/**
* nextTick
* nextTick是Vue的一个核心实现
*
* JS运行机制
* JS执行是单线程的，它是基于事件循环的。
*
* 事件循环
* 1）所有同步任务都在主线程上执行，形成一个执行栈
* 2）主线程之外，还存在一个“任务队列”。只要异步任务有了运行结果，就在“任务队列”之中放置一个事件。
* 3）一旦“执行栈”中的所有同步任务执行完毕，系统就会读取“任务队列”，看看里面有哪些事件。那些对应的异步任务，
* 于是结束等待状态，进入执行栈，开始执行。
* 4）主线程不断重复上面的第三步。
*/
/**
* nextTick
* src/core/util/next-tick.js
*/
import { noop } from 'shared/util'
import { handleError } from './error'
import { isIOS, isNative } from './env'
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]()
}
}
// Herea 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 hight 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 evnet handlers attached by v-on).
// 分别对应的micro task的函数和macro task 的函数
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 */
/**
* 先监测是否支持原生setImmediate
* 再检查是否支持原生MessageChannel
* 否则setTimeout
*/
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.
*
* 它是对函数做一层包装，确保函数执行过程中对数据任意的修改，触发变化执行nextTick的时候强制走
* macroTimerFunc。比如对于一些DOM交互事件，如v-on绑定的事件回调函数的处理，会强制走macro task。
*/
export function withMacroTask (fn: Function): Function{
return fn._withTask || (fn._withTask = function () {
useMacroTask = true
const res = fn.apply(null, arguments)
useMacroTask = false
return res
})
}
/*
* 把传入的回调函数cb俨如callbacks数组，再根据条件执行宏任务或者微任务
* 在下一个tick执行flushCallbacks
*/
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()
}
}
/**
* 当nextTick不传cb参数的时候，提供一个Promise化的调用
*/
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}
/**
* tick执行flushCallbacks
* 这里使用callbacks而不是直接在nextTick中执行回调函数的原因是保证在同一个tick内多次执行
* nextTick，不会开启多个异步任务，而把这些异步任务都压成一个同步任务，在下一个tick执行完毕。
*/
/**
* 如果我们的某些方法依赖类数据修改后的DOM变化，我们就必须在nextTick后执行
*
* Vue.js 提供类2种调用nextTick的方式。
* 一种是全局API Vue.nextTick 一种是实例上的方法vm.$nextTick
*/
getData(res).then(() => {
this.xxx = res.data
this.$nextTick(() => {
})
})