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- <h1 id="mutation">Mutation</h1>
- <p>更改 Vuex 的 store 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 <strong>事件类型 (type)</strong> 和 一个 <strong>回调函数 (handler)</strong>。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:</p>
- <pre class="language-"><code class="lang-js"><span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Vuex<span class="token punctuation">.</span>Store</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
- state<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- count<span class="token punctuation">:</span> <span class="token number">1</span>
- <span class="token punctuation">}</span><span class="token punctuation">,</span>
- mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- increment <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token comment" spellcheck="true">// 变更状态</span>
- state<span class="token punctuation">.</span>count<span class="token operator">++</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- </code></pre>
- <p>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 <strong>store.commit</strong> 方法:</p>
- <pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token string">'increment'</span><span class="token punctuation">)</span>
- </code></pre>
- <h3 id="提交载荷(payload)">提交载荷(Payload)</h3>
- <p>你可以向 <code>store.commit</code> 传入额外的参数,即 mutation 的 <strong>载荷(payload)</strong>:</p>
- <pre class="language-"><code class="lang-js"><span class="token comment" spellcheck="true">// ...</span>
- mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- increment <span class="token punctuation">(</span>state<span class="token punctuation">,</span> n<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- state<span class="token punctuation">.</span>count <span class="token operator">+</span><span class="token operator">=</span> n
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- </code></pre>
- <pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token string">'increment'</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span>
- </code></pre>
- <p>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:</p>
- <pre class="language-"><code class="lang-js"><span class="token comment" spellcheck="true">// ...</span>
- mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- increment <span class="token punctuation">(</span>state<span class="token punctuation">,</span> payload<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- state<span class="token punctuation">.</span>count <span class="token operator">+</span><span class="token operator">=</span> payload<span class="token punctuation">.</span>amount
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- </code></pre>
- <pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token string">'increment'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
- amount<span class="token punctuation">:</span> <span class="token number">10</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- </code></pre>
- <h3 id="对象风格的提交方式">对象风格的提交方式</h3>
- <p>提交 mutation 的另一种方式是直接使用包含 <code>type</code> 属性的对象:</p>
- <pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
- type<span class="token punctuation">:</span> <span class="token string">'increment'</span><span class="token punctuation">,</span>
- amount<span class="token punctuation">:</span> <span class="token number">10</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- </code></pre>
- <p>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:</p>
- <pre class="language-"><code class="lang-js">mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- increment <span class="token punctuation">(</span>state<span class="token punctuation">,</span> payload<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- state<span class="token punctuation">.</span>count <span class="token operator">+</span><span class="token operator">=</span> payload<span class="token punctuation">.</span>amount
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- </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>
- <p>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:</p>
- <ol>
- <li><p>最好提前在你的 store 中初始化好所有所需属性。</p>
- </li>
- <li><p>当需要在对象上添加新属性时,你应该</p>
- <ul>
- <li><p>使用 <code>Vue.set(obj, 'newProp', 123)</code>, 或者</p>
- </li>
- <li><p>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:</p>
- <pre class="language-"><code class="lang-js">state<span class="token punctuation">.</span>obj <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span>state<span class="token punctuation">.</span>obj<span class="token punctuation">,</span> newProp<span class="token punctuation">:</span> <span class="token number">123</span> <span class="token punctuation">}</span>
- </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>
- <p>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:</p>
- <pre class="language-"><code class="lang-js"><span class="token comment" spellcheck="true">// mutation-types.js</span>
- <span class="token keyword">export</span> <span class="token keyword">const</span> SOME_MUTATION <span class="token operator">=</span> <span class="token string">'SOME_MUTATION'</span>
- </code></pre>
- <pre class="language-"><code class="lang-js"><span class="token comment" spellcheck="true">// store.js</span>
- <span class="token keyword">import</span> Vuex <span class="token keyword">from</span> <span class="token string">'vuex'</span>
- <span class="token keyword">import</span> <span class="token punctuation">{</span> SOME_MUTATION <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'./mutation-types'</span>
- <span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Vuex<span class="token punctuation">.</span>Store</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
- state<span class="token punctuation">:</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
- mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- <span class="token comment" spellcheck="true">// 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</span>
- <span class="token punctuation">[</span>SOME_MUTATION<span class="token punctuation">]</span> <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- <span class="token comment" spellcheck="true">// mutate state</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- </code></pre>
- <p>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。</p>
- <h3 id="mutation-必须是同步函数">Mutation 必须是同步函数</h3>
- <p>一条重要的原则就是要记住 <strong>mutation 必须是同步函数</strong>。为什么?请参考下面的例子:</p>
- <pre class="language-"><code class="lang-js">mutations<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- someMutation <span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token punctuation">{</span>
- api<span class="token punctuation">.</span><span class="token function">callAsyncMethod</span><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>
- state<span class="token punctuation">.</span>count<span class="token operator">++</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- </code></pre>
- <p>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。</p>
- <h3 id="在组件中提交-mutation">在组件中提交 Mutation</h3>
- <p>你可以在组件中使用 <code>this.$store.commit('xxx')</code> 提交 mutation,或者使用 <code>mapMutations</code> 辅助函数将组件中的 methods 映射为 <code>store.commit</code> 调用(需要在根节点注入 <code>store</code>)。</p>
- <pre class="language-"><code class="lang-js"><span class="token keyword">import</span> <span class="token punctuation">{</span> mapMutations <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'vuex'</span>
- <span class="token keyword">export</span> <span class="token keyword">default</span> <span class="token punctuation">{</span>
- <span class="token comment" spellcheck="true">// ...</span>
- methods<span class="token punctuation">:</span> <span class="token punctuation">{</span>
- <span class="token operator">...</span><span class="token function">mapMutations</span><span class="token punctuation">(</span><span class="token punctuation">[</span>
- <span class="token string">'increment'</span><span class="token punctuation">,</span> <span class="token comment" spellcheck="true">// 将 `this.increment()` 映射为 `this.$store.commit('increment')`</span>
- <span class="token comment" spellcheck="true">// `mapMutations` 也支持载荷:</span>
- <span class="token string">'incrementBy'</span> <span class="token comment" spellcheck="true">// 将 `this.incrementBy(amount)` 映射为 `this.$store.commit('incrementBy', amount)`</span>
- <span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
- <span class="token operator">...</span><span class="token function">mapMutations</span><span class="token punctuation">(</span><span class="token punctuation">{</span>
- add<span class="token punctuation">:</span> <span class="token string">'increment'</span> <span class="token comment" spellcheck="true">// 将 `this.add()` 映射为 `this.$store.commit('increment')`</span>
- <span class="token punctuation">}</span><span class="token punctuation">)</span>
- <span class="token punctuation">}</span>
- <span class="token punctuation">}</span>
- </code></pre>
- <h3 id="下一步:action">下一步:Action</h3>
- <p>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,<strong>mutation 都是同步事务</strong>:</p>
- <pre class="language-"><code class="lang-js">store<span class="token punctuation">.</span><span class="token function">commit</span><span class="token punctuation">(</span><span class="token string">'increment'</span><span class="token punctuation">)</span>
- <span class="token comment" spellcheck="true">// 任何由 "increment" 导致的状态变更都应该在此刻完成。</span>
- </code></pre>
- <p>为了处理异步操作,让我们来看一看 <a href="actions.html">Action</a>。</p>
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