event_base_loop是核心代码，用于监听事件以实现实现高性能和高并发的网络服务。 17kk 有空再完善

event_base_loop源码如下：

int event_base_loop(struct event_base *base, int flags)
        {
    const struct eventop *evsel = base->evsel;
    struct timeval tv;
    struct timeval *tv_p;
    int res, done, retval = 0;
    
    EVBASE_ACQUIRE_LOCK(base, th_base_lock);
    if (base->running_loop) {
        event_warnx("%s: reentrant invocation.  Only one event_base_loop"
                                       " can run on each event_base at once.", __func__);
        EVBASE_RELEASE_LOCK(base, th_base_lock);
        return -1;
    }
    base->running_loop = 1;
    clear_time_cache(base);
    if (base->sig.ev_signal_added && base->sig.ev_n_signals_added)
                            evsig_set_base(base);
    done = 0;
    #ifndef _EVENT_DISABLE_THREAD_SUPPORT
                            base->th_owner_id = EVTHREAD_GET_ID();
    #endif
                    base->event_gotterm = base->event_break = 0;
    while (!done) {
        base->event_continue = 0;
        
        if (base->event_gotterm) {
            break;
        }
        if (base->event_break) {
            break;
        }
        timeout_correct(base, &tv);
        tv_p = &tv;
        if (!N_ACTIVE_CALLBACKS(base) && !(flags & EVLOOP_NONBLOCK)) {
            timeout_next(base, &tv_p);
        } else {
            
            evutil_timerclear(&tv);
        }
        
        if (!event_haveevents(base) && !N_ACTIVE_CALLBACKS(base)) {
            event_debug(("%s: no events registered.", __func__));
            retval = 1;
            goto done;
        }
        
        gettime(base, &base->event_tv);
        clear_time_cache(base);
        res = evsel->dispatch(base, tv_p);
        if (res == -1) {
            event_debug(("%s: dispatch returned unsuccessfully.",
                                                __func__));
            retval = -1;
            goto done;
        }
        update_time_cache(base);
        timeout_process(base);
        if (N_ACTIVE_CALLBACKS(base)) {
            int n = event_process_active(base);
            if ((flags & EVLOOP_ONCE)
                                           && N_ACTIVE_CALLBACKS(base) == 0
                                           && n != 0)
                                                done = 1;
        } else if (flags & EVLOOP_NONBLOCK)
                                    done = 1;
    }
    event_debug(("%s: asked to terminate loop.", __func__));
    done:
                clear_time_cache(base);
    base->running_loop = 0;
    EVBASE_RELEASE_LOCK(base, th_base_lock);
    return (retval);
}