关于appear的用法和enter的用法相似，它只是在第一次渲染的时候才会起作用。看完整的代码：
别忘了引用vue.js
appear appear-class="custom-appear" appear-to-class="custom-appear-to" appear-active-class="custom-appear-active" > appear
但是这里有一些问题：关于appear-class、 appear-to-class、 appear-active-class的相同属性那个起作用的问题。
四种情况：（与他们在style中的排列顺序有关系）
1、appear-class、 appear-to-class、 appear-active-class或者 appear-to-class、appear-class、 appear-active-class的排列顺序，此时只有appear-active-class的属性起作用。
2、appear-active-class、appear-class、 appear-to-class
此时appear-active-class的不起作用，由appear-class过渡到appear-to-class属性。
3、appear-class、appear-active-class、 appear-to-class
此时appear-class属性不起作用，由appear-active-class过渡到 appear-to-class属性。
4、 appear-to-class、 appear-active-class、appear-class
此时appear-to-class不起作用，由appear-class过渡到 appear-active-class属性。
enter也有相似的问题