Android Handler机制8之消息的取出与消息的其他操作

  /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
         //第1步
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        //第2步
        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();

         //第3步
        for (;;) {
            //第四步
            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 traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                // 第5步
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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);
            }
            // 第6步
            msg.recycleUnchecked();
        }
    }

    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.
        // 如果消息循环已经退出了。则直接在这里return。因为调用disposed()方法后mPtr=0
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }
        //记录空闲时处理的IdlerHandler的数量
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        // native层用到的变量 ，如果消息尚未到达处理时间，则表示为距离该消息处理事件的总时长，
        // 表明Native Looper只需要block到消息需要处理的时间就行了。 所以nextPollTimeoutMillis>0表示还有消息待处理
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                //刷新下Binder命令，一般在阻塞前调用
                Binder.flushPendingCommands();
            }
            // 调用native层进行消息标示，nextPollTimeoutMillis 为0立即返回，为-1则阻塞等待。
            nativePollOnce(ptr, nextPollTimeoutMillis);
            //加上同步锁
            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                // 获取开机到现在的时间
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                // 获取MessageQueue的链表表头的第一个元素
                Message msg = mMessages;
                 // 判断Message是否是障栅，如果是则执行循环，拦截所有同步消息，直到取到第一个异步消息为止
                if (msg != null && msg.target == null) {
                     // 如果能进入这个if，则表面MessageQueue的第一个元素就是障栅(barrier)
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    // 循环遍历出第一个异步消息，这段代码可以看出障栅会拦截所有同步消息
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                       //如果msg==null或者msg是异步消息则退出循环，msg==null则意味着已经循环结束
                    } while (msg != null && !msg.isAsynchronous());
                }
                 // 判断是否有可执行的Message
                if (msg != null) {  
                    // 判断该Mesage是否到了被执行的时间。
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        // 当Message还没有到被执行时间的时候，记录下一次要执行的Message的时间点
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Message的被执行时间已到
                        // Got a message.
                        // 从队列中取出该Message，并重新构建原来队列的链接
                        // 刺客说明说有消息，所以不能阻塞
                        mBlocked = false;
                        // 如果还有上一个元素
                        if (prevMsg != null) {
                            //上一个元素的next(越过自己)直接指向下一个元素
                            prevMsg.next = msg.next;
                        } else {
                           //如果没有上一个元素，则说明是消息队列中的头元素，直接让第二个元素变成头元素
                            mMessages = msg.next;
                        }
                        // 因为要取出msg，所以msg的next不能指向链表的任何元素，所以next要置为null
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        // 标记该Message为正处于使用状态，然后返回Message
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    // 没有任何可执行的Message，重置时间
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                // 关闭消息队列，返回null，通知Looper停止循环
                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.
                // 当第一次循环的时候才会在空闲的时候去执行IdleHandler，从代码可以看出所谓的空闲状态
                // 指的就是当队列中没有任何可执行的Message，这里的可执行有两要求，
                // 即该Message不会被障栅拦截，且Message.when到达了执行时间点
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                
                // 这里是消息队列阻塞( 死循环) 的重点，消息队列在阻塞的标示是消息队列中没有任何消息，
                // 并且所有的 IdleHandler 都已经执行过一次了
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }
    
                // 初始化要被执行的IdleHandler，最少4个
                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.
            // 开始循环执行所有的IdleHandler，并且根据返回值判断是否保留IdleHandler
            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.
            // 重点代码，IdleHandler只会在消息队列阻塞之前执行一次，执行之后改标示设置为0，
            // 之后就不会再执行，一直到下一次调用MessageQueue.next() 方法。
            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.
            // 当执行了IdleHandler 的 处理之后，会消耗一段时间，这时候消息队列里的可能有消息已经到达 
             // 可执行时间，所以重置该变量回去重新检查消息队列。
            nextPollTimeoutMillis = 0;
        }
    }

//****************  第一部分  ***************
                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());
                }

//============分割线==============


//****************  第二部分  ***************
                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;
                }

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            //当Message存在回调方法，回调msg.callback.run()方法；
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                //当Handler存在Callback成员变量时，回调方法handleMessage()；
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            //Handler自身的回调方法handleMessage()
            handleMessage(msg);
        }
    }

// Handler.java
public final void removeCallbacks(Runnable r) {
    mQueue.removeMessages(this, r, null);
}

public final void removeCallbacks(Runnable r, Object token) {
    mQueue.removeMessages(this, r, token);
}

public final void removeMessages(int what) {
    mQueue.removeMessages(this, what, null);
}

public final void removeMessages(int what, Object object) {
    mQueue.removeMessages(this, what, object);
}

public final void removeCallbacksAndMessages(Object token) {
    mQueue.removeCallbacksAndMessages(this, token);
}

  void removeMessages(Handler h, int what, Object object) {
        // 第1步
        if (h == null) {
            return;
        }
         // 第2步
        synchronized (this) {
           // 第3步
            Message p = mMessages;

            // Remove all messages at front.

