前两篇教程介绍了 Redux 的基本用法和异步操作,今天是最后一部分,介绍如何在 React 项目中使用 Redux。
为了方便使用,Redux 的作者封装了一个 React 专用的库 React-Redux,本文主要介绍它。
这个库是可以选用的。实际项目中,你应该权衡一下,是直接使用 Redux,还是使用 React-Redux。后者虽然提供了便利,但是需要掌握额外的 API,并且要遵守它的组件拆分规范。
一、UI 组件
React-Redux 将所有组件分成两大类:UI 组件(presentational component)和容器组件(container component)。
UI 组件有以下几个特征。
- 只负责 UI 的呈现,不带有任何业务逻辑
- 没有状态(即不使用
this.state
这个变量)- 所有数据都由参数(
this.props
)提供- 不使用任何 Redux 的 API
下面就是一个 UI 组件的例子。
const Title <span class="token operator">=</span> value <span class="token operator">=</span><span class="token operator">></span> <span class="token operator"><</span>h1<span class="token operator">></span><span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token operator"><</span><span class="token operator">/</span>h1<span class="token operator">></span><span class="token punctuation">;</span>
因为不含有状态,UI 组件又称为”纯组件”,即它纯函数一样,纯粹由参数决定它的值。
二、容器组件
容器组件的特征恰恰相反。
- 负责管理数据和业务逻辑,不负责 UI 的呈现
- 带有内部状态
- 使用 Redux 的 API
总之,只要记住一句话就可以了:UI 组件负责 UI 的呈现,容器组件负责管理数据和逻辑。
你可能会问,如果一个组件既有 UI 又有业务逻辑,那怎么办?回答是,将它拆分成下面的结构:外面是一个容器组件,里面包了一个UI 组件。前者负责与外部的通信,将数据传给后者,由后者渲染出视图。
React-Redux 规定,所有的 UI 组件都由用户提供,容器组件则是由 React-Redux 自动生成。也就是说,用户负责视觉层,状态管理则是全部交给它。
三、connect()
React-Redux 提供connect
方法,用于从 UI 组件生成容器组件。connect
的意思,就是将这两种组件连起来。
import <span class="token punctuation">{</span> connect <span class="token punctuation">}</span> from <span class="token string">'react-redux'</span> const VisibleTodoList <span class="token operator">=</span> <span class="token function">connect<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">(</span>TodoList<span class="token punctuation">)</span><span class="token punctuation">;</span>
上面代码中,TodoList
是 UI 组件,VisibleTodoList
就是由 React-Redux 通过connect
方法自动生成的容器组件。
但是,因为没有定义业务逻辑,上面这个容器组件毫无意义,只是 UI 组件的一个单纯的包装层。为了定义业务逻辑,需要给出下面两方面的信息。
(1)输入逻辑:外部的数据(即
state
对象)如何转换为 UI 组件的参数(2)输出逻辑:用户发出的动作如何变为 Action 对象,从 UI 组件传出去。
因此,connect
方法的完整 API 如下。
import <span class="token punctuation">{</span> connect <span class="token punctuation">}</span> from <span class="token string">'react-redux'</span> const VisibleTodoList <span class="token operator">=</span> <span class="token function">connect<span class="token punctuation">(</span></span> mapStateToProps<span class="token punctuation">,</span> mapDispatchToProps <span class="token punctuation">)</span><span class="token punctuation">(</span>TodoList<span class="token punctuation">)</span>
上面代码中,connect
方法接受两个参数:mapStateToProps
和mapDispatchToProps
。它们定义了 UI 组件的业务逻辑。前者负责输入逻辑,即将state
映射到 UI 组件的参数(props
),后者负责输出逻辑,即将用户对 UI 组件的操作映射成 Action。
四、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。
const mapStateToProps <span class="token operator">=</span> <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> todos<span class="token punctuation">:</span> <span class="token function">getVisibleTodos<span class="token punctuation">(</span></span>state<span class="token punctuation">.</span>todos<span class="token punctuation">,</span> state<span class="token punctuation">.</span>visibilityFilter<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span>
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。
下面就是getVisibleTodos
的一个例子,用来算出todos
。
const getVisibleTodos <span class="token operator">=</span> <span class="token punctuation">(</span>todos<span class="token punctuation">,</span> filter<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> switch <span class="token punctuation">(</span>filter<span class="token punctuation">)</span> <span class="token punctuation">{</span> case <span class="token string">'SHOW_ALL'</span><span class="token punctuation">:</span> <span class="token keyword">return</span> todos case <span class="token string">'SHOW_COMPLETED'</span><span class="token punctuation">:</span> <span class="token keyword">return</span> todos<span class="token punctuation">.</span><span class="token function">filter<span class="token punctuation">(</span></span>t <span class="token operator">=</span><span class="token operator">></span> t<span class="token punctuation">.</span>completed<span class="token punctuation">)</span> case <span class="token string">'SHOW_ACTIVE'</span><span class="token punctuation">:</span> <span class="token keyword">return</span> todos<span class="token punctuation">.</span><span class="token function">filter<span class="token punctuation">(</span></span>t <span class="token operator">=</span><span class="token operator">></span> <span class="token operator">!</span>t<span class="token punctuation">.</span>completed<span class="token punctuation">)</span> default<span class="token punctuation">:</span> <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">'Unknown filter: '</span> <span class="token operator">+</span> filter<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span>
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。
mapStateToProps
的第一个参数总是state
对象,还可以使用第二个参数,代表容器组件的props
对象。
