Redux: Reducer Composition

20th August 2020 at 2:19pm
Redux: Patterns and Recipes

大 reducer 可以拆分成小 reducer 并组合起来。比如对于教程中的 To-do app,大 reducer 如下:

function todoApp(state = initialState, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return Object.assign({}, state, {
        visibilityFilter: action.filter
      })
    case ADD_TODO:
      return Object.assign({}, state, {
        todos: [
          ...state.todos,
          {
            text: action.text,
            completed: false
          }
        ]
      })
    case TOGGLE_TODO:
      return Object.assign({}, state, {
        todos: state.todos.map((todo, index) => {
          if (index === action.index) {
            return Object.assign({}, todo, {
              completed: !todo.completed
            })
          }
          return todo
        })
      })
    default:
      return state
  }
}

拆分成小 reducer 后:

function todos(state = [], action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        ...state,
        {
          text: action.text,
          completed: false
        }
      ]
    case TOGGLE_TODO:
      return state.map((todo, index) => {
        if (index === action.index) {
          return Object.assign({}, todo, {
            completed: !todo.completed
          })
        }
        return todo
      })
    default:
      return state
  }
}

function visibilityFilter(state = SHOW_ALL, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return action.filter
    default:
      return state
  }
}

function todoApp(state = {}, action) {
  return {
    visibilityFilter: visibilityFilter(state.visibilityFilter, action),
    todos: todos(state.todos, action)
  }
}

注意 todos 接受的 state 不再是带 visibilityFilter 的整个应用的 state 数据,而只有其中的 todos 部分。同时观察 todoApp 在这个过程中的变化。

Redux 提供了一个工具函数 combineReducers() 可以完成跟上述 todoApp 一样的功能:

import { combineReducers } from 'redux'

const todoApp = combineReducers({
  visibilityFilter,
  todos
})

各类 Redux 框架(如 react-redux)或者 helper 可能提供了一些更易用的 API,使你不用自己关心这个拆分。