目前为止，自己只在指令中使用到这个方法，贴下源码解读。
代码语言：javascript复制"use strict";
var vueFocus = {}
vueFocus.config = {}
vueFocus.install = function(Vue) {
Vue.directive('focus',{
bind: function(el,binding) {
},
update: function(el,binding) {
if(binding.value && binding.value.flag) {
Vue.nextTick(function () {
el.focus();
//el.value = binding.value.value //使用后会无法编辑input
});
}
}
})
}
module.exports = vueFocus;源码部分：其实源码上的注释已经讲的比较明白了,首先返回一个闭包函数，它会将 nextTick 传入的函数 缓存进 callback 的数组中，如果没有其它函数
代码语言：javascript复制/**
* Defer a task to execute it asynchronously.
*
*/
export const nextTick = (function () {
const callbacks = [] //缓存回调数组
let pending = false //是否正在执行flag
let timerFunc //要执行的函数
function nextTickHandler () {
pending = false
const copies = callbacks.slice(0) //复制函数
callbacks.length = 0
for (let i = 0; i < copies.length; i ) {
copies[i]() //依次执行
}
}
// 此处做了兼容处理，首先调用promise.then 来达到延迟调用
// 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 (typeof MutationObserver !== 'undefined' && ( //其次使用 html5 新增的 MutationObserver 来监听 DOM 操作结束
isNative(MutationObserver) ||
// PhantomJS and iOS 7.x
MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
// use MutationObserver where native Promise is not available,
// e.g. PhantomJS IE11, 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 { //利用setTimeout,setTimeout(func, 0)会将func函数延迟到下一次函数调用栈的开始，也就是当前函数执行完毕后再执行该函数，因此完成了延迟功能。
// 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) { //如果没有函数在执行，则进行下一步，执行传入的函数
pending = true
timerFunc()
}
if (!cb && typeof Promise !== 'undefined') { //如果不传函数，则可以使用 $nextTick().then()的这种方法
return new Promise((resolve, reject) => {
_resolve = resolve
})
}
}
})()
