreadLocal.只要是这个线程里保存的这个对象只有一个，sThreadLocal实现一个线程对应着唯一一个对象。

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {//我当前这个线程里已经有了一个对象了
            throw new RuntimeException("Only one Looper may be created per thread");//如果之前的线程已经有了一个Looper这个代码就走不下去了.马上就抛异常
            //所以这里就保证了我的一个线程对应着唯一的一个Looper.
        }
        sThreadLocal.set(new Looper(quitAllowed));//把创建好的Looper通过线程级的单例进行保存,这就实现了一个线程对应着唯一的一个Looper.
    }

Looper的构造是一个私有的，不允许你去调，所以说如果你直接去new出来这个东西是不行的。

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//创建这个Looper先创建了一个MessageQueue.new了一个MessageQueue,通过mQueue保存起来
        mRun = true;
        mThread = Thread.currentThread();
    }

 想创建新的Looper就得调prepare这个方法。prepareMainLooper和prepare方法它们俩是public static void。

    public static void prepare() {
        prepare(true);
    }

    /**
     * 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();
        }
    }

    final MessageQueue mQueue;

mQueue是一个final类型.final类型只要赋值了之后无法修改了.由于一个线程通过线程级的单例对应起了一个唯一的Looper.在创建这个Looper的时候又搞了一个MessageQueue,而且MessageQueue还是final的.所以一个Looper又对应起了唯一的MessageQueue.这就保证了一个线程对应一个消息队列.一个消息队列用一个消息泵取消息.所以说只要你这个消息队列创建好了之后,它就跟当前的线程对应起来了.只要是你在这一个线程里进行操作，不管是通过谁去丢消息，肯定丢不错。你丢到的都是同一个消息队列里。因为一个线程只有这么一个消息队列。

---

通过构造创建好了Looper还不够，下一步得让Looper转起来。真正的得不断地从消息队列里面去取消息，为什么子线程一sendMessage()这边主线程就取出来了？实际上有一段代码在不断地取消息。怎么实现不断地取消息？

 /**
     * 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();//实际上就是获取一下这个Looper.
        if (me == null) {//如果没有Looper,就抛出异常.所以这个方法不能先调,得先准备好一个Looper,然后才能去调它的Looper.loop().
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;//拿出Looper之后把消息队列拿出来了.

        // 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 (;;) {//死循环,相当于while(true)
            Message msg = queue.next(); // 拿下一条消息,一条一条拿消息.might block 有可能会阻塞.当前消息队列是空的没消息那它就一直在那儿等.一旦有消息进来就马上把消息取出来.
            if (msg == null) {//取出一条消息就是Message.
                // 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);//Message还有一个属性是target.

            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.recycle();
        }
    }

 运行当前线程的消息队列。

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {
        return sThreadLocal.get();//通过线程级的单例保存这个Looper.直接从这个对象里面就可以把它获取到.
    }

实际上消息一取出来它就调了Handler的dispatchMessage.来看看Handler的dispatchMessage.

    /**
     * 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);//重写了handleMessage方法
        }
    }

 我的主线程一运行起来就准备了这么一个Looper,并且准备了一个消息队列.然后调了Looper的loop方法.loop方法不断地调从消息队列里不断地去取消息.一旦有消息取出来马上去调dispatchMessage.一调dispatchMessage实际上就走到了这个handleMessage这个方法.所以说咱们就实现了这个地儿你重写handleMessage往这儿一丢马上就能取出来.

---

发消息是怎么搞的？

    /**
     * Pushes a message onto the end of the message queue after all pending messages
     * before the current time. It will be received in {@link #handleMessage},
     * in the thread attached to this handler.
     *  
     * @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 sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);//发消息调的是sendMessageDelayed.sendMessageDelayed(msg,0)表示是马上执行的.
    }

    /**
     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @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.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);//SystemClock.uptimeMillis()获取系统当前的时间.拿着开机开多长时间+延迟多长的时间.
    }

Open Declaration long android.os.SystemClock.uptimeMillis()


Returns milliseconds since boot, not counting time spent in deep sleep.

Returns:
milliseconds of non-sleep uptime since boot.

since boot从开机之后.机器开了多久那就获取一下这个时间.

