angular-forms

What this skill does

# Angular Signal Forms

Build type-safe, reactive forms using Angular's Signal Forms API. Signal Forms provide automatic two-way binding, schema-based validation, and reactive field state.

**Note:** Signal Forms are experimental in Angular v21. For production apps requiring stability, see [references/form-patterns.md](references/form-patterns.md) for Reactive Forms patterns.

## Basic Setup

```typescript
import { Component, signal } from '@angular/core';
import { form, FormField, required, email } from '@angular/forms/signals';

interface LoginData {
  email: string;
  password: string;
}

@Component({
  selector: 'app-login',
  imports: [FormField],
  template: `
    <form (submit)="onSubmit($event)">
      <label>
        Email
        <input type="email" [formField]="loginForm.email" />
      </label>
      @if (loginForm.email().touched() && loginForm.email().invalid()) {
        <p class="error">{{ loginForm.email().errors()[0].message }}</p>
      }
      
      <label>
        Password
        <input type="password" [formField]="loginForm.password" />
      </label>
      @if (loginForm.password().touched() && loginForm.password().invalid()) {
        <p class="error">{{ loginForm.password().errors()[0].message }}</p>
      }
      
      <button type="submit" [disabled]="loginForm().invalid()">Login</button>
    </form>
  `,
})
export class Login {
  // Form model - a writable signal
  loginModel = signal<LoginData>({
    email: '',
    password: '',
  });
  
  // Create form with validation schema
  loginForm = form(this.loginModel, (schemaPath) => {
    required(schemaPath.email, { message: 'Email is required' });
    email(schemaPath.email, { message: 'Enter a valid email address' });
    required(schemaPath.password, { message: 'Password is required' });
  });
  
  onSubmit(event: Event) {
    event.preventDefault();
    if (this.loginForm().valid()) {
      const credentials = this.loginModel();
      console.log('Submitting:', credentials);
    }
  }
}
```

## Form Models

Form models are writable signals that serve as the single source of truth:

```typescript
// Define interface for type safety
interface UserProfile {
  name: string;
  email: string;
  age: number | null;
  preferences: {
    newsletter: boolean;
    theme: 'light' | 'dark';
  };
}

// Create model signal with initial values
const userModel = signal<UserProfile>({
  name: '',
  email: '',
  age: null,
  preferences: {
    newsletter: false,
    theme: 'light',
  },
});

// Create form from model
const userForm = form(userModel);

// Access nested fields via dot notation
userForm.name                    // FieldTree<string>
userForm.preferences.theme       // FieldTree<'light' | 'dark'>
```

### Reading Values

```typescript
// Read entire model
const data = this.userModel();

// Read field value via field state
const name = this.userForm.name().value();
const theme = this.userForm.preferences.theme().value();
```

### Updating Values

```typescript
// Replace entire model
this.userModel.set({
  name: 'Alice',
  email: '[email protected]',
  age: 30,
  preferences: { newsletter: true, theme: 'dark' },
});

// Update single field
this.userForm.name().value.set('Bob');
this.userForm.age().value.update(age => (age ?? 0) + 1);
```

## Field State

Each field provides reactive signals for validation, interaction, and availability:

```typescript
const emailField = this.form.email();

// Validation state
emailField.valid()      // true if passes all validation
emailField.invalid()    // true if has validation errors
emailField.errors()     // array of error objects
emailField.pending()    // true if async validation in progress

// Interaction state
emailField.touched()    // true after focus + blur
emailField.dirty()      // true after user modification

// Availability state
emailField.disabled()   // true if field is disabled
emailField.hidden()     // true if field should be hidden
emailField.readonly()   // true if field is readonly

// Value
emailField.value()      // current field value (signal)
```

### Form-Level State

The form itself is also a field with aggregated state:

```typescript
// Form is valid when all interactive fields are valid
this.form().valid()

// Form is touched when any field is touched
this.form().touched()

// Form is dirty when any field is modified
this.form().dirty()
```

