feat(Form): add validateField method

src/form/README.md

@@ -420,7 +420,8 @@ Use [ref](https://vuejs.org/v2/api/#ref) to get Form instance and call instance
 
 | Name | Description | Attribute | Return value |
 |------|------|------|------|
-| validate | Trigger validation | - | *Promise\<void\>* |
+| validate | Validate all fields | - | *Promise* |
+| validateField | Validate a filed | *name: string* | *Promise* |
 | resetValidation | Reset all validation | - | - |
 
 ### Slots

src/form/README.zh-CN.md

@@ -452,7 +452,8 @@ export default {
 
 | 方法名 | 说明 | 参数 | 返回值 |
 |------|------|------|------|
-| validate | 触发表单验证 | - | *Promise\<void\>* |
+| validate | 验证所有表单项，返回 Promise，验证不通过时 reject | - | *Promise* |
+| validateField | 验证单个表单项，返回 Promise，验证不通过时 reject | *name: string* | *Promise* |
 | resetValidation | 重置所有表单验证提示 | - | - |
 
 ### Slots

src/form/demo/BasicUsage.vue

@@ -1,6 +1,6 @@
 <template>
   <demo-block :title="$t('basicUsage')">
-    <van-form @submit="onSubmit" @failed="onFailed">
+    <van-form ref="form" @submit="onSubmit" @failed="onFailed">
       <van-field
         v-model="username"
         name="username"

src/form/index.js

@@ -67,6 +67,25 @@ export default createComponent({
       return this.validateFirst ? this.validateSeq() : this.validateAll();
     },
 
+    // @exposed-api
+    validateField(name) {
+      const matched = this.fields.filter(item => item.name === name);
+
+      if (matched.length) {
+        return new Promise((resolve, reject) => {
+          matched[0].validate().then(error => {
+            if (error) {
+              reject(error);
+            } else {
+              resolve();
+            }
+          });
+        });
+      }
+
+      return Promise.reject();
+    },
+
     // @exposed-api
     resetValidation() {
       this.fields.forEach(item => {