<span class="token comment" spellcheck="true"> // 容器组件的代码 </span><span class="token comment" spellcheck="true">// <FilterLink filter="SHOW_ALL"> </span><span class="token comment" spellcheck="true">// All </span><span class="token comment" spellcheck="true">// </FilterLink> </span> const mapStateToProps <span class="token operator">=</span> <span class="token punctuation">(</span>state<span class="token punctuation">,</span> ownProps<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> active<span class="token punctuation">:</span> ownProps<span class="token punctuation">.</span>filter <span class="token operator">===</span> state<span class="token punctuation">.</span>visibilityFilter <span class="token punctuation">}</span> <span class="token punctuation">}</span>
使用ownProps
作为参数后,如果容器组件的参数发生变化,也会引发 UI 组件重新渲染。
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。
五、mapDispatchToProps()
mapDispatchToProps
是connect
函数的第二个参数,用来建立 UI 组件的参数到store.dispatch
方法的映射。也就是说,它定义了哪些用户的操作应该当作 Action,传给 Store。它可以是一个函数,也可以是一个对象。
如果mapDispatchToProps
是一个函数,会得到dispatch
和ownProps
(容器组件的props
对象)两个参数。
const mapDispatchToProps <span class="token operator">=</span> <span class="token punctuation">(</span> dispatch<span class="token punctuation">,</span> ownProps <span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> onClick<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> <span class="token function">dispatch<span class="token punctuation">(</span></span><span class="token punctuation">{</span> type<span class="token punctuation">:</span> <span class="token string">'SET_VISIBILITY_FILTER'</span><span class="token punctuation">,</span> filter<span class="token punctuation">:</span> ownProps<span class="token punctuation">.</span>filter <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
从上面代码可以看到,mapDispatchToProps
作为函数,应该返回一个对象,该对象的每个键值对都是一个映射,定义了 UI 组件的参数怎样发出 Action。
如果mapDispatchToProps
是一个对象,它的每个键名也是对应 UI 组件的同名参数,键值应该是一个函数,会被当作 Action creator ,返回的 Action 会由 Redux 自动发出。举例来说,上面的mapDispatchToProps
写成对象就是下面这样。
const mapDispatchToProps <span class="token operator">=</span> <span class="token punctuation">{</span> onClick<span class="token punctuation">:</span> <span class="token punctuation">(</span>filter<span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> type<span class="token punctuation">:</span> <span class="token string">'SET_VISIBILITY_FILTER'</span><span class="token punctuation">,</span> filter<span class="token punctuation">:</span> filter <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
六、 组件
connect
方法生成容器组件以后,需要让容器组件拿到state
对象,才能生成 UI 组件的参数。
一种解决方法是将state
对象作为参数,传入容器组件。但是,这样做比较麻烦,尤其是容器组件可能在很深的层级,一级级将state
传下去就很麻烦。
React-Redux 提供Provider
组件,可以让容器组件拿到state
。
import <span class="token punctuation">{</span> Provider <span class="token punctuation">}</span> from <span class="token string">'react-redux'</span> import <span class="token punctuation">{</span> createStore <span class="token punctuation">}</span> from <span class="token string">'redux'</span> import todoApp from <span class="token string">'./reducers'</span> import App from <span class="token string">'./components/App'</span> <span class="token keyword">let</span> store <span class="token operator">=</span> <span class="token function">createStore<span class="token punctuation">(</span></span>todoApp<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token function">render<span class="token punctuation">(</span></span> <span class="token operator"><</span>Provider store<span class="token operator">=</span><span class="token punctuation">{</span>store<span class="token punctuation">}</span><span class="token operator">></span> <span class="token operator"><</span>App <span class="token operator">/</span><span class="token operator">></span> <span class="token operator"><</span><span class="token operator">/</span>Provider<span class="token operator">></span><span class="token punctuation">,</span> document<span class="token punctuation">.</span><span class="token function">getElementById<span class="token punctuation">(</span></span><span class="token string">'root'</span><span class="token punctuation">)</span> <span class="token punctuation">)</span>
上面代码中,Provider
在根组件外面包了一层,这样一来,App
的所有子组件就默认都可以拿到state
了。
它的原理是React
组件的context
属性,请看源码。
class <span class="token class-name">Provider</span> extends <span class="token class-name">Component</span> <span class="token punctuation">{</span> <span class="token function">getChildContext<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> store<span class="token punctuation">:</span> this<span class="token punctuation">.</span>props<span class="token punctuation">.</span>store <span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">render<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> this<span class="token punctuation">.</span>props<span class="token punctuation">.</span>children<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> Provider<span class="token punctuation">.</span>childContextTypes <span class="token operator">=</span> <span class="token punctuation">{</span> store<span class="token punctuation">:</span> React<span class="token punctuation">.</span>PropTypes<span class="token punctuation">.</span>object <span class="token punctuation">}</span>