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @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.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    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);
    }

    /**
     * Use the {@link Looper} for the current thread with the specified callback interface
     * and set whether the handler should be asynchronous.
     *
     * Handlers are synchronous by default unless this constructor is used to make
     * one that is strictly asynchronous.
     *
     * Asynchronous messages represent interrupts or events that do not require global ordering
     * with represent to synchronous messages.  Asynchronous messages are not subject to
     * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
     *
     * @param callback The callback interface in which to handle messages, or null.
     * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
     * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
     *
     * @hide
     */
    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();// 把当前线程的Looper给它获取到.
        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;
    }

 当Handler一构造起来之后,为什么说Handler发消息发不错？我在主线程创建了Handler,一定是丢给了主线程。首先在构造Handler的时候调的是无参的构造，

    /**
     * Default constructor associates this handler with the {@link Looper} for the
     * current thread.
     *
     * If this thread does not have a looper, this handler won't be able to receive messages
     * so an exception is thrown.
     */
    public Handler() {
        this(null, false);
    }

 无参构造调的是两个参数的构造。两个参数的有参构造首先是从 Looper.myLooper()开始.刚才已经看了myLooper()的源码.

Open Declaration Looper android.os.Looper.myLooper()


Return the Looper object associated with the current thread. Returns null if the calling thread is not associated with a Looper.

 把当前线程的Looper给它获取到.所以Handler一创建起来首先通过成员变量mLooper拿到了当前线程的Looper.拿到之后通过mLooper又拿到了mQueue.所以说Handler拿到了,你在哪个线程创建的Handler,最终你的消息就发到哪个线程对应的消息队列。最终对这个线程搞的Looper进行轮询。

    /**
     * Enqueue a message into the message queue after all pending messages
     * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
     * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
     * You will receive it in {@link #handleMessage}, in the thread attached
     * to this handler.
     * 
     * @param uptimeMillis The absolute time at which the message should be
     *         delivered, using the
     *         {@link android.os.SystemClock#uptimeMillis} time-base.
     *         
     * @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.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;//拿到消息队列
        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);//把消息队列传给enqueueMessage
    }

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);//queue就是当前线程的消息队列,Handler是在主线程创建的,那么它就是主线程的消息队列.
    }

    boolean enqueueMessage(Message msg, long when) {//when=当前系统开机的时间+延迟的时间
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {
            throw new AndroidRuntimeException("Message must have a target.");
        }

        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;
            }

            msg.when = when;//when是消息要执行的时间.通过消息要执行的时间进行排序.
            Message p = mMessages;//只记住了mMessages,只有一条消息
            if (p == null || when == 0 || when < p.when) {//p=null说明当前的消息队列是空的,when=0说明我这条消息需要立即执行.when<p.when都说明我这条新加进来的消息应该优先执行.
                它应该排在消息队列的第一条.// New head, wake up the event queue if blocked.
                msg.next = p;//每一条消息都有一个成员变量next
                mMessages = msg;//mMessages等于传进来的msg
                needWake = mBlocked;
            } else {//当前传进去的消息不是优先执行的.比当前消息队列记住的第一条消息这个when要大.
                // 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 (;;) {//for循环是给这个消息找到一个合适的位置
                    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;
            }
        }
        if (needWake) {
            nativeWake(mPtr);
        }
        return true;
    }

 虽然MessageQueue是消息队列，但是并没有什么队列的东西在里面。没有什么集合HashMap之类的。

   Message mMessages;//只记住了一条消息.通过Message类型的成员变量mMessages把消息队列里的第一条消息给它记住了

    // sometimes we store linked lists of these things
    /*package*/ Message next;//每一条消息都有一个成员变量next

 这个消息队列怎么去排的？就是这么去排的。

 sendMessageAtTime最终调的是enqueueMessage(把消息进行入队)。把消息进行一下排序,怎么去排序呢？就是通过要执行的时间when进行排序。谁先执行谁就排在消息队列的前面，谁后执行谁就排在后面。怎么去排？实际上就是按照这个时间来。

最终就按照消息要执行的先后时间把这个消息就入队了。

---

丢到这里这个Looper就不断地在转，

  /**
     * 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  Looper不断地在转
            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.recycle();
        }
    }

一旦有消息丢进来了，检查一下这个时间，如果这个时间需要立即执行，这就涉及到一个next()方法。next()就是从消息队列里去取消息。

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

        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            nativePollOnce(mPtr, nextPollTimeoutMillis);//nativePollOnce后面还会介绍，是通过C来实现的.

            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) {//拿到消息之后比一下时间,与第一条消息比一下时间,是不是当前的时间.如果不是当前时间的话说明我这个消息不需要立即执行.now>msg.when是需要立即执行的.立即执行就给它拽出来.如果now<msg.when的话说明还得接着等,接着等的话记一下时间然后就当前的这个Looper就接着顺下去了.
                        // 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);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuiting) {
                    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;
        }
    }

究竟这个消息什么时候执行，是取决于时间的.