vue监听数组变化的源码inserted = args.slice(2)，为什么要slice(2)?

Question

/*

• not type checking this file because flow doesn't play well with
• dynamically accessing methods on Array prototype
  */

import { def } from '../util/index'

const arrayProto = Array.prototype
export const arrayMethods = Object.create(arrayProto)

const methodsToPatch = [
'push',
'pop',
'shift',
'unshift',
'splice',
'sort',
'reverse'
]

/**

• Intercept mutating methods and emit events
  */

methodsToPatch.forEach(function (method) {
// cache original method
const original = arrayProto[method]
def(arrayMethods, method, function mutator (...args) {

const result = original.apply(this, args)
const ob = this.__ob__
let inserted
switch (method) {
  case 'push':
  case 'unshift':
    inserted = args
    break
  case 'splice':
    inserted = args.slice(2)
    break
}
if (inserted) ob.observeArray(inserted)
// notify change
ob.dep.notify()
return result

})
})

这里的inserted = args.slice(2)，为什么要slice(2)?

Answer 1

splice 前俩个参数是被替换的位置和长度，第三个参数开始是新增的被插入的值。
做响应式监听的时候，主要是为了对新增加的值做监听，所以拿到slice(2) 就拿到了新增加的数据项，执行ob.observeArray(inserted)就可以了