How to understand Mutations in Vuex? how to use?

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The popular understanding of mutations contains a collection of methods for changing data. This is Vuex A very important point in the design is to put all logical methods for processing data in mutations to separate data and views.

How to use mutations?

Mutation structure: Each mutation has a string type event type (type) and callback function (handler), which can also be understood as {type:handler()}, which is somewhat similar to subscription publishing. Register the event first, and call handker() when the response type is triggered. When calling type, you need to use the store.commit method.

 const store = new Vuex.Store({
    state: {
        count: 1
        },
    mutations: {
   		 increment (state) {      //注册事件，type:increment，handler第一个参数是state；
  		  // 变更状态
   		state.count++}}})
   store.commit('increment')   //调用type，触发handler（state）

Load (payload): The simple understanding is to pass the parameter handler(stage, payload) to handler(stage); usually it is an object.

  mutations: {
 increment (state, n) {
     state.count += n}}
 store.commit('increment', 10)

mutation-types: Place constants in separate files to facilitate collaborative development.

mutation-types.js

export const SOME_MUTATION = 'SOME_MUTATION'

store.js

import Vuex from 'vuex'
import { SOME_MUTATION } from './mutation-types'
  const store = new Vuex.Store({
    state: { ... },
    mutations: {
     // 我们可以使用 ES2015 风格的计算属性命名功能来使用一个常量作为函数名
    [SOME_MUTATION] (state) {
    // mutate state
  }
}
})

commit: Commit can be used in components this.$store.commit('xxx') Submit mutations, or use the mapMutations auxiliary function to map the methods in the component to store.commit calls (store needs to be injected into the root node).

import { mapMutations } from 'vuex'
export default {
methods: {
  ...mapMutations([
    'increment' // 映射 this.increment() 为 this.$store.commit('increment')]),
  ...mapMutations({
    add: 'increment' // 映射 this.add() 为 this.$store.commit('increment')
  })}}

Source code analysis

function registerMutation (store, type, handler, path = []) {
 //4个参数，store是Store实例，type为mutation的type，handler，path为当前模块路径
    const entry = store._mutations[type] || (store._mutations[type] = 
[])  //通过type拿到对应的mutation对象数组
     entry.push(function wrappedMutationHandler (payload) {
     //将mutation包装成函数push到数组中，同时添加载荷payload参数    
     handler(getNestedState(store.state, path), payload)
     //通过getNestedState()得到当前的state，同时添加载荷payload参数
   })
 }

commit: call mutation

commit (type, payload, options) {
  // 3个参数，type是mutation类型，payload载荷，options配置
    if (isObject(type) && type.type) {
       // 当type为object类型，
      options = payload
      payload = type
      type = type.type
  }
 const mutation = { type, payload }
 const entry = this._mutations[type]
   // 通过type查找对应的mutation
 if (!entry) {
  //找不到报错
   console.error(`[vuex] unknown mutation type: ${type}`)
   return
 }
 this._withCommit(() => {
   entry.forEach(function commitIterator (handler) {
   // 遍历type对应的mutation对象数组，执行handle（payload）方法
   //也就是开始执行wrappedMutationHandler(handler)
     handler(payload)
   })
 })
 if (!options || !options.silent) {
   this._subscribers.forEach(sub => sub(mutation, this.state))
    //把mutation和根state作为参数传入
 }
}

subscribers: subscribe to store’s mutation

subscribe (fn) {
const subs = this._subscribers
if (subs.indexOf(fn) < 0) {
  subs.push(fn)
  }
return () => {
  const i = subs.indexOf(fn)
  if (i > -1) {
    subs.splice(i, 1)
    }
  }
 }

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