react 计算衍生数据

import React from 'react'
import { connect } from 'react-redux'
import TodoList from '../components/TodoList'
import {toggleTodo,VisibilityFilters} from '../actions/index'
import { createSelector } from 'reselect'
const getVisibleTodos = (todos,filter)=>{
  switch (filter) {
    case 'SHOW_ALL':
      return todos
      break;
    case 'SHOW_COMPLETED':
      return todos.filter((todo)=>todo.completed)
      break;
    case 'SHOW_ACTIVE':
      return todos.filter(t=>!t.completed)
      break;
    default:
      throw new Error('Unknown filter: ' + filter)
  }
}
const mapStateToProps = (state, ownProps) => {
  console.log(state)
  return {
     todos: getVisibleTodos(state.todos,state.visibilityFilter)
  }
}
const mapDispatchToProps = (dispatch, ownProps) => {
  return {
    toggleTodo: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}
export default connect(mapStateToProps,mapDispatchToProps)(TodoList)

 从上面的例子可以看出每次组件更新时 todos列表都会被从新计算，如果 state tree 非常大，或者计算量非常大，每次更新都重新计算可能会带来性能问题。Reselect 能帮你省去这些没必要的重新计算。

import React from 'react'
import { connect } from 'react-redux'
import TodoList from '../components/TodoList'
import {toggleTodo,VisibilityFilters} from '../actions/index'
import { createSelector } from 'reselect'
const getTodos = (state)=>state.todos;
const visibilityFilter = (state)=>state.visibilityFilter

//计算衍生数据

createSelector接受二个参数 第一个参数为input-select 意思是当那些状态变更时才会从新计算todos列表 是一个数组函数。
第二个参数是一个函数 参数是：将前一个参数的返回值作为参数

const getVisibleTodos = createSelector(
  [getTodos,visibilityFilter],(todos,filter)=>{
    switch (filter) {
      case 'SHOW_ALL':
        return todos
        break;
      case 'SHOW_COMPLETED':
        return todos.filter((todo)=>todo.completed)
        break;
      case 'SHOW_ACTIVE':
        return todos.filter(t=>!t.completed)
        break;
      default:
        throw new Error('Unknown filter: ' + filter)
  }
})

const mapStateToProps = (state, ownProps) => {
  console.log(state)
  return {
    // todos: getVisibleTodos(state.todos,state.visibilityFilter)
    todos:getVisibleTodos(state)
  }
}
const mapDispatchToProps = (dispatch, ownProps) => {
  return {
    toggleTodo: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}
export default connect(mapStateToProps,mapDispatchToProps)(TodoList)