[Improvement] uniform fade animation (#410)

packages/mixins/popup/Modal.vue

@@ -1,11 +1,14 @@
 <template>
-  <div
-    class="van-modal"
-    :class="className"
-    :style="style"
-    @touchmove.prevent.stop
-    @click="$emit('click', $event)"
-  />
+  <transition name="van-fade">
+    <div
+      v-show="visible"
+      class="van-modal"
+      :class="className"
+      :style="style"
+      @touchmove.prevent.stop
+      @click="$emit('click', $event)"
+    />
+  </transition>
 </template>
 
 <script>
@@ -13,6 +16,7 @@ export default {
   name: 'van-modal',
 
   props: {
+    visible: Boolean,
     zIndex: Number,
     className: String,
     customStyle: Object