# mapStateToProps

`mapStateToProps`是一个函数。它的作用就是像它的名字那样，建立一个从（外部的）`state`对象到（UI 组件的）`props`对象的映射关系。

作为函数，`mapStateToProps`执行后应该返回一个对象，里面的每一个键值对就是一个映射。请看下面的例子。

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

上面代码中，`mapStateToProps`是一个函数，它接受`state`作为参数，返回一个对象。这个对象有一个`todos`属性，代表 UI 组件的同名参数，后面的`getVisibleTodos`也是一个函数，可以从`state`算出 `todos` 的值。

`mapStateToProps`会订阅 Store，每当`state`更新的时候，就会自动执行，重新计算 UI 组件的参数，从而触发 UI 组件的重新渲染。

`connect`方法可以省略`mapStateToProps`参数，那样的话，UI 组件就不会订阅Store，就是说 Store 的更新不会引起 UI 组件的更新。


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