[Improvement] Dialog: add before-close prop (#881)

docs/markdown/zh-CN/dialog.md

@@ -78,13 +78,18 @@ export default {
 | overlay | 是否展示蒙层 | `Boolean` | `true` | - |
 | closeOnClickOverlay | 点击蒙层时是否关闭弹窗 | `Boolean` | `false` | - |
 | lockScroll | 是否锁定背景滚动 | `Boolean` | `true` | - |
+| beforeClose | 关闭前的回调函数，调用 done 后关闭弹窗 | (action: string, done: function) => void | - | - |
 
 
 #### 高级用法
 如果需要在弹窗内实现更复杂的交互，可以通过组件形式来调用 Dialog
 
 ```html
-<van-dialog v-model="show" :asyncConfirm="onClickConfirmAsync">
+<van-dialog
+  v-model="show"
+  show-cancel-button
+  :before-close="beforeClose"
+>
   <van-field
     v-model="username"
     label="用户名"
@@ -112,12 +117,12 @@ export default {
   },
   
   methods: {
-    onClickConfirmAsync() {
-      return new Promise((resolve, reject) => {
-        setTimeout(() => {
-          resolve();
-        }, 1000)
-      });
+    beforeClose() {
+      if (action === 'confirm') {
+        setTimeout(done, 1000);
+      } else {
+        done();
+      }
     }
   }
 }
@@ -130,7 +135,6 @@ export default {
 | v-model | 是否显示弹窗 | `Boolean` | - | - |
 | title | 标题 | `String` | - | - |
 | message | 内容 | `String` | - | - |
-| async-confirm | dialog是否异步关闭，传入一个返回Promise的函数，在点击确定时触发。 | `Function` | - | - |
 | show-confirm-button | 是否展示确认按钮 | `Boolean` |  `true` | - |
 | show-cancel-button | 是否展示取消按钮 | `Boolean` |  `false` | - |
 | confirm-button-text | 确认按钮的文案 | `String` |  `确认` | - |
@@ -138,6 +142,7 @@ export default {
 | overlay | 是否展示蒙层 | `Boolean` | `true` | - |
 | close-on-click-overlay | 点击蒙层时是否关闭弹窗 | `Boolean` | `false` | - |
 | lock-scroll | 是否锁定背景滚动 | `Boolean` | `true` | - |
+| before-close | 关闭前的回调函数，调用 done 后关闭弹窗 | (action: string, done: function) => void | - | - |
 
 ### Event