feat(forms): provide generic Signal Forms error messages

projects/kit/forms/src/kit-ionic-form-field.spec.ts

@@ -15,8 +15,10 @@ import { provideKitIonicSignalForms } from './provide-kit-ionic-signal-forms';
     <ion-checkbox [formField]="fields.checkbox"></ion-checkbox>
     <ion-radio-group [formField]="fields.radio"></ion-radio-group>
     <ion-toggle [formField]="fields.toggle"></ion-toggle>
-    <ion-input [formField]="fields.explicit" errorText="Cross-field error"></ion-input>
-    <ion-input [formField]="fields.explicit" [errorText]="boundError"></ion-input>
+    <ion-input [formField]="fields.custom"></ion-input>
+    <ion-input [formField]="fields.unknown"></ion-input>
+    <ion-input data-testid="static-error" [formField]="fields.explicit" errorText="Cross-field error"></ion-input>
+    <ion-input data-testid="bound-error" [formField]="fields.explicit" [errorText]="boundError"></ion-input>
   `,
 })
 class Host {
@@ -29,19 +31,19 @@ class Host {
     checkbox: false,
     radio: '',
     toggle: false,
+    custom: '',
+    unknown: '',
     explicit: '',
   });
   readonly fields = form(this.model, (path) => {
-    validate(path.input, ({ value }) =>
-      value() ? undefined : { kind: 'non-string', message: 123 as unknown as string },
-    );
-    validate(path.input, ({ value }) => (value() ? undefined : { kind: 'empty', message: '  ' }));
-    required(path.input, { message: 'Input required' });
-    required(path.textarea, { message: 'Textarea required' });
-    required(path.select, { message: 'Select required' });
-    required(path.checkbox, { message: 'Checkbox required' });
-    required(path.radio, { message: 'Radio required' });
-    required(path.toggle, { message: 'Toggle required' });
+    required(path.input);
+    required(path.textarea);
+    required(path.select);
+    required(path.checkbox);
+    required(path.radio);
+    required(path.toggle);
+    required(path.custom, { message: 'Localized required' });
+    validate(path.unknown, ({ value }) => (value() ? undefined : { kind: 'domain-error' }));
     required(path.explicit, { message: 'Generated error' });
   });
 }
@@ -54,11 +56,20 @@ describe('KitIonicFormField', () => {
 
     expect(
       [...fixture.nativeElement.querySelectorAll('ion-input, ion-textarea, ion-select, ion-checkbox, ion-radio-group, ion-toggle')]
-        .slice(0, 6)
+        .slice(0, 8)
         .map((element: { errorText?: string }) => element.errorText),
-    ).toEqual(['Input required', 'Textarea required', 'Select required', 'Checkbox required', 'Radio required', 'Toggle required']);
-    expect(fixture.nativeElement.querySelectorAll('ion-input')[1].errorText).toBe('Cross-field error');
-    expect(fixture.nativeElement.querySelectorAll('ion-input')[2].errorText).toBe('Bound cross-field error');
+    ).toEqual([
+      'This field is required.',
+      'This field is required.',
+      'This field is required.',
+      'This field is required.',
+      'This field is required.',
+      'This field is required.',
+      'Localized required',
+      'Enter a valid value.',
+    ]);
+    expect(fixture.nativeElement.querySelector('[data-testid="static-error"]').errorText).toBe('Cross-field error');
+    expect(fixture.nativeElement.querySelector('[data-testid="bound-error"]').errorText).toBe('Bound cross-field error');
 
     const input = fixture.nativeElement.querySelector('ion-input');
     expect(input.classList.contains('ion-invalid')).toBe(true);

projects/kit/forms/src/kit-ionic-form-field.ts

@@ -1,5 +1,6 @@
 import { Directive, effect, ElementRef, inject, Renderer2 } from '@angular/core';
 import { FORM_FIELD } from '@angular/forms/signals';
+import { KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER } from './kit-signal-form-error-message';
 