上面代码中,store
放在了上下文对象context
上面。然后,子组件就可以从context
拿到store
,代码大致如下。
class <span class="token class-name">VisibleTodoList</span> extends <span class="token class-name">Component</span> <span class="token punctuation">{</span> <span class="token function">componentDidMount<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> const <span class="token punctuation">{</span> store <span class="token punctuation">}</span> <span class="token operator">=</span> this<span class="token punctuation">.</span>context<span class="token punctuation">;</span> this<span class="token punctuation">.</span>unsubscribe <span class="token operator">=</span> store<span class="token punctuation">.</span><span class="token function">subscribe<span class="token punctuation">(</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> this<span class="token punctuation">.</span><span class="token function">forceUpdate<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token function">render<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> const props <span class="token operator">=</span> this<span class="token punctuation">.</span>props<span class="token punctuation">;</span> const <span class="token punctuation">{</span> store <span class="token punctuation">}</span> <span class="token operator">=</span> this<span class="token punctuation">.</span>context<span class="token punctuation">;</span> const state <span class="token operator">=</span> store<span class="token punctuation">.</span><span class="token function">getState<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment" spellcheck="true"> // ... </span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> VisibleTodoList<span class="token punctuation">.</span>contextTypes <span class="token operator">=</span> <span class="token punctuation">{</span> store<span class="token punctuation">:</span> React<span class="token punctuation">.</span>PropTypes<span class="token punctuation">.</span>object <span class="token punctuation">}</span>
React-Redux
自动生成的容器组件的代码,就类似上面这样,从而拿到store
。
七、实例:计数器
我们来看一个实例。下面是一个计数器组件,它是一个纯的 UI 组件。
class <span class="token class-name">Counter</span> extends <span class="token class-name">Component</span> <span class="token punctuation">{</span> <span class="token function">render<span class="token punctuation">(</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span> const <span class="token punctuation">{</span> value<span class="token punctuation">,</span> onIncreaseClick <span class="token punctuation">}</span> <span class="token operator">=</span> this<span class="token punctuation">.</span>props <span class="token keyword">return</span> <span class="token punctuation">(</span> <span class="token operator"><</span>div<span class="token operator">></span> <span class="token operator"><</span>span<span class="token operator">></span><span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token operator"><</span><span class="token operator">/</span>span<span class="token operator">></span> <span class="token operator"><</span>button onClick<span class="token operator">=</span><span class="token punctuation">{</span>onIncreaseClick<span class="token punctuation">}</span><span class="token operator">></span>Increase<span class="token operator"><</span><span class="token operator">/</span>button<span class="token operator">></span> <span class="token operator"><</span><span class="token operator">/</span>div<span class="token operator">></span> <span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span>
上面代码中,这个 UI 组件有两个参数:value
和onIncreaseClick
。前者需要从state
计算得到,后者需要向外发出 Action。
接着,定义value
到state
的映射,以及onIncreaseClick
到dispatch
的映射。
<span class="token keyword">function</span> <span class="token function">mapStateToProps<span class="token punctuation">(</span></span>state<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> value<span class="token punctuation">:</span> state<span class="token punctuation">.</span>count <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token keyword">function</span> <span class="token function">mapDispatchToProps<span class="token punctuation">(</span></span>dispatch<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> onIncreaseClick<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token function">dispatch<span class="token punctuation">(</span></span>increaseAction<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token comment" spellcheck="true"> // Action Creator </span>const increaseAction <span class="token operator">=</span> <span class="token punctuation">{</span> type<span class="token punctuation">:</span> <span class="token string">'increase'</span> <span class="token punctuation">}</span>
然后,使用connect
方法生成容器组件。
const App <span class="token operator">=</span> <span class="token function">connect<span class="token punctuation">(</span></span> mapStateToProps<span class="token punctuation">,</span> mapDispatchToProps <span class="token punctuation">)</span><span class="token punctuation">(</span>Counter<span class="token punctuation">)</span>
然后,定义这个组件的 Reducer。
