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<!DOCTYPE HTML> <html lang="zh-cn" > <head> <meta charset="UTF-8"> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <title>Mutation · Vuex</title> <meta http-equiv="X-UA-Compatible" content="IE=edge" /> <meta name="description" content=""> <meta name="generator" content="GitBook 3.2.3"> <link rel="stylesheet" href="../gitbook/style.css"> <link rel="stylesheet" href="../gitbook/gitbook-plugin-prism/prism.css"> <link rel="stylesheet" href="../gitbook/gitbook-plugin-search/search.css"> <link rel="stylesheet" href="../gitbook/gitbook-plugin-theme-vuejs/vue.css"> <meta name="HandheldFriendly" content="true"/> <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no"> <meta name="apple-mobile-web-app-capable" content="yes"> <meta name="apple-mobile-web-app-status-bar-style" content="black"> <link rel="apple-touch-icon-precomposed" sizes="152x152" href="../gitbook/images/apple-touch-icon-precomposed-152.png"> <link rel="shortcut icon" 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中的状态的唯一方法是提交 mutation。Vuex 中的 mutation 非常类似于事件：每个 mutation 都有一个字符串的 事件类型 (type) 和 一个 回调函数 (handler)。这个回调函数就是我们实际进行状态更改的地方，并且它会接受 state 作为第一个参数： <pre class="language-"><code class="lang-js">const store = new Vuex.Store({ state: { count: 1 }, mutations: { increment (state) { // 变更状态 state.count++ } } }) </code></pre> 你不能直接调用一个 mutation handler。这个选项更像是事件注册：“当触发一个类型为 <code>increment</code> 的 mutation 时，调用此函数。”要唤醒一个 mutation handler，你需要以相应的 type 调用 store.commit 方法： <pre class="language-"><code class="lang-js">store.commit('increment') </code></pre> <h3 id="提交载荷（payload）">提交载荷（Payload）</h3> 你可以向 <code>store.commit</code> 传入额外的参数，即 mutation 的 载荷（payload）： <pre class="language-"><code class="lang-js">// ... mutations: { increment (state, n) { state.count += n } } </code></pre> <pre class="language-"><code class="lang-js">store.commit('increment', 10) </code></pre> 在大多数情况下，载荷应该是一个对象，这样可以包含多个字段并且记录的 mutation 会更易读： <pre class="language-"><code class="lang-js">// ... mutations: { increment (state, payload) { state.count += payload.amount } } </code></pre> <pre class="language-"><code class="lang-js">store.commit('increment', { amount: 10 }) </code></pre> <h3 id="对象风格的提交方式">对象风格的提交方式</h3> 提交 mutation 的另一种方式是直接使用包含 <code>type</code> 属性的对象： <pre class="language-"><code class="lang-js">store.commit({ type: 'increment', amount: 10 }) </code></pre> 当使用对象风格的提交方式，整个对象都作为载荷传给 mutation 函数，因此 handler 保持不变： <pre class="language-"><code class="lang-js">mutations: { increment (state, payload) { state.count += payload.amount } } </code></pre> <h3 id="mutation-需遵守-vue-的响应规则">Mutation 需遵守 Vue 的响应规则</h3> 既然 Vuex 的 store 中的状态是响应式的，那么当我们变更状态时，监视状态的 Vue 组件也会自动更新。这也意味着 Vuex 中的 mutation 也需要与使用 Vue 一样遵守一些注意事项： <ol> <li>最好提前在你的 store 中初始化好所有所需属性。 </li> <li>当需要在对象上添加新属性时，你应该 <ul> <li>使用 <code>Vue.set(obj, 'newProp', 123)</code>, 或者 </li> <li>以新对象替换老对象。