当启动Node.js的时候,会初始化事件循环(event loop),事件循环包含6个阶段:
libuv中的源码:
int uv_run(uv_loop_t* loop, uv_run_mode mode) {
int timeout;
int r;
int ran_pending;
r = uv__loop_alive(loop);
if (!r)
uv__update_time(loop);
while (r != 0 && loop->stop_flag == 0) {
uv__update_time(loop);
uv__run_timers(loop);
ran_pending = uv__run_pending(loop);
uv__run_idle(loop);
uv__run_prepare(loop);
timeout = 0;
if ((mode == UV_RUN_ONCE && !ran_pending) || mode == UV_RUN_DEFAULT)
timeout = uv_backend_timeout(loop);
uv__io_poll(loop, timeout);
uv__run_check(loop);
uv__run_closing_handles(loop);
if (mode == UV_RUN_ONCE) {
/* UV_RUN_ONCE implies forward progress: at least one callback must have
* been invoked when it returns. uv__io_poll() can return without doing
* I/O (meaning: no callbacks) when its timeout expires - which means we
* have pending timers that satisfy the forward progress constraint.
*
* UV_RUN_NOWAIT makes no guarantees about progress so it's omitted from
* the check.
*/
uv__update_time(loop);
uv__run_timers(loop);
}
r = uv__loop_alive(loop);
if (mode == UV_RUN_ONCE || mode == UV_RUN_NOWAIT)
break;
}
/* The if statement lets gcc compile it to a conditional store. Avoids
* dirtying a cache line.
*/
if (loop->stop_flag != 0)
loop->stop_flag = 0;
return r;
}
- timer阶段:对应的是源码中的uv_run_timer(loop)方法,主要执行setTimeout()和setInterval()方法中到期的callback;
- pending callbacks(I/O)阶段:对应的是源码中的uv_run_pending(loop)方法,主要是执行推迟到下一循环I/O操作的callbacks;
- idle,prepare阶段:对应源码中的uv_run_idle(loop)和uv_run_prepare(loop)方法,主要是Node自己内部使用(不用考虑);
- poll阶段:对应源码中的uv_io_poll(loop,timeout)方法,检索新的I/O事件;执行I/O相关的callbacks;除此之外,poll还负责检测是否有timer的callback到达时间,并且还未执行那么就循环至timer阶段执行timer的callback;
- check阶段:对应源码中的uv_run_check(loop)方法,主要执行setImmediate()方法的callback;
- close callbacks阶段:对应源码中的uv_run_closing_handles(loop)方法,主要执行一些close事件的callback.
const {readFile} = require('fs');
const {resolve} = require('path');
setImmediate(() => console.log(`[check阶段:] setImmediate 回调 1`));
setTimeout(() => console.log(`[timer阶段:] setTimeout 回调 1`), 0);
setTimeout(() => console.log(`[timer阶段:] setTimeout 回调 2`), 0);
setImmediate(() => console.log(`[check阶段:] setImmediate 回调 2`));
setTimeout(() => console.log(`[timer阶段:] setTimeout 回调 3`), 0);
readFile(resolve(__dirname, './exec.js'), 'utf-8', data => {
console.log(`[I/O阶段] readFile 回调 1`);
});
Promise.resolve()
.then(() => {
console.log(`[...待切入下一阶段] Promise 回调 1`);
console.log(`[...待切入下一阶段] Promise 回调 1`);
console.log(`[...待切入下一阶段] Promise 回调 1`);
console.log(`[...待切入下一阶段] Promise 回调 1`);
console.log(`[...待切入下一阶段] Promise 回调 1`);
})
.then(() => {
console.log(`[...待切入下一阶段] Promise 回调 3`);
console.log(`[...待切入下一阶段] Promise 回调 3`);
console.log(`[...待切入下一阶段] Promise 回调 3`);
});
process.nextTick(() => console.log(`[...待切入下一阶段] nextTick 回调 1`));
process.nextTick(() => {
console.log(`[...待切入下一阶段] nextTick 回调 2`);
Promise.resolve()
.then(() => {
console.log(`[...待切入下一阶段] nextTick中的Promise 回调 2`);
setImmediate(() => console.log(`[check阶段:] nextTick中的setImmediate 回调 5`));
setTimeout(() => console.log(`[timer阶段:] nextTick中的setTimeout 回调 5`), 0);
})
.then(() => {
console.log(`[...待切入下一阶段] nextTick中的Promise 回调 4`);
process.nextTick(() => console.log(`[...待切入下一阶段] nextTick中的Promise中的nextTick 回调 3`));
})
});
运行结果:
[...待切入下一阶段] nextTick 回调 1
[...待切入下一阶段] nextTick 回调 2
[...待切入下一阶段] Promise 回调 1
[...待切入下一阶段] Promise 回调 1
[...待切入下一阶段] Promise 回调 1
[...待切入下一阶段] Promise 回调 1
[...待切入下一阶段] Promise 回调 1
[...待切入下一阶段] nextTick中的Promise 回调 2
[...待切入下一阶段] Promise 回调 3
[...待切入下一阶段] Promise 回调 3
[...待切入下一阶段] Promise 回调 3
[...待切入下一阶段] nextTick中的Promise 回调 4
[...待切入下一阶段] nextTick中的Promise中的nextTick 回调 3
[timer阶段:] setTimeout 回调 1
[timer阶段:] setTimeout 回调 2
[timer阶段:] setTimeout 回调 3
[timer阶段:] nextTick中的setTimeout 回调 5
[check阶段:] setImmediate 回调 1
[check阶段:] setImmediate 回调 2
[check阶段:] nextTick中的setImmediate 回调 5
[I/O阶段] readFile 回调 1
参考文章：
https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick/