计算衍生数据

Reselect 库可以创建可记忆的(Memoized)、可组合的 selector 函数。Reselect selectors 可以用来高效地计算 Redux store 里的衍生数据。

可记忆的 Selectors 初衷

首先访问 Todos 列表示例:

containers/VisibleTodoList.js

import { connect } from 'react-redux'
import { toggleTodo } from '../actions'
import TodoList from '../components/TodoList'

const getVisibleTodos = (todos, filter) => {
  switch (filter) {
    case 'SHOW_ALL':
      return todos
    case 'SHOW_COMPLETED':
      return todos.filter(t => t.completed)
    case 'SHOW_ACTIVE':
      return todos.filter(t => !t.completed)
  }
}

const mapStateToProps = (state) => {
  return {
    todos: getVisibleTodos(state.todos, state.visibilityFilter)
  }
}

const mapDispatchToProps = (dispatch) => {
  return {
    onTodoClick: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}

const VisibleTodoList = connect(
  mapStateToProps,
  mapDispatchToProps
)(TodoList)

export default VisibleTodoList

上面的示例中,mapStateToProps 调用了 getVisibleTodos 来计算 todos。运行没问题,但有一个缺点:每当组件更新时都会重新计算 todos。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。

创建可记忆的 Selector

我们需要一个可记忆的 selector 来替代这个 getVisibleTodos,只在 state.todos or state.visibilityFilter 变化时重新计算 todos,而在其它部分(非相关)变化时不做计算。

Reselect 提供 createSelector 函数来创建可记忆的 selector。createSelector 接收一个 input-selectors 数组和一个转换函数作为参数。如果 state tree 的改变会引起 input-selector 值变化,那么 selector 会调用转换函数,传入 input-selectors 作为参数,并返回结果。如果 input-selectors 的值和前一次的一样,它将会直接返回前一次计算的数据,而不会再调用一次转换函数。

定义一个可记忆的 selector getVisibleTodos 来替代上面的无记忆版本:

selectors/index.js

import { createSelector } from 'reselect'

const getVisibilityFilter = (state) => state.visibilityFilter
const getTodos = (state) => state.todos

export const getVisibleTodos = createSelector(
  [ getVisibilityFilter, getTodos ],
  (visibilityFilter, todos) => {
    switch (visibilityFilter) {
      case 'SHOW_ALL':
        return todos
      case 'SHOW_COMPLETED':
        return todos.filter(t => t.completed)
      case 'SHOW_ACTIVE':
        return todos.filter(t => !t.completed)
    }
  }
)

在上例中,getVisibilityFiltergetTodos 是 input-selector。因为他们并不转换数据,所以被创建成普通的非记忆的 selector 函数。但是,getVisibleTodos 是一个可记忆的 selector。他接收 getVisibilityFiltergetTodos 为 input-selector,还有一个转换函数来计算过滤的 todos 列表。

组合 Selector

可记忆的 selector 自身可以作为其它可记忆的 selector 的 input-selector。下面的 getVisibleTodos 被当作另一个 selector 的 input-selector,来进一步通过关键字(keyword)过滤 todos。

const getKeyword = (state) => state.keyword

const getVisibleTodosFilteredByKeyword = createSelector(
  [ getVisibleTodos, getKeyword ],
  (visibleTodos, keyword) => visibleTodos.filter(
    todo => todo.text.indexOf(keyword) > -1
  )
)

连接 Selector 和 Redux Store

如果你在使用 React Redux,你可以在 mapStateToProps() 中当正常函数来调用 selectors

containers/VisibleTodoList.js

import { connect } from 'react-redux'
import { toggleTodo } from '../actions'
import TodoList from '../components/TodoList'
import { getVisibleTodos } from '../selectors'

const mapStateToProps = (state) => {
  return {
    todos: getVisibleTodos(state)
  }
}

const mapDispatchToProps = (dispatch) => {
  return {
    onTodoClick: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}

const VisibleTodoList = connect(
  mapStateToProps,
  mapDispatchToProps
)(TodoList)

export default VisibleTodoList

在 selectors 中访问 React Props

到目前为止,我们只看到 selector 接收 Redux store state 作为参数,然而,selector 也可以接收 props。

例如,我们来实现包含多个 Todo List 的应用。我们需要改写 state 来支持多个 Todo List,每个 Todo List 分别有各自的 todosvisibilityFilter state。

