docs: form add preserve

Author: tangjinzhou

components/form/demo/dynamic-form-item.vue

@@ -58,7 +58,7 @@ let id = 0;
 export default {
   beforeCreate () {
     this.form = this.$form.createForm(this);
-    this.form.getFieldDecorator('keys', { initialValue: [] });
+    this.form.getFieldDecorator('keys', { initialValue: [], preserve: true });
   },
   data () {
     return {

components/form/index.en-US.md

@@ -154,6 +154,7 @@ After wrapped by `getFieldDecorator` or `v-decorator`, `value`(or other property
 | options.getValueProps | Get the component props according to field value. | function(value): any | [reference](https://github.com/react-component/form#option-object)
 | options.initialValue | You can specify initial value, type, optional value of children node. (Note: Because `Form` will test equality with `===` internally, we recommend to use variable as `initialValue`, instead of literal) |  | n/a |
 | options.normalize | Normalize value to form component, [a select-all example](https://codesandbox.io/s/kw4l2vqqmv) | function(value, prevValue, allValues): any | - |
+| options.preserve | Keep the field even if field removed | boolean | false |
 | options.rules | Includes validation rules. Please refer to "Validation Rules" part for details. | object\[] | n/a |
 | options.trigger | When to collect the value of children node | string | 'change' |
 | options.validateFirst | Whether stop validate on first rule of error for this field. | boolean | false |

components/form/index.zh-CN.md

@@ -152,6 +152,7 @@ validateFields(['field1', 'field2'], options, (errors, values) => {
 | options.getValueFromEvent | 可以把 onChange 的参数（如 event）转化为控件的值 | function(..args) | [reference](https://github.com/react-component/form#option-object) |
 | options.initialValue | 子节点的初始值，类型、可选值均由子节点决定(注意：由于内部校验时使用 `===` 判断是否变化，建议使用变量缓存所需设置的值而非直接使用字面量)) |  |  |
 | options.normalize | 转换默认的 value 给控件，[一个选择全部的例子](https://codesandbox.io/s/kw4l2vqqmv) | function(value, prevValue, allValues): any | - |
+| options.preserve | 即便字段不再使用，也保留该字段的值 | boolean | false |
 | options.rules | 校验规则，参考下方文档 | object\[] |  |
 | options.trigger | 收集子节点的值的时机 | string | 'change' |
 | options.validateFirst | 当某一规则校验不通过时，是否停止剩下的规则的校验 | boolean | false |

types/form/form.d.ts

@@ -143,6 +143,13 @@ declare interface FieldDecoratorOptions {
    */
   normalize?: (value: any, prevValue: any, allValues: any) => any;
 
+  /**
+   * Keep the field even if field removed
+   * @default false
+   * @type boolean
+   */
+  preserve?: boolean;
+
   /**
    * Includes validation rules. Please refer to "Validation Rules" part for details.
    * @default n/a