perf[el-dragDialog]: add dragDialog callback function

src/views/components-demo/dragDialog.vue

@@ -1,7 +1,11 @@
 <template>
   <div class="components-container">
     <el-button type="primary" @click="dialogTableVisible = true">open a Drag Dialog</el-button>
-    <el-dialog v-el-drag-dialog title="Shipping address" :visible.sync="dialogTableVisible">
+    <el-dialog v-el-drag-dialog @dragDialog="handleDrag" title="Shipping address" :visible.sync="dialogTableVisible">
+      <el-select ref="select" v-model="value" placeholder="请选择">
+        <el-option v-for="item in options" :key="item.value" :label="item.label" :value="item.value">
+        </el-option>
+      </el-select>
       <el-table :data="gridData">
         <el-table-column property="date" label="Date" width="150"></el-table-column>
         <el-table-column property="name" label="Name" width="200"></el-table-column>
@@ -20,6 +24,13 @@ export default {
   data() {
     return {
       dialogTableVisible: false,
+      options: [
+        { value: '选项1', label: '黄金糕' },
+        { value: '选项2', label: '双皮奶' },
+        { value: '选项3', label: '蚵仔煎' },
+        { value: '选项4', label: '龙须面' }
+      ],
+      value: '',
       gridData: [{
         date: '2016-05-02',
         name: 'John Smith',
@@ -38,6 +49,12 @@ export default {
         address: 'No.1518,  Jinshajiang Road, Putuo District'
       }]
     }
+  },
+  methods: {
+    // v-el-drag-dialog onDrag callback function
+    handleDrag() {
+      this.$refs.select.blur()
+    }
   }
 }
 </script>