Handler解析

Handler存在的意义

• 跟web开发的ajax有异曲同工之妙
• 使得Android开发难度大大降低（弱化了android线程间通信，根本原理是内存共享）
• 几乎看不到多线程死锁问题

消息处理流程

Handler.sendMessage()
>> Handler.enqueueMessage() 
>> MessageQueue.enqueueMessage() 
>> Looper.loop()
>> MessageQueue.next()
>> Handler.dispatchMessage()
>> Handler.handlerMessage()

Handler主要函数

问题

系统启动: zygote > system_server > ActivityManagerService > ActivityThread
ActivityThread类里的main()里调用了Looper.prepareMainLooper()，所以主线程已经帮我们创建好了Looper，然后继续调用Looper.loop()启动for死循环，表示只要手机启动了则主线程的Looper就已经在死循环了。为何死循环不会导致ANR呢？为什么Looper在Handler里面的初始化不是直接new？为什么Queue要在Looper里面初始化？

  public Handler(Callback callback, boolean async) {
        ......
        mLooper = Looper.myLooper();
        ......
        mQueue = mLooper.mQueue;
        ......
    }

Looper.prepare()

• Looper的构造函数是私有的，但是Looper并不是一个单例模式，而是在prepare()里new出looper。
• ThreadLocal是一个线程隔离工具类,sThreadLocal.set(new Looper(quitAllowed))就是把looper存放到当前thread的ThreadLocalMap里面，sThreadLocal.get() != null得到的就是一个looper，如果不为空就会抛出一个异常。
• 从而保证了一个thread对应一个looper，一个looper又对应一个messageQueue，messageQueue里的message又对应一个handler （target变量）。
• 当在子线程创建handler使用getMainLooper()时，利用thread与looper一对一绑定的特点，进行线程的切换。不得不佩服Handler设计的巧妙！

  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));
    }

Looper.loop()

• MessageQueue是一个具有优先级的先进先出消息队列。
• sendMessage()的时候是有一个延迟时间的，消息队列会根据延迟时间进行排序。
• 入队：当队列满的时候，阻塞，直到用户通过next取出消息。当next方法被调用，通知MessagQueue可以进行消息的入队。
• 出队：由Looper.loop(),启动轮询器，对queue进行轮询。当消息达到执行时间就取出来。当message queue为空的时候，队列阻塞，等消息队列调enqueueMessage的时候，通知队列，可以取出消息，停止阻塞。
• loop()最后会调用msg.recycleUnchecked()来进行消息的回收，使用的享元模式，里面会把message重置来进行再利用，而不是重新new一个message，再通过Message.obtain()重复利用，节省资源。

消息排序

boolean enqueueMessage(Message msg, long when) {
        //判断该消息是否有效，必须有一个target(handler)
        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 可能多个线程插入消息，保证按顺序排列
        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;
           //如果当前队列没有消息，或者该消息需要马上执行，或者该消息，小于最前端消息
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                //是否唤醒取决于mBlocked状态
                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.
               //此时会先判断是否被阻塞，是否handler为空，该消息是否是异步消息
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                //通过for循环来轮询消息队列
                for (;;) {
                    prev = p;
                    p = p.next;
                    //找到时间相符合的条件
                    if (p == null || when < p.when) {
                        break;
                    }
                   //如果需要唤醒，前面已经有一条异步消息则取消唤醒。
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                //把消息添加进来
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

           //最后再判断是否需要唤醒
            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();
            }
            //把线程的等待放到Native层，释放当前线程，当前线程的时间片释放出去就不会出现ANR了，如果nextPollTimeoutMillis=0则执行后续方法
            nativePollOnce(ptr, nextPollTimeoutMillis);
           
           //同样保证线程安全，保证一个消息只被一个线程取出
            synchronized (this) {
                //找到当前的时间
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
               //获取消息队列头部消息
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    //一直向后寻找，直到碰到一个异步消息，跳出 while 循环
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // 如果执行时间没到，消息执行需要等待的时间
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        //获取了一条可执行的消息，此时阻塞需要改为 false
                        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 {
                    // 没有消息了，下次循环进入等待
                    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)) {
                    //mIdleHandlers消息队列为空时的额外处理集合
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                //跳出当次循环很重要
                if (pendingIdleHandlerCount <= 0) {
                    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;
        }
    }