vant/packages/vant-use/src/useCustomFieldValue/index.ts

import { watch, inject, InjectionKey, Ref } from 'vue';

export type CustomFieldInjectionValue = {
  customValue: Ref<(() => unknown) | undefined>;
  resetValidation: () => void;
  validateWithTrigger: (trigger: 'onBlur' | 'onChange' | 'onSubmit') => void;
};

export const CUSTOM_FIELD_INJECTION_KEY: InjectionKey<CustomFieldInjectionValue> =
  Symbol('van-field');

export function useCustomFieldValue(customValue: () => unknown) {
  const field = inject(CUSTOM_FIELD_INJECTION_KEY, null);

  if (field && !field.customValue.value) {
    field.customValue.value = customValue;

    watch(customValue, () => {
      field.resetValidation();
      field.validateWithTrigger('onChange');
    });
  }
}