我们还需要改写 reducer,现在 todosvisibilityFilter 分别在各自的 Todo List state里, 所以我们只需要一个 todoLists reducer 来进行我们的state 管理。

reducers/index.js

import { combineReducers } from 'redux'
import todoLists from './todoLists'

export default combineReducers({
  todoLists
});

reducers/todoLists.js

// Note that we're hard coding three lists here just as an example.
// In the real world, we'd have a feature to add/remove lists,
// and this would be empty initially.
const initialState = {
  1: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  },
  2: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  },
  3: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  }
}

const addTodo = (state, action) => {
  const todoList = state[action.listId]
  const { todos } = todoList

  return {
    ...state,
    [action.listId]: {
      ...todoList,
      todos: [
        ...todos,
        {
          id: action.id,
          text: action.text,
          completed: false
        }
      ]
    }
  }
}

const toggleTodo = (state, action) => {
  const todoList = state[action.listId]
  const { todos } = todoList

  return {
    ...state,
    [action.listId]: {
      ...todoList,
      todos: todos.map(todo =>
        (todo.id === action.id)
          ? {...todo, completed: !todo.completed}
          : todo
      )
    }
  }
}

const setVisibilityFilter = (state, action) => {
  const todoList = state[action.listId]
  return {
    ...state,
    [action.listId]: {
      ...todoList,
      visibilityFilter: action.filter
    }
  }
}

export default const todoLists = (state = initialState, action) => {
  // make sure a list with the given id exists
  if (!state[action.listId]) {
    return state;
  }

  switch (action.type) {
    case 'ADD_TODO':
      return addTodo(state, action)

    case 'TOGGLE_TODO':
      return toggleTodo(state, action)

    case 'SET_VISIBILITY_FILTER':
      return setVisibilityFilter(state, action)

    default:
      return state
  }
}

上面的例子中,我们使用 todoLists reducer 来处理全部三个 action, 所以我们需要向 action creator 传入一个 listId 参数

actions/index.js

let nextTodoId = 0
export const addTodo = (text, listId) => ({
  type: 'ADD_TODO',
  id: nextTodoId++,
  text,
  listId
})export const setVisibilityFilter = (filter, listId) => ({
  type: 'SET_VISIBILITY_FILTER',
  filter,
  listId
})export const toggleTodo = (id, listId) => ({
  type: 'TOGGLE_TODO',
  id,
  listId
})export const VisibilityFilters = {
  SHOW_ALL: 'SHOW_ALL',
  SHOW_COMPLETED: 'SHOW_COMPLETED',
  SHOW_ACTIVE: 'SHOW_ACTIVE'
}

components/TodoList.js

import React from 'react'
import PropTypes from 'prop-types'
import Todo from './Todo'const TodoList = ({ todos, toggleTodo, listId }) => (
  <ul>
    {todos.map(todo =>
      <Todo
        key={todo.id}
        {...todo}
        onClick={() => toggleTodo(todo.id, listId)}
      />
    )}
  </ul>
)

export default TodoList

以下是渲染三个 VisibleTodoList components 的 App , 每个VisibleTodoList 都有一个 listId prop。

components/App.js

import React from 'react'
import Footer from './Footer'
import AddTodo from '../containers/AddTodo'
import VisibleTodoList from '../containers/VisibleTodoList'

const App = () => (
  <div>
    <VisibleTodoList listId="1" />
    <VisibleTodoList listId="2" />
    <VisibleTodoList listId="3" />
  </div>
)

每个 VisibleTodoList 容器根据 listId props 的值选择不同的 state 切片,让我们修改 getVisibilityFiltergetTodos 来接收 props。

selectors/todoSelectors.js

import { createSelector } from 'reselect'

const getVisibilityFilter = (state, props) =>
  state.todoLists[props.listId].visibilityFilter

const getTodos = (state, props) => state.todoLists[props.listId].todos

const getVisibleTodos = createSelector(
  [getVisibilityFilter, getTodos],
  (visibilityFilter, todos) => {
    switch (visibilityFilter) {
      case 'SHOW_COMPLETED':
        return todos.filter(todo => todo.completed)
      case 'SHOW_ACTIVE':
        return todos.filter(todo => !todo.completed)
      default:
        return todos
    }
  }
)

export default getVisibleTodos

props 可以通过 mapStateToProps 传递给 getVisibleTodos:

const mapStateToProps = (state, props) => {
  return {
    todos: getVisibleTodos(state, props)
  }
}

现在,getVisibleTodos 可以访问 props,一切看上去都是如此的美好。

但是这儿有一个问题!

