本篇文章试图来介绍envoy的事件处理部分的代码，对于envoy来说是基于libevent做了简单封装来实现的异步调度。
本篇文章会从下面两部分来进行讲解，libevent的基础知识介绍，envoy中event的类的实现和event在envoy中的调度逻辑。
一、libevent的基础知识篇
1.事件flags:
#define EV_TIMEOUT 0x01 Indicates that a timeout has occurred. #define EV_READ 0x02 Wait for a socket or FD to become readable. #define EV_WRITE 0x04 Wait for a socket or FD to become writeable. #define EV_SIGNAL 0x08 Wait for a POSIX signal to be raised. #define EV_PERSIST 0x10 Persistent event: won't get removed automatically when activated. #define EV_ET 0x20 Select edge-triggered behavior, if supported by the backend. #define EV_FINALIZE 0x40 If this option is provided, then event_del() will not block in one thread while waiting for the event callback to complete in another thread. #define EV_CLOSED 0x80 Detects connection close events.
2.信号量对事件的封装
对于事件，libevent分成了三类，分别是:signal、timer和文件，同时libevent也对他们做了函数层面的重定义，如下所示:
#define evsignal_add(ev, tv) event_add((ev), (tv))
#define evsignal_assign(ev, b, x, cb, arg) event_assign((ev), (b), (x), EV_SIGNAL|EV_PERSIST, cb, (arg))
#define evsignal_new(b, x, cb, arg) event_new((b), (x), EV_SIGNAL|EV_PERSIST, (cb), (arg))
#define evsignal_del(ev) event_del(ev)
#define evsignal_pending(ev, tv) event_pending((ev), EV_SIGNAL, (tv))
#define evsignal_initialized(ev) event_initialized(ev)
Timer对事件的封装： #define evtimer_assign(ev, b, cb, arg) event_assign((ev), (b), -1, 0, (cb), (arg))
#define evtimer_new(b, cb, arg) event_new((b), -1, 0, (cb), (arg))
#define evtimer_add(ev, tv) event_add((ev), (tv))
#define evtimer_del(ev) event_del(ev)
#define evtimer_pending(ev, tv) event_pending((ev), EV_TIMEOUT, (tv))
#define evtimer_initialized(ev) event_initialized(ev)
用户对事件的封装： #define evuser_new(b, cb, arg) event_new((b), -1, 0, (cb), (arg))
#define evuser_del(ev) event_del(ev)#define evuser_pending(ev, tv) event_pending((ev), 0, (tv))
#define evuser_initialized(ev) event_initialized(ev)
#define evuser_trigger(ev) event_active((ev), 0, 0
3.基础函数介绍
对于单个事件来说，主要具备下面几种状态，而这些状态之间的转换基本就是下面的这个图描述的样子。
event_assign() 描述:Prepare a new, already-allocated event structure to be added.
event_new() 描述:Allocate and assign a new event structure, ready to be added.
event_add() 描述:Add an event to the set of pending events.
event_del() 描述:Remove an event from the set of monitored events.
event_free() 描述:Deallocate a struct event * returned by event_new().
event_active() 描述:Make an event active.You can use this function on a pending or a non-pending event to make it active,so that its callback will be run by event_base_dispatch() or event_base_loop(). One common use in multithreaded programs is to wake the thread running event_base_loop() from another thread.
event_pending() 描述:Checks if a specific event is pending or scheduled.
event_remove_timer() 描述:Remove a timer from a pending event without removing the event itself.
event_initialized() 描述:Test if an event structure might be initialized.
event_init() 描述:Initialize the event API.
event_loop() 描述: Handle events.This function behaves like event_base_loop(), but uses the "current" base
event_loopbreak() 描述: Abort the active event_loop() immediately.This function behaves like event_base_loopbreakt(), except that it uses the "current" base.
event_loopexit() 描述:Exit the event loop after the specified time.This function behaves like event_base_loopexit(), except that it uses the "current" base. event_once() 描述:Schedule a one-time event to occur.
event_set() 描述:Prepare an event structure to be added.
event_dispatch() 描述: Loop to process events.Like event_base_dispatch(), but uses the "current" base.
event_base_active_by_fd() 描述: Activates all pending events for the given fd and event mask. This function activates pending events only. Events which have not been added will not become active. event_base_active_by_signal() 描述:Activates all pending signals with a given signal number. This function activates pending events only. Events which have not been added will not become active. event_base_dispatch() 描述:Event dispatching loop. This loop will run the event base until either there are no more pending or active, or until something calls event_base_loopbreak() or event_base_loopexit(). event_base_loop() 描述:Wait for events to become active, and run their callbacks.This is a more flexible version of event_base_dispatch(). By default, this loop will run the event base until either there are no more pending or active events, or until something calls event_base_loopbreak() or event_base_loopexit(). You can override this behavior with the 'flags' argument.
备注:第二部分，请在下一篇文章查看。
参考文档:
https://libevent.org/doc/event_8h.html#acd7da32086d1e37008e7c76c9ea54fc4
