feat(widget-list): add min max configuration (#4394)

packages/netlify-cms-lib-widgets/src/index.ts

@@ -1,6 +1,8 @@
 import * as stringTemplate from './stringTemplate';
+import * as validations from './validations';
 
 export const NetlifyCmsLibWidgets = {
   stringTemplate,
+  validations,
 };
-export { stringTemplate };
+export { stringTemplate, validations };

packages/netlify-cms-lib-widgets/src/validations.ts

@@ -0,0 +1,28 @@
+import { isNumber } from 'lodash';
+import { List } from 'immutable';
+
+export const validateMinMax = (
+  t: (key: string, options: unknown) => string,
+  fieldLabel: string,
+  value?: List<unknown>,
+  min?: number,
+  max?: number,
+) => {
+  const minMaxError = (messageKey: string) => ({
+    type: 'RANGE',
+    message: t(`editor.editorControlPane.widget.${messageKey}`, {
+      fieldLabel,
+      minCount: min,
+      maxCount: max,
+      count: min,
+    }),
+  });
+
+  if ([min, max, value?.size].every(isNumber) && (value!.size < min! || value!.size > max!)) {
+    return minMaxError(min === max ? 'rangeCountExact' : 'rangeCount');
+  } else if (isNumber(min) && min > 0 && value?.size && value.size < min) {
+    return minMaxError('rangeMin');
+  } else if (isNumber(max) && value?.size && value.size > max) {
+    return minMaxError('rangeMax');
+  }
+};