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Document 'own form controls' application

tags/part_1
Kevin C. Coram 1 year ago
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# Components Creating Own Form Controls

When a component becomes sufficiently complex, or the developer wishes to be able to reuse parts of it elsewhere, the component can be refactored into sub-components. One approach to binding such sub-components to their containing parent component is to pass the parent component's `FormGroup` in as an `@Input` parameter. Additionally, the data that each child sub-component needs is passed into an `@Input`. The children create their own `FormControls` as needed, and add them to the parent `FormGroup` provided to them. This approach greatly simplifies the code and template of the parent component:

```typescript
export class AppComponent implements OnInit, OnDestroy {
contact: Contact;
form: FormGroup;

private subscription: Subscription

constructor(private fb: FormBuilder, private service: ContactService) {
this.form = this.fb.group({});
}

public ngOnInit() {
this.subscription = this.service.loadContact().subscribe((data: Contact) => {
this.contact = data;
});
}
}
```

```html
<form [formGroup]="form">
<nested-forms-name
[name]="contact.name"
[parent]="form"
></nested-forms-name>
<nested-forms-address-list
[addresses]="contact.addresses"
[parent]="form"
></nested-forms-address-list>
</form>
```

The `nested-forms-name` component is responsible for creating the form controls binding to the Contact's name, and the `nested-forms-address-list` component is responsible for iterating over the Contact's addresses and binding to them using the `nested-forms-address` (singular) component. For example, the `nested-forms-name` would be implented as so:

```typescript
@Component({
selector: 'nested-forms-name',
templateUrl: './name.component.html',
styleUrls: ['./name.component.css']
})
export class NameComponent implements OnInit {

@Input() name: Name;
@Input() parent: FormGroup;

group: FormGroup;

constructor(private fb: FormBuilder) {
}

ngOnInit() {
this.group = this.fb.group({
firstName: new FormControl(this.name ? this.name.firstName : ''),
lastName: new FormControl(this.name ? this.name.lastName : ''),
middleName: new FormControl(this.name ? this.name.middleName : ''),
prefix: new FormControl(this.name ? this.name.prefix : ''),
suffix: new FormControl(this.name ? this.name.suffix : ''),
});

if (this.parent) {
this.parent.addControl('name', this.group);
}
}
}
```

Calling `this.parent.addControl(....)` is what ensures that the controls created in the child component are made part of the over-all form.

## Pros

- The parent component is easy to understand and maintain
- Each child component encapsulates its form controls and template
- The child components can easily be re-used in other "parent" components

## Cons

- The creation of the form controls is tightly coupled with the templates
- Since each child component encapsulates its form controls, the overall shape of the form data is not always clear


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