使用带有多个 visibleTodoList 容器实例的 getVisibleTodos selector 不能正常使用函数记忆功能。

containers/VisibleTodoList.js

import { connect } from 'react-redux'
import { toggleTodo } from '../actions'
import TodoList from '../components/TodoList'
import { getVisibleTodos } from '../selectors'

const mapStateToProps = (state, props) => {
  return {
    // 警告:下面的 selector 不会正确记忆
    todos: getVisibleTodos(state, props)
  }
}

const mapDispatchToProps = (dispatch) => {
  return {
    onTodoClick: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}

const VisibleTodoList = connect(
  mapStateToProps,
  mapDispatchToProps
)(TodoList)

export default VisibleTodoList

createSelector 创建的 selector 只有在参数集与之前的参数集相同时才会返回缓存的值。如果我们交替的渲染 VisibleTodoList listId="1" />VisibleTodoList listId="2" />,共享的 selector 将交替的接收 listId: 1listId: 2。这会导致每次调用时传入的参数不同,因此 selector 将始终重新计算而不是返回缓存的值。我们将在下一节了解如何解决这个限制。

跨多组件的共享 Selector

这节中的例子需要 React Redux v4.3.0 或者更高的版本

为了跨越多个 VisibleTodoList 组件共享 selector,同时实现正确记忆。每个组件的实例需要有拷贝 selector 的私有版本。

我们创建一个 makeGetVisibleTodos 的函数,在每个调用的时候返回一个 getVisibleTodos selector 的新拷贝。

selectors/todoSelectors.js

import { createSelector } from 'reselect'

const getVisibilityFilter = (state, props) =>
  state.todoLists[props.listId].visibilityFilter

const getTodos = (state, props) =>
  state.todoLists[props.listId].todos

const makeGetVisibleTodos = () => {
  return createSelector(
    [ getVisibilityFilter, getTodos ],
    (visibilityFilter, todos) => {
      switch (visibilityFilter) {
        case 'SHOW_COMPLETED':
          return todos.filter(todo => todo.completed)
        case 'SHOW_ACTIVE':
          return todos.filter(todo => !todo.completed)
        default:
          return todos
      }
    }
  )
}

我们还需要一种每个容器访问自己私有 selector 的方式。connectmapStateToProps 函数可以帮助我们。

如果 connectmapStateToProps 返回的不是一个对象而是一个函数,他将被用做为每个容器的实例创建一个单独的 mapStateToProps 函数。

下面例子中的 makeMapStateToProps 创建一个新的 getVisibleTodos selectors,返回一个独占新 selector 的权限的 mapStateToProps 函数。

const makeMapStateToProps = () => {
  const getVisibleTodos = makeGetVisibleTodos()
  const mapStateToProps = (state, props) => {
    return {
      todos: getVisibleTodos(state, props)
    }
  }
  return mapStateToProps
}

如果我们通过 makeMapStateToPropsconnectVisibleTodosList 容器的每个组件都会拥有含私有 getVisibleTodos selector 的 mapStateToProps。不论 VisibleTodosList 容器的展现顺序如何,记忆功能都会正常工作。

container/VisibleTodosList.js

import { connect } from 'react-redux'
import { toggleTodo } from '../actions'
import TodoList from '../components/TodoList'
import { makeGetVisibleTodos } from '../selectors'

const makeMapStateToProps = () => {
  const getVisibleTodos = makeGetVisibleTodos()
  const mapStateToProps = (state, props) => {
    return {
      todos: getVisibleTodos(state, props)
    }
  }
  return mapStateToProps
}

const mapDispatchToProps = (dispatch) => {
  return {
    onTodoClick: (id) => {
      dispatch(toggleTodo(id))
    }
  }
}

const VisibleTodoList = connect(
  makeMapStateToProps,
  mapDispatchToProps
)(TodoList)

export default VisibleTodoList

下一步

查看 官方文档FAQ。当因为太多的衍生计算和重复渲染导致出现性能问题时,大多数的 Redux 项目会开始使用 Reselect。所以在你创建一个大型项目的时候确保你对 reselect 是熟悉的。你也可以去研究他的 源码,这样你就不认为他是黑魔法了。

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