<span class="token comment" spellcheck="true"> // Reducer </span><span class="token keyword">function</span> <span class="token function">counter<span class="token punctuation">(</span></span>state <span class="token operator">=</span> <span class="token punctuation">{</span> count<span class="token punctuation">:</span> <span class="token number">0</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> action<span class="token punctuation">)</span> <span class="token punctuation">{</span> const count <span class="token operator">=</span> state<span class="token punctuation">.</span>count switch <span class="token punctuation">(</span>action<span class="token punctuation">.</span>type<span class="token punctuation">)</span> <span class="token punctuation">{</span> case <span class="token string">'increase'</span><span class="token punctuation">:</span> <span class="token keyword">return</span> <span class="token punctuation">{</span> count<span class="token punctuation">:</span> count <span class="token operator">+</span> <span class="token number">1</span> <span class="token punctuation">}</span> default<span class="token punctuation">:</span> <span class="token keyword">return</span> state <span class="token punctuation">}</span> <span class="token punctuation">}</span>
最后,生成store
对象,并使用Provider
在根组件外面包一层。
import <span class="token punctuation">{</span> loadState<span class="token punctuation">,</span> saveState <span class="token punctuation">}</span> from <span class="token string">'./localStorage'</span><span class="token punctuation">;</span> const persistedState <span class="token operator">=</span> <span class="token function">loadState<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">;</span> const store <span class="token operator">=</span> <span class="token function">createStore<span class="token punctuation">(</span></span> todoApp<span class="token punctuation">,</span> persistedState <span class="token punctuation">)</span><span class="token punctuation">;</span> store<span class="token punctuation">.</span><span class="token function">subscribe<span class="token punctuation">(</span></span><span class="token function">throttle<span class="token punctuation">(</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">{</span> <span class="token function">saveState<span class="token punctuation">(</span></span><span class="token punctuation">{</span> todos<span class="token punctuation">:</span> store<span class="token punctuation">.</span><span class="token function">getState<span class="token punctuation">(</span></span><span class="token punctuation">)</span><span class="token punctuation">.</span>todos<span class="token punctuation">,</span> <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">1000</span><span class="token punctuation">)</span><span class="token punctuation">)</span> ReactDOM<span class="token punctuation">.</span><span class="token function">render<span class="token punctuation">(</span></span> <span class="token operator"><</span>Provider store<span class="token operator">=</span><span class="token punctuation">{</span>store<span class="token punctuation">}</span><span class="token operator">></span> <span class="token operator"><</span>App <span class="token operator">/</span><span class="token operator">></span> <span class="token operator"><</span><span class="token operator">/</span>Provider<span class="token operator">></span><span class="token punctuation">,</span> document<span class="token punctuation">.</span><span class="token function">getElementById<span class="token punctuation">(</span></span><span class="token string">'root'</span><span class="token punctuation">)</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
完整的代码看这里。
八、React-Router 路由库
使用React-Router
的项目,与其他项目没有不同之处,也是使用Provider
在Router
外面包一层,毕竟Provider
的唯一功能就是传入store
对象。
const Root <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">{</span> store <span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token operator">=</span><span class="token operator">></span> <span class="token punctuation">(</span> <span class="token operator"><</span>Provider store<span class="token operator">=</span><span class="token punctuation">{</span>store<span class="token punctuation">}</span><span class="token operator">></span> <span class="token operator"><</span>Router<span class="token operator">></span> <span class="token operator"><</span>Route path<span class="token operator">=</span><span class="token string">"/"</span> component<span class="token operator">=</span><span class="token punctuation">{</span>App<span class="token punctuation">}</span> <span class="token operator">/</span><span class="token operator">></span> <span class="token operator"><</span><span class="token operator">/</span>Router<span class="token operator">></span> <span class="token operator"><</span><span class="token operator">/</span>Provider<span class="token operator">></span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
(完)
文章转载于:阮一峰的网络日志
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