 @Directive({
   // Angular's FormField owns [formField]. This sibling directive only adapts its validation message to Ionic.
@@ -16,12 +17,18 @@ export class KitIonicFormField {
   readonly #field = inject(FORM_FIELD, { self: true });
   readonly #element = inject(ElementRef<HTMLElement>);
   readonly #renderer = inject(Renderer2);
+  readonly #resolveErrorMessage = inject(KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER);
 
   constructor() {
     effect(() => {
       const message = this.#field
         .errors()
-        .map((error) => error.message)
+        .map((error) => {
+          if (typeof error.message === 'string' && error.message.trim().length > 0) {
+            return error.message;
+          }
+          return this.#resolveErrorMessage(error);
+        })
         .find((candidate): candidate is string => typeof candidate === 'string' && candidate.trim().length > 0);
       this.#renderer.setProperty(this.#element.nativeElement, 'errorText', message);
     });

projects/kit/forms/src/kit-signal-form-error-message.spec.ts

@@ -0,0 +1,30 @@
+import {
+  emailError,
+  maxDateError,
+  maxError,
+  maxLengthError,
+  minDateError,
+  minError,
+  minLengthError,
+  patternError,
+  requiredError,
+} from '@angular/forms/signals';
+import { kitDefaultSignalFormErrorMessage } from './kit-signal-form-error-message';
+
+describe('kitDefaultSignalFormErrorMessage', () => {
+  it.each([
+    [requiredError(), 'This field is required.'],
+    [emailError(), 'Enter a valid email address.'],
+    [minError(2), 'Enter a value of at least 2.'],
+    [maxError(5), 'Enter a value of no more than 5.'],
+    [minLengthError(3), 'Enter at least 3 characters.'],
+    [maxLengthError(10), 'Enter no more than 10 characters.'],
+    [minDateError(new Date('2026-01-02T00:00:00.000Z')), 'Enter a date on or after 2026-01-02.'],
+    [maxDateError(new Date('2026-12-31T00:00:00.000Z')), 'Enter a date on or before 2026-12-31.'],
+    [minDateError(new Date(Number.NaN)), 'Enter a date on or after a valid date.'],
+    [patternError(/\d+/u), 'Enter a value in the required format.'],
+    [{ kind: 'custom' }, 'Enter a valid value.'],
+  ])('resolves $kind without a configured message', (error, expected) => {
+    expect(kitDefaultSignalFormErrorMessage(error)).toBe(expected);
+  });
+});

projects/kit/forms/src/kit-signal-form-error-message.ts

@@ -0,0 +1,49 @@
+import { InjectionToken } from '@angular/core';
+import type {
+  MaxLengthValidationError,
+  MaxValidationError,
+  MinLengthValidationError,
+  MinValidationError,
+  ValidationError,
+} from '@angular/forms/signals';
+
+export type KitSignalFormErrorMessageResolver = (error: ValidationError) => string | undefined;
+
+const formatDate = (value: Date): string => (Number.isNaN(value.getTime()) ? 'a valid date' : value.toISOString().slice(0, 10));
+
+/** Resolves Angular Signal Forms built-in validation errors to generic English messages. */
+export const kitDefaultSignalFormErrorMessage: KitSignalFormErrorMessageResolver = (error) => {
+  switch (error.kind) {
+    case 'required':
+      return 'This field is required.';
+    case 'email':
+      return 'Enter a valid email address.';
+    case 'min':
+      return `Enter a value of at least ${(error as MinValidationError).min}.`;
+    case 'max':
+      return `Enter a value of no more than ${(error as MaxValidationError).max}.`;
+    case 'minLength':
+      return `Enter at least ${(error as MinLengthValidationError).minLength} characters.`;
+    case 'maxLength':
+      return `Enter no more than ${(error as MaxLengthValidationError).maxLength} characters.`;
+    case 'minDate':
+      return `Enter a date on or after ${formatDate((error as ValidationError & { minDate: Date }).minDate)}.`;
+    case 'maxDate':
+      return `Enter a date on or before ${formatDate((error as ValidationError & { maxDate: Date }).maxDate)}.`;
+    case 'pattern':
+      return 'Enter a value in the required format.';
+    case 'parse':
+      return 'Enter a valid value.';
+    default:
+      return 'Enter a valid value.';
+  }
+};
+
+/** Override to localize or otherwise customize Signal Forms fallback messages. */
+export const KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER = new InjectionToken<KitSignalFormErrorMessageResolver>(
+  'KIT_SIGNAL_FORM_ERROR_MESSAGE_RESOLVER',
+  {
+    providedIn: 'root',
+    factory: () => kitDefaultSignalFormErrorMessage,
+  },
+);

projects/kit/forms/src/public-api.ts

@@ -1,2 +1,3 @@
 export * from './kit-ionic-form-field';
+export * from './kit-signal-form-error-message';
 export * from './provide-kit-ionic-signal-forms';