例如，利用 stage-3 的<a href="https://github.com/sebmarkbage/ecmascript-rest-spread" target="_blank">对象展开运算符</a>我们可以这样写： <pre class="language-"><code class="lang-js">state.obj = { ...state.obj, newProp: 123 } </code></pre> </li> </ul> </li> </ol> <h3 id="使用常量替代-mutation-事件类型">使用常量替代 Mutation 事件类型</h3> 使用常量替代 mutation 事件类型在各种 Flux 实现中是很常见的模式。这样可以使 linter 之类的工具发挥作用，同时把这些常量放在单独的文件中可以让你的代码合作者对整个 app 包含的 mutation 一目了然： <pre class="language-"><code class="lang-js">// mutation-types.js export const SOME_MUTATION = 'SOME_MUTATION' </code></pre> <pre class="language-"><code class="lang-js">// store.js import Vuex from 'vuex' import { SOME_MUTATION } from './mutation-types' const store = new Vuex.Store({ state: { ... }, mutations: { // 我们可以使用 ES2015 风格的计算属性命名功能来使用一个常量作为函数名 [SOME_MUTATION] (state) { // mutate state } } }) </code></pre> 用不用常量取决于你——在需要多人协作的大型项目中，这会很有帮助。但如果你不喜欢，你完全可以不这样做。 <h3 id="mutation-必须是同步函数">Mutation 必须是同步函数</h3> 一条重要的原则就是要记住 mutation 必须是同步函数。为什么？请参考下面的例子： <pre class="language-"><code class="lang-js">mutations: { someMutation (state) { api.callAsyncMethod(() => { state.count++ }) } } </code></pre> 现在想象，我们正在 debug 一个 app 并且观察 devtool 中的 mutation 日志。每一条 mutation 被记录，devtools 都需要捕捉到前一状态和后一状态的快照。然而，在上面的例子中 mutation 中的异步函数中的回调让这不可能完成：因为当 mutation 触发的时候，回调函数还没有被调用，devtools 不知道什么时候回调函数实际上被调用——实质上任何在回调函数中进行的状态的改变都是不可追踪的。 <h3 id="在组件中提交-mutation">在组件中提交 Mutation</h3> 你可以在组件中使用 <code>this.$store.commit('xxx')</code> 提交 mutation，或者使用 <code>mapMutations</code> 辅助函数将组件中的 methods 映射为 <code>store.commit</code> 调用（需要在根节点注入 <code>store</code>）。 <pre class="language-"><code class="lang-js">import { mapMutations } from 'vuex' export default { // ... methods: { ...mapMutations([ 'increment', // 将 `this.increment()` 映射为 `this.$store.commit('increment')` // `mapMutations` 也支持载荷： 'incrementBy' // 将 `this.incrementBy(amount)` 映射为 `this.$store.commit('incrementBy', amount)` ]), ...mapMutations({ add: 'increment' // 将 `this.add()` 映射为 `this.$store.commit('increment')` }) } } </code></pre> <h3 id="下一步：action">下一步：Action</h3> 在 mutation 中混合异步调用会导致你的程序很难调试。例如，当你调用了两个包含异步回调的 mutation 来改变状态，你怎么知道什么时候回调和哪个先回调呢？这就是为什么我们要区分这两个概念。在 Vuex 中，mutation 都是同步事务： <pre class="language-"><code class="lang-js">store.commit('increment') // 任何由 "increment" 导致的状态变更都应该在此刻完成。 </code></pre> 为了处理异步操作，让我们来看一看 <a href="actions.html">Action</a>。 </section> </div> <div class="search-results"> <div class="has-results"> <h1 class="search-results-title"> results matching ""</h1> <ul class="search-results-list"></ul> </div> <div class="no-results"> <h1 class="search-results-title">No results matching ""</h1> </div> </div> </div> </div> </div> <script src="//m.servedby-buysellads.com/monetization.js" type="text/javascript"></script> <div class="bsa-cpc"></div> <script> (function(){ if(typeof _bsa !== 'undefined' && _bsa) { _bsa.init('default', 'CKYD62QM', 'placement:vuejsorg', { target: '.bsa-cpc', align: 'horizontal', disable_css: 'true' }); } })(); </script> </div> <a href="getters.html" class="navigation navigation-prev " aria-label="Previous page: Getter"> </a> <a href="actions.html" class="navigation navigation-next " aria-label="Next 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