          
            //第4步
            while (p != null && p.target == h && p.what == what
                   && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            //第5步
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.what == what
                        && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    void removeMessages(Handler h, Runnable r, Object object) {
        if (h == null || r == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h && p.callback == r
                   && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && n.callback == r
                        && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

    void removeCallbacksAndMessages(Handler h, Object object) {
        if (h == null) {
            return;
        }

        synchronized (this) {
            Message p = mMessages;

            // Remove all messages at front.
            while (p != null && p.target == h
                    && (object == null || p.obj == object)) {
                Message n = p.next;
                mMessages = n;
                p.recycleUnchecked();
                p = n;
            }

            // Remove all messages after front.
            while (p != null) {
                Message n = p.next;
                if (n != null) {
                    if (n.target == h && (object == null || n.obj == object)) {
                        Message nn = n.next;
                        n.recycleUnchecked();
                        p.next = nn;
                        continue;
                    }
                }
                p = n;
            }
        }
    }

   private void removeAllMessagesLocked() {
        Message p = mMessages;
        while (p != null) {
            Message n = p.next;
            p.recycleUnchecked();
            p = n;
        }
        mMessages = null;
    }

    private void removeAllFutureMessagesLocked() {
         // 第1步
        final long now = SystemClock.uptimeMillis();
         // 第2步
        Message p = mMessages;
        if (p != null) {
            // 第3步
            if (p.when > now) {
                removeAllMessagesLocked();
            } else {
                // 第4步
                Message n;
                for (;;) {
                    n = p.next;
                    if (n == null) {
                        return;
                    }
                    if (n.when > now) {
                        break;
                    }
                    p = n;
                }
               // 第5步
                p.next = null;
                do {
                    p = n;
                    n = p.next;
                    p.recycleUnchecked();
                } while (n != null);
            }
        }
    }

    void quit(boolean safe) {
         // 第1步
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }
        // 第2步
        synchronized (this) {
            // 第3步
            if (mQuitting) {
                return;
            }
            mQuitting = true;
            // 第4步
            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            // 第5步
            nativeWake(mPtr);
        }
    }

// Handler.java
public final boolean hasMessages(int what) {
    return mQueue.hasMessages(this, what, null);
}

public final boolean hasMessages(int what, Object object) {
    return mQueue.hasMessages(this, what, object);
}

public final boolean hasCallbacks(Runnable r) {
    return mQueue.hasMessages(this, r, null);
}

    boolean hasMessages(Handler h, int what, Object object) {
        //第1步
        if (h == null) {
            return false;
        }
        //第2步
        synchronized (this) {
            //第3步
            Message p = mMessages;
            //第4步
            while (p != null) {
                if (p.target == h && p.what == what && (object == null || p.obj == object)) {
                    return true;
                }
                p = p.next;
            }
            return false;
        }
    }

    /**
     * Runs the specified task synchronously.
     * <p>
     * If the current thread is the same as the handler thread, then the runnable
     * runs immediately without being enqueued.  Otherwise, posts the runnable
     * to the handler and waits for it to complete before returning.
     * </p><p>
     * This method is dangerous!  Improper use can result in deadlocks.
     * Never call this method while any locks are held or use it in a
     * possibly re-entrant manner.
     * </p><p>
     * This method is occasionally useful in situations where a background thread
     * must synchronously await completion of a task that must run on the
     * handler's thread.  However, this problem is often a symptom of bad design.
     * Consider improving the design (if possible) before resorting to this method.
     * </p><p>
     * One example of where you might want to use this method is when you just
     * set up a Handler thread and need to perform some initialization steps on
     * it before continuing execution.
     * </p><p>
     * If timeout occurs then this method returns <code>false</code> but the runnable
     * will remain posted on the handler and may already be in progress or
     * complete at a later time.
     * </p><p>
     * When using this method, be sure to use {@link Looper#quitSafely} when
     * quitting the looper.  Otherwise {@link #runWithScissors} may hang indefinitely.
     * (TODO: We should fix this by making MessageQueue aware of blocking runnables.)
     * </p>
     *
     * @param r The Runnable that will be executed synchronously.
     * @param timeout The timeout in milliseconds, or 0 to wait indefinitely.
     *
     * @return Returns true if the Runnable was successfully executed.
     *         Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     *
     * @hide This method is prone to abuse and should probably not be in the API.
     * If we ever do make it part of the API, we might want to rename it to something
     * less funny like runUnsafe().
     */
    public final boolean runWithScissors(final Runnable r, long timeout) {
         // 第1步
        if (r == null) {
            throw new IllegalArgumentException("runnable must not be null");
        }
         // 第2步
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout must be non-negative");
        }

          // 第3步
         // 如果为同一个线程，则直接执行runnable，而不需要加入到消息队列。
        if (Looper.myLooper() == mLooper) {
            r.run();
            return true;
        }
 
        // 第4步
        BlockingRunnable br = new BlockingRunnable(r);
        return br.postAndWait(this, timeout);
    }

  private static final class BlockingRunnable implements Runnable {
        private final Runnable mTask;
        private boolean mDone;

        public BlockingRunnable(Runnable task) {
            mTask = task;
        }

        @Override
        public void run() {
            try {
                mTask.run();
            } finally {
                synchronized (this) {
                    mDone = true;
                     // runnable 执行完之后，会通知wait的线程不再wait
                    notifyAll();
                }
            }
        }

        public boolean postAndWait(Handler handler, long timeout) {
            if (!handler.post(this)) {
                return false;
            }

            synchronized (this) {
                if (timeout > 0) {
                    final long expirationTime = SystemClock.uptimeMillis() + timeout;
                    while (!mDone) {
                        long delay = expirationTime - SystemClock.uptimeMillis();
                        if (delay <= 0) {
                            return false; // timeout
                        }
                       // post runnable 之后，将调用线程变为wait状态
                        try {
                            wait(delay);
                        } catch (InterruptedException ex) {
                        }
                    }
                } else {
                    while (!mDone) {
                         // post runnable 之后，将调用线程变为wait状态
                        try {
                            wait();
                        } catch (InterruptedException ex) {
                        }
                    }
                }
            }
            return true;
        }
    }