为什么使用 v-if 不进入转换但离开工作的元素内部的转换？

Question

标题几乎解释了一切。我已经在 StackOverflow 上搜索了解决方案，但 neither of 他们提供了帮助。

一个使用经典模态组件的例子：

Vue.component('modal', {
  template: `
  <transition name="modal">
    <div class="modal-wrapper">
      <div class="modal-dialog">
        <slot></slot>
      </div>
    </div>
  </transition>
  `,
})

const app = new Vue({
  el: '#app',
  data: {
    showModal: false,
  },
})

/* transition */
.modal-enter-active,
.modal-leave-active {
  transition: opacity .5s;
}

.modal-enter,
.modal-leave-to {
  opacity: 0;
}

.modal-wrapper {
  position: absolute;
  left: 0; right: 0;
  top: 0; bottom: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, .25);
}

.modal-dialog {
  max-width: 90%;
  padding: 1em;
  background: white;
}

<script src="https://vuejs.org/js/vue.js"></script>
<div id="app">
  <p><button @click="showModal = true">Show modal</button></p>
  <modal v-if="showModal">
    <h3>Hello world</h3>
    <p>Amet quam alias amet incidunt voluptatum sapiente Mollitia</p>
    <p><button @click="showModal = false">Close</button></p>
  </modal>
</div>

（控制台也没有错误）

但是，使用v-show 时一切正常。但我不能在我的项目中真正使用它来代替 v-if。

Vue.component('modal', {
  template: `
  <transition name="modal">
    <div class="modal-wrapper">
      <div class="modal-dialog">
        <slot></slot>
      </div>
    </div>
  </transition>
  `,
})

const app = new Vue({
  el: '#app',
  data: {
    showModal: false,
  },
})

/* transition */
.modal-enter-active,
.modal-leave-active {
  transition: opacity .5s;
}

.modal-enter,
.modal-leave-to {
  opacity: 0;
}

.modal-wrapper {
  position: absolute;
  left: 0; right: 0;
  top: 0; bottom: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, .25);
}

.modal-dialog {
  max-width: 90%;
  padding: 1em;
  background: white;
}

<script src="https://vuejs.org/js/vue.js"></script>
<div id="app">
  <p><button @click="showModal = true">Show modal</button></p>
  <modal v-show="showModal">
    <h3>Hello world</h3>
    <p>Amet quam alias amet incidunt voluptatum sapiente Mollitia</p>
    <p><button @click="showModal = false">Close</button></p>
  </modal>
</div>

也就是说，我必须将 <modal> 与 <transition> 包装在任何地方使用并从模态本身移除过渡（这听起来不太好）

<transition name="modal">
  <modal>
  ...
  </modal>
</transition>

为什么会这样以及如何使进入动画工作（v-if 和<transition> 在模态组件中？

我注意到 Vue 2.5（而不是 Vue 2.6）没有这样的问题。从那以后肯定发生了一些变化。

Answer 1

您缺少appear 属性。

默认情况下，第一次插入元素时，Vue 不会动画。根据文档：

如果您还想在节点的初始渲染上应用过渡，可以添加出现属性：

<transition appear>
  <!-- ... -->
</transition>

默认情况下，这将使用为进入和离开指定的转换。但是，如果您愿意，也可以指定自定义 CSS 类：

<transition
  appear
  appear-class="custom-appear-class"
  appear-to-class="custom-appear-to-class"
  appear-active-class="custom-appear-active-class"
>
<!-- ... -->
</transition>

所以只需添加appear 即可解决问题。

Answer 2

这是因为 v-if 插入/销毁元素而 v-show 隐藏了它们（不会从 DOM 中删除它们）。在您的示例中，如果 v-if 设置在过渡元素上/上方，则 transition 不存在。如果您将v-if 移动到转换下方，那么它将起作用。

Vue.component('modal', {
  props: ['showModal'],
  template: `
  <transition name="modal">
    <div class="modal-wrapper" v-if="showModal">
      <div class="modal-dialog">
        <slot></slot>
      </div>
    </div>
  </transition>
  `,
})

const app = new Vue({
  el: '#app',
  data: {
    showModal: false,
  },
})

/* transition */
.modal-enter-active,
.modal-leave-active {
  transition: opacity .5s;
}

.modal-enter,
.modal-leave-to {
  opacity: 0;
}

.modal-wrapper {
  position: absolute;
  left: 0; right: 0;
  top: 0; bottom: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, .25);
}

.modal-dialog {
  max-width: 90%;
  padding: 1em;
  background: white;
}

<script src="https://vuejs.org/js/vue.js"></script>
<div id="app">
  <p><button @click="showModal = true">Show modal</button></p>
  <modal :show-modal="showModal">
    <h3>Hello world</h3>
    <p>Amet quam alias amet incidunt voluptatum sapiente Mollitia</p>
    <p><button @click="showModal = false">Close</button></p>
  </modal>
</div>

Answer 3

这应该可以解决问题：

{
  template: `
  <transition name="modal">
    <div v-if="show">...</div>
  </transition>
  `,
  data() {return {
    show: false,
  }},
  mounted() {
    this.show = true
  },
}

在我看来（我相信确实如此）这种方法会将动画延迟 1 个 Vue 滴答声，并且大量嵌套此类动画元素会显着降低速度。

Vue.component('modal', {
  template: `
  <transition name="modal">
    <div class="modal-wrapper" v-if="show">
      <div class="modal-dialog">
        <slot></slot>
      </div>
    </div>
  </transition>
  `,
  data() {return {
    show: false,
  }},
  mounted() {
    this.show = true
  }
})

const app = new Vue({
  el: '#app',
  data: {
    showModal: false,
  },
})

/* transition */
.modal-enter-active,
.modal-leave-active {
  transition: opacity .5s;
}

.modal-enter,
.modal-leave-to {
  opacity: 0;
}

.modal-wrapper {
  position: absolute;
  left: 0; right: 0;
  top: 0; bottom: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, .25);
}

.modal-dialog {
  max-width: 90%;
  padding: 1em;
  background: white;
}

<script src="https://vuejs.org/js/vue.js"></script>
<div id="app">
  <p><button @click="showModal = true">Show modal</button></p>
  <modal v-if="showModal">
    <h3>Hello world</h3>
    <p>Amet quam alias amet incidunt voluptatum sapiente Mollitia</p>
    <p><button @click="showModal = false">Close</button></p>
  </modal>
</div>