instance/index.js
function Vue (options) {
 if (process.env.NODE_ENV !== 'production' &&
 !(this instanceof Vue)
 ) {
 warn('Vue is a constructor and should be called with the `new` keyword')
 }
 this._init(options)
}
renderMixin(Vue)

renderMixin
installRenderHelpers( 將一些渲染的工具函數放在Vue 原型上)

Vue.prototype.$nextTick = function (fn: Function) {
 return nextTick(fn, this)
 }

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
 })
 }
}

function flushCallbacks () {
 pending = false
 const copies = callbacks.slice(0)
 callbacks.length = 0
 for (let i = 0; i < copies.length; i++) {
 copies[i]()
 }
}

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

vnode = render.call(vm._renderProxy, vm.$createElement)