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docs(forms): add applyWhenValue example for union validation #68853

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40 changes: 40 additions & 0 deletions adev/src/content/guide/forms/signals/validation.md
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Expand Up @@ -332,6 +332,46 @@ export class OrderComponent {
}
```

## Validation for unions and variants

Some forms use union types where validation rules depend on the current variant. Use `applyWhenValue()` to conditionally apply validation rules based on another field's value.

This pattern is useful for discriminated unions where different variants require different validation rules.

```ts
import {applyWhenValue, form, minLength, required} from '@angular/forms/signals';

type Login =
| {
type: 'email';
email: string;
password: string;
}
| {
type: 'guest';
nickname: string;
};

loginForm = form(this.loginModel, (schemaPath) => {
applyWhenValue(schemaPath, {
when: ({valueOf}) => valueOf(schemaPath.type) === 'email',
apply: (path) => {
required(path.email);
minLength(path.password, 8);
},
});

applyWhenValue(schemaPath, {
when: ({valueOf}) => valueOf(schemaPath.type) === 'guest',
apply: (path) => {
required(path.nickname);
},
});
});
```

`applyWhenValue()` only applies validation rules when the `when` condition returns `true`. This lets each variant define its own validation requirements while keeping validation logic inside the schema function.

## Validation errors

When validation rules fail, they produce error objects that describe what went wrong. Understanding error structure helps you provide clear feedback to users.
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