# 计算衍生数据
[Reselect](https://github.com/faassen/reselect.git) 是用来创建可记忆的(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/App.js`
~~~
import React, { Component, PropTypes } from 'react';
import { connect } from 'react-redux';
import { addTodo, completeTodo, setVisibilityFilter, VisibilityFilters } from '../actions';
import AddTodo from '../components/AddTodo';
import TodoList from '../components/TodoList';
import Footer from '../components/Footer';
class App extends Component {
render() {
// 通过 connect() 注入：
const { dispatch, visibleTodos, visibilityFilter } = this.props;
return (
<div>
<AddTodo
onAddClick={text =>
dispatch(addTodo(text))
} />
<TodoList
todos={this.props.visibleTodos}
onTodoClick={index =>
dispatch(completeTodo(index))
} />
<Footer
filter={visibilityFilter}
onFilterChange={nextFilter =>
dispatch(setVisibilityFilter(nextFilter))
} />
</div>
);
}
}
App.propTypes = {
visibleTodos: PropTypes.arrayOf(PropTypes.shape({
text: PropTypes.string.isRequired,
completed: PropTypes.bool.isRequired
})),
visibilityFilter: PropTypes.oneOf([
'SHOW_ALL',
'SHOW_COMPLETED',
'SHOW_ACTIVE'
]).isRequired
};
function selectTodos(todos, filter) {
switch (filter) {
case VisibilityFilters.SHOW_ALL:
return todos;
case VisibilityFilters.SHOW_COMPLETED:
return todos.filter(todo => todo.completed);
case VisibilityFilters.SHOW_ACTIVE:
return todos.filter(todo => !todo.completed);
}
}
function select(state) {
return {
visibleTodos: selectTodos(state.todos, state.visibilityFilter),
visibilityFilter: state.visibilityFilter
};
}
// 把组件包起来，以此来注入 dispatch 和 state
export default connect(select)(App);
~~~
上面的示例中，`select` 调用了 `selectTodos` 来计算 `visibleTodos`。运行没问题，但有一个缺点：每当组件更新时都会计算 `visibleTodos`。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 `select`，只在 `state.todos` or `state.visibilityFilter` 变化时重新计算 `visibleTodos`，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 的值的前一次的一样，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。
让我们定义一个可记忆的 selector `visibleTodosSelector` 来替代 `select`：
#### `selectors/TodoSelectors.js`
~~~
import { createSelector } from 'reselect';
import { VisibilityFilters } from './actions';
function selectTodos(todos, filter) {
switch (filter) {
case VisibilityFilters.SHOW_ALL:
return todos;
case VisibilityFilters.SHOW_COMPLETED:
return todos.filter(todo => todo.completed);
case VisibilityFilters.SHOW_ACTIVE:
return todos.filter(todo => !todo.completed);
}
}
const visibilityFilterSelector = (state) => state.visibilityFilter;
const todosSelector = (state) => state.todos;
export const visibleTodosSelector = createSelector(
[visibilityFilterSelector, todosSelector],
(visibilityFilter, todos) => {
return {
visibleTodos: selectTodos(todos, visibilityFilter),
visibilityFilter
};
}
);
~~~
在上例中，`visibilityFilterSelector` 和 `todosSelector` 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，`visibleTodosSelector` 是一个可记忆的 selector。他接收 `visibilityFilterSelector` 和 `todosSelector` 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 `visibleTodosSelector` 被当作另一个 selector 的 input-selector，来进一步通过关键字（keyword）过滤 todos。
~~~
const keywordSelector = (state) => state.keyword;
const keywordFilterSelector = createSelector(
[visibleTodosSelector, keywordSelector],
(visibleTodos, keyword) => visibleTodos.filter(
todo => todo.indexOf(keyword) > -1
)
);
~~~
### 连接 Selector 和 Redux Store
如果你在使用 react-redux，你可以使用 connect 来连接可忘记的 selector 和 Redux store。
#### `containers/App.js`
~~~
import React, { Component, PropTypes } from 'react';
import { connect } from 'react-redux';
import { addTodo, completeTodo, setVisibilityFilter } from '../actions';
import AddTodo from '../components/AddTodo';
import TodoList from '../components/TodoList';
import Footer from '../components/Footer';
import { visibleTodosSelector } from '../selectors/todoSelectors.js';
class App extends Component {
render() {
// 通过 connect() 注入：
const { dispatch, visibleTodos, visibilityFilter } = this.props;
return (
<div>
<AddTodo
onAddClick={text =>
dispatch(addTodo(text))
} />
<TodoList
todos={this.props.visibleTodos}
onTodoClick={index =>
dispatch(completeTodo(index))
} />
<Footer
filter={visibilityFilter}
onFilterChange={nextFilter =>
dispatch(setVisibilityFilter(nextFilter))
} />
</div>
);
}
}
App.propTypes = {
visibleTodos: PropTypes.arrayOf(PropTypes.shape({
text: PropTypes.string.isRequired,
completed: PropTypes.bool.isRequired
})),
visibilityFilter: PropTypes.oneOf([
'SHOW_ALL',
'SHOW_COMPLETED',
'SHOW_ACTIVE'
]).isRequired
};
// 把 selector 传递给连接的组件
export default connect(visibleTodosSelector)(App);
~~~