## Validation

### Built-in Validators

```typescript
import { 
  form, required, email, min, max, 
  minLength, maxLength, pattern 
} from '@angular/forms/signals';

const userForm = form(this.userModel, (schemaPath) => {
  // Required field
  required(schemaPath.name, { message: 'Name is required' });
  
  // Email format
  email(schemaPath.email, { message: 'Invalid email' });
  
  // Numeric range
  min(schemaPath.age, 18, { message: 'Must be 18+' });
  max(schemaPath.age, 120, { message: 'Invalid age' });
  
  // String/array length
  minLength(schemaPath.password, 8, { message: 'Min 8 characters' });
  maxLength(schemaPath.bio, 500, { message: 'Max 500 characters' });
  
  // Regex pattern
  pattern(schemaPath.phone, /^\d{3}-\d{3}-\d{4}$/, {
    message: 'Format: 555-123-4567',
  });
});
```

### Conditional Validation

```typescript
const orderForm = form(this.orderModel, (schemaPath) => {
  required(schemaPath.promoCode, {
    message: 'Promo code required for discounts',
    when: ({ valueOf }) => valueOf(schemaPath.applyDiscount),
  });
});
```

### Custom Validators

```typescript
import { validate } from '@angular/forms/signals';

const signupForm = form(this.signupModel, (schemaPath) => {
  // Custom validation logic
  validate(schemaPath.username, ({ value }) => {
    if (value().includes(' ')) {
      return { kind: 'noSpaces', message: 'Username cannot contain spaces' };
    }
    return null;
  });
});
```

### Cross-Field Validation

```typescript
const passwordForm = form(this.passwordModel, (schemaPath) => {
  required(schemaPath.password);
  required(schemaPath.confirmPassword);
  
  // Compare fields
  validate(schemaPath.confirmPassword, ({ value, valueOf }) => {
    if (value() !== valueOf(schemaPath.password)) {
      return { kind: 'mismatch', message: 'Passwords do not match' };
    }
    return null;
  });
});
```

### Async Validation

```typescript
import { validateHttp } from '@angular/forms/signals';

const signupForm = form(this.signupModel, (schemaPath) => {
  validateHttp(schemaPath.username, {
    request: ({ value }) => `/api/check-username?u=${value()}`,
    onSuccess: (response: { taken: boolean }) => {
      if (response.taken) {
        return { kind: 'taken', message: 'Username already taken' };
      }
      return null;
    },
    onError: () => ({
      kind: 'networkError',
      message: 'Could not verify username',
    }),
  });
});
```

## Conditional Fields

### Hidden Fields

```typescript
import { hidden } from '@angular/forms/signals';

const profileForm = form(this.profileModel, (schemaPath) => {
  hidden(schemaPath.publicUrl, ({ valueOf }) => !valueOf(schemaPath.isPublic));
});
```

```html
@if (!profileForm.publicUrl().hidden()) {
  <input [formField]="profileForm.publicUrl" />
}
```

### Disabled Fields

```typescript
import { disabled } from '@angular/forms/signals';

const orderForm = form(this.orderModel, (schemaPath) => {
  disabled(schemaPath.couponCode, ({ valueOf }) => valueOf(schemaPath.total) < 50);
});
```

### Readonly Fields

```typescript
import { readonly } from '@angular/forms/signals';

const accountForm = form(this.accountModel, (schemaPath) => {
  readonly(schemaPath.username); // Always readonly
});
```

## Form Submission

```typescript
import { submit } from '@angular/forms/signals';

@Component({
  template: `
    <form (submit)="onSubmit($event)">
      <input [formField]="form.email" />
      <input [formField]="form.password" />
      <button type="submit" [disabled]="form().invalid()">Submit</button>
    </form>
  `,
})
export class Login {
  model = signal({ email: '', password: '' });