Add Internationalizable interface and type guard

Author: lucas-koehler

packages/core/src/i18n/i18nUtil.ts

@@ -1,5 +1,5 @@
 import { ErrorObject } from 'ajv';
-import { isControlElement, UISchemaElement } from '../models';
+import { isInternationalized, UISchemaElement } from '../models';
 import { getControlPath } from '../reducers';
 import { formatErrorMessage } from '../util';
 import { i18nJsonSchema, ErrorTranslator, Translator } from './i18nTypes';
@@ -8,7 +8,7 @@ export const getI18nKeyPrefixBySchema = (
   schema: i18nJsonSchema | undefined,
   uischema: UISchemaElement | undefined
 ): string | undefined => {
-  if (uischema && isControlElement(uischema) && uischema.i18n) {
+  if (isInternationalized(uischema)) {
     return uischema.i18n;
   }
   return schema?.i18n ?? undefined;

packages/core/src/models/uischema.ts

@@ -36,6 +36,15 @@ export interface Scopable {
   scope: string;
 }
 
+/**
+ * Interface for describing an UI schema element that can provide an internationalization base key.
+ * If defined, this key is suffixed to derive applicable message keys for the UI schema element.
+ * For example, such suffixes are `.label` or `.description` to derive the corresponding message keys for a control element.
+ */
+export interface Internationalizable {
+  i18n?: string;
+}
+
 /**
  * A rule that may be attached to any UI schema element.
  */
@@ -207,17 +216,12 @@ export interface LabelElement extends UISchemaElement {
  * A control element. The scope property of the control determines
  * to which part of the schema the control should be bound.
  */
-export interface ControlElement extends UISchemaElement, Scopable {
+export interface ControlElement extends UISchemaElement, Scopable, Internationalizable {
   type: 'Control';
   /**
    * An optional label that will be associated with the control
    */
   label?: string | boolean | LabelDescription;
-  /**
-   * The i18n key for the control. It is used to identify the string that needs to be translated.
-   * It is suffixed with `.label`, `.description` and `.error.<keyword>` to derive the corresponding message keys for the control.
-   */
-  i18n?: string;
 }
 
 /**
@@ -249,8 +253,9 @@ export interface Categorization extends UISchemaElement {
   elements: (Category | Categorization)[];
 }
 
-export const isControlElement = (element: UISchemaElement): element is ControlElement =>
-  element.type === 'Control';
+export const isInternationalized = (element: unknown): element is Required<Internationalizable> => {
+  return typeof element === 'object' && element !== null && typeof (element as Internationalizable).i18n === 'string';
+}
 
 export const isGroup = (layout: Layout): layout is GroupLayout =>
   layout.type === 'Group';