Vue 的 nextTick 函数大家都不陌生，做什么的：
将注入的回调函数，以微任务的形式放到本次事件循环队列的末尾上去，如果不支持就放到下一次事件队列中
本文以 vue 2.x 最后一个版本(2.6.14)讲解源码
第一步: 创建flushCallbacks回调函数执行器
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]()
}
}
callbacks用于存储$nextTick传入的函数
flushCallbacks用于遍历执行回调函数
第二步: 创建微任务包装器，并判断浏览器支持的微任务API赋予timerFunc
// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc
由vue源码注释第一句，意思是：这里我们有使用微任务的异步延迟包装器。
- 先判断浏览器是否支持Promise，如果支持，则使用Promise来实现
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
timerFunc = () => {
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)
}
isUsingMicroTask = true
}
此处 Promise.resolve()
等价于 new Promise(resolve => resolve())
then函数注册回调
- 在判断浏览器是否支持MutationObserver，如果支持，则使用MutationObserver来实现
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
// (#6466 MutationObserver is unreliable in IE11)
let counter = 1
const observer = new MutationObserver(flushCallbacks)
const textNode = document.createTextNode(String(counter))
observer.observe(textNode, {
characterData: true
})
timerFunc = () => {
counter = (counter + 1) % 2
textNode.data = String(counter)
}
isUsingMicroTask = true
}
MutationObserver API在MDN描述是：
提供了监视对DOM树所做更改的能力，它会在指定的DOM发生变化时被调用。
vue是创建一个文本节点，通过监听文本节点变化来执行MutationObserver的注册函数调用
至于vue这么做的原理是什么，和MutationObserver监听DOM的变化和微任务又有什么联系？
请看我对MutationObserver的详解（编写中）
- 如果浏览器对上述微任务API都不支持，则使用setImmediate/setTimeout来异步执行回调
else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
// Fallback to setImmediate.
// Technically it leverages the (macro) task queue,
// but it is still a better choice than setTimeout.
timerFunc = () => {
setImmediate(flushCallbacks)
}
} else {
// Fallback to setTimeout.
timerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}
vue 源码说setImmediate是一个更好的选择相比setTimeout，因为性能高于setTimeout
但setImmediate目前只有最新版本的 Internet Explorer 和Node.js 0.10+实现了该方法
- 判断出浏览器支持的API后，最后一步便是执行
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
timerFunc()
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}
导出nextTick方法，可接受两个参数cb是回调函数，ctx执行上下文
首先判断cb是否存在，存在则将绑定了ctx的回调函数push入callbacks中
pending默认值false，然后调用timerFunc微任务包装器来执行我们传入的回调
总结
- $nextTick微任务包装器的shim降级顺序：
Promise > MutationObserver > setImmediate > setTimeout
(Tips: 尤大在2.x多个版本中多次改变shim的顺序，最终在2.x latest版本中定为此顺序)
