“滑稽”的事件循环

最近在写 Vue 时遇到了一个问题,大体场景是:有一个组件 A 用于对话框中,也可用在对话框之外。A 初始化后会请求数据,当它用于对话框中时,可能会接收外部传入的参数,需要根据这些参数去请求数据。

我们知道大部分 Dialog 组件在实现时,只有当对话框首次被打开时才会去渲染其内部的 content 部分。于是问题是:当组件 A 处于对话框中,并且首次打开对话框时,A 先被初始化,然后对话框向 A 透传外部传入的参数,由 A 发起数据请求。

事故现场

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export const A = {
methods: {
reload() {
// 省略...请求数据
},
receiveContext({ params, data } = {}) {
// 省略...
this.reload();
},
},
mounted() {
// this.reload();
nextTick(() => this.reload(), 0);
}
};

A 的 mounted 先被触发,调用 reload 函数;接着外部开始调用 receiveContext,再次调用 reload,也就出现了在对话框首次打开时会重复请求。

P.S. 别问我为什么不用 props 透传,因为我用 dispatch action 触发 receiveContext

无论我在哪儿怎么加 nextTick,都避免不了在第一次渲染时重复请求的问题。于是发现自己对 nextTick 的本质还没搞透。

修复后

先直接给出修复后的代码,再慢慢探究原因。

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export const A = {
methods: {
reload() {
// NOTE: 防止初次渲染 receiveContext 后触发 2次 reload
if (this._macroTimer) {
clearTimeout(this._macroTimer);
this._macroTimer = null;
}
// 省略...请求数据
},
receiveContext({ params, data } = {}) {
// 省略...
this.reload();
},
},
mounted() {
// NOTE: setTimeout 为 macrotask, 可以保证在 nextTick (microtask) 之后执行
this._macroTimer = setTimeout(() => this.reload(), 0);
}
};

Event Loop

都知道 js 是单线程的,为什么要设计成单线程,读者不妨可以思考一下🤔

同时 js 又是非阻塞的,这就是 Event Loop 的功劳了。

  1. 主线程运行的时候会生成堆(heap)和栈(stack);
  2. js 从上到下解析方法,将其中的同步任务按照执行顺序排列到执行栈中;
  3. 当程序调用外部的API时,比如 ajax、setTimeout 等,会将此类异步任务挂起,继续执行执行栈中的任务,等异步任务返回结果后,再按照执行顺序排列到事件队列中;
  4. 主线程先将执行栈中的同步任务清空,然后检查事件队列中是否有任务,如果有,就将第一个事件对应的回调推到执行栈中执行,若在执行过程中遇到异步任务,则继续将这个异步任务排列到事件队列中。
  5. 主线程每次将执行栈清空后,就去事件队列中检查是否有任务,如果有,就每次取出一个推到执行栈中执行,这个过程是循环往复的,就叫做 Event Loop 事件循环。

macro task + micro task

在 Event Loop 为了区分异步任务的执行优先级,js 设计出了 macro task (宏任务) 与 micro task (微任务) 这两个概念。

常见的 task 有

  • macro task: setTimeout,setInterval,setImmediate,I/O (磁盘读写或网络通信),UI 交互事件

  • micro task: process.nextTick,Promise.then

事件循环会将遇到的异步任务排列到对应的 macro task 及 micro task 队列中:

  • 当执行栈中的任务清空,主线程会先检查 micro task 队列中是否有任务,如果有,就将 micro task 队列中的任务依次执行,直到队列为空;
  • 然后再检查 macro task 队列中是否有任务,如果有,则每次取出【第一个】macro task 加入到执行栈中;
  • 再次清空执行栈,重新检查微任务(重复第一步)

测试代码

test 1

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console.log(1);
setTimeout(() => {
console.log(6);
}, 0);
new Promise(resolve => {
console.log(2);
resolve();
}).then(() => {
console.log(4);
}).then(() => {
console.log(5);
});
console.log(3);

test 2

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console.log(1);
setTimeout(() => {
console.log(5);
}, 0);
new Promise(resolve => {
console.log(2);
resolve();
}).then(() => {
setTimeout(() => console.log(6), 0);
}).then(() => {
console.log(4);
});
console.log(3);

test 3

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console.log(1);
setTimeout(() => {
Promise.resolve().then(() => console.log(5));
}, 0);
new Promise(resolve => {
console.log(2);
resolve();
}).then(() => {
setTimeout(() => console.log(6), 0);
}).then(() => {
console.log(4);
});
console.log(3);

test 4

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console.log(1);
setTimeout(() => {
Promise.resolve().then(() => console.log(6));
}, 0);
new Promise(resolve => {
console.log(2);
resolve();
}).then(() => {
Promise.resolve().then(() => console.log(4), 0);
}).then(() => {
console.log(5);
});
console.log(3);

