Android:Handler总结

handler 总结

handler架构包含那几个实体

handler：插入消息到messagequeue
looper：从messagequeue中去消息
messagequeue：存储消息，链表结构

如何保证每个线程有一个looper

使用ThreadLocal<Looper>

如何唤醒messagequeue

插入消息的时候去唤醒它，底层使用epoll，往epoll写入消息唤醒他。

空闲消息处理器和同步屏障机制听过吗？

都是优先级相关，同步屏障机制优先级最高，空闲消息处理器优先级最低。
https://blog.csdn.net/asdgbc/article/details/79148180
https://blog.csdn.net/kc58236582/article/details/52919904

分析一下去消息的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);  //第一次取消息会马上返回，因为nextPollTimeoutMillis=0

            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) {   //如果没有target，就优先处理
                    // 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) {    //消息还没到，再等等，这个等待时间会传到epoll
                        // 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;
                }

                // 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.
               //处理空闲handler
                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;   //没有消息，也没有空闲的handler，block住，mBlocked为true的话，插入message就会唤醒epoll
                    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.
            //处理空闲handler
            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.
             //能到这里，说明处理了空闲handler，下次nativepollonce也不需要等待，直接返回
            nextPollTimeoutMillis = 0;
        }
    }

解析一些epoll函数

int epoll_create(int size) 创建epoll
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
第一个参数是epoll_create返回值，第二个参数代表操作，第三个参数代表监听的fd，第四个参数是告诉内核需要监听什么事
int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout)

等待事件的产生，类似于select()调用。参数events用来从内核得到事件的集合，maxevents表示每次能处理的最大事件数，告之内核这个events有多大，这个maxevents的值不能大于创建epoll_create()时的size，参数timeout是超时时间（毫秒，0会立即返回，-1将不确定，也有说法说是永久阻塞）。该函数返回需要处理的事件数目，如返回0表示已超时。

handler里面都是怎样的
epoll_create：
mEpollFd = epoll_create(EPOLL_SIZE_HINT);

epoll_ctl：
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);

epoll_wait：
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

native looper机制是怎样的？

通过addfd来实现的。也是通过epoll来实现等待，可以有callback。
https://blog.csdn.net/chwan_gogogo/article/details/46953563