以上所有示例都按 1 2 3 4 5 6 的顺序输出。

最后一例中,关于 45 的输出顺序,还是有点“滑稽”。

Vue nextTick

v2.4.4

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/**
* Defer a task to execute it asynchronously.
*/
export const nextTick = (function () {
const callbacks = []
let pending = false
let timerFunc
function nextTickHandler () {
pending = false
const copies = callbacks.slice(0)
callbacks.length = 0
for (let i = 0; i < copies.length; i++) {
copies[i]()
}
}
// the nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore if */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
var p = Promise.resolve()
var logError = err => { console.error(err) }
timerFunc = () => {
p.then(nextTickHandler).catch(logError)
// in problematic UIWebViews, Promise.then doesn't completely break, but
// it can get stuck in a weird state where callbacks are pushed into the
// microtask queue but the queue isn't being flushed, until the browser
// needs to do some other work, e.g. handle a timer. Therefore we can
// "force" the microtask queue to be flushed by adding an empty timer.
if (isIOS) setTimeout(noop)
}
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
isNative(MutationObserver) ||
// PhantomJS and iOS 7.x
MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
// use MutationObserver where native Promise is not available,
// e.g. PhantomJS, iOS7, Android 4.4
var counter = 1
var observer = new MutationObserver(nextTickHandler)
var textNode = document.createTextNode(String(counter))
observer.observe(textNode, {
characterData: true
})
timerFunc = () => {
counter = (counter + 1) % 2
textNode.data = String(counter)
}
} else {
// fallback to setTimeout
/* istanbul ignore next */
timerFunc = () => {
setTimeout(nextTickHandler, 0)
}
}
return function queueNextTick (cb?: Function, ctx?: Object) {
let _resolve
callbacks.push(() => {
if (cb) {
try {
cb.call(ctx)
} catch (e) {
handleError(e, ctx, 'nextTick')
}
} else if (_resolve) {
_resolve(ctx)
}
})
if (!pending) {
pending = true
timerFunc()
}
if (!cb && typeof Promise !== 'undefined') {
return new Promise((resolve, reject) => {
_resolve = resolve
})
}
}
})()

可以看到,Vue 优先使用 Promise.resolve() 来实现 nextTick,对于不支持 Promise 的设备则使用 MutationObserver,再次之则降级为 setTimeout

v2.5.17

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const callbacks = []
let pending = false
function flushCallbacks () {
pending = false
const copies = callbacks.slice(0)
callbacks.length = 0
for (let i = 0; i < copies.length; i++) {
copies[i]()
}
}
// Here we have async deferring wrappers using both microtasks and (macro) tasks.
// In < 2.4 we used microtasks everywhere, but there are some scenarios where
// microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690) or even between bubbling of the same
// event (#6566). However, using (macro) tasks everywhere also has subtle problems
// when state is changed right before repaint (e.g. #6813, out-in transitions).
// Here we use microtask by default, but expose a way to force (macro) task when
// needed (e.g. in event handlers attached by v-on).
let microTimerFunc
let macroTimerFunc
let useMacroTask = false
// Determine (macro) task defer implementation.
// Technically setImmediate should be the ideal choice, but it's only available
// in IE. The only polyfill that consistently queues the callback after all DOM
// events triggered in the same loop is by using MessageChannel.
/* istanbul ignore if */
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
macroTimerFunc = () => {
setImmediate(flushCallbacks)
}
} else if (typeof MessageChannel !== 'undefined' && (
isNative(MessageChannel) ||
// PhantomJS
MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
const channel = new MessageChannel()
const port = channel.port2
channel.port1.onmessage = flushCallbacks
macroTimerFunc = () => {
port.postMessage(1)
}
} else {
/* istanbul ignore next */
macroTimerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}
// Determine microtask defer implementation.
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
microTimerFunc = () => {
p.then(flushCallbacks)
// in problematic UIWebViews, Promise.then doesn't completely break, but
// it can get stuck in a weird state where callbacks are pushed into the
// microtask queue but the queue isn't being flushed, until the browser
// needs to do some other work, e.g. handle a timer. Therefore we can
// "force" the microtask queue to be flushed by adding an empty timer.
if (isIOS) setTimeout(noop)
}
} else {
// fallback to macro
microTimerFunc = macroTimerFunc
}
/**
* Wrap a function so that if any code inside triggers state change,
* the changes are queued using a (macro) task instead of a microtask.
*/
export function withMacroTask (fn: Function): Function {
return fn._withTask || (fn._withTask = function () {
useMacroTask = true
const res = fn.apply(null, arguments)
useMacroTask = false
return res
})
}
export function nextTick (cb?: Function, ctx?: Object) {
let _resolve
callbacks.push(() => {
if (cb) {
try {
cb.call(ctx)
} catch (e) {
handleError(e, ctx, 'nextTick')
}
} else if (_resolve) {
_resolve(ctx)
}
})
if (!pending) {
pending = true
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}

【参考资料】

http://hcysun.me/vue-design/art/8vue-reactive-dep-watch.html#nexttick-的实现

https://developer.mozilla.org/zh-CN/docs/Web/API/MutationObserver