Reuse components in angular2 model driven forms

Question

I'm fairly new to angular2 and for the past few days I have been trying to create reusable form components using model driven forms

So lets say we have a component componentA.component.ts

@Component({
    selector: 'common-a',
    template: `
    <div [formGroup]="_metadataIdentifier">
        <div class="form-group">
        <label>Common A[1]</label>
        <div>
            <input type="text" formControlName="valueA1">
            <small>Description 1</small>
        </div>
        <div class="form-group">
        <label>Common A[2]</label>
        <div>
            <input type="text" formControlName="valueA2">
            <small>Description 2</small>
        </div>
    </div>
    `
})


export class ComponentA implements OnInit{

    @Input('group')
    public myForm: FormGroup;

    constructor(private _fb: FormBuilder) {
    }

    ngOnInit() {
        this.myForm = this._fb.group({
            valueA1 : ['', [Validators.required]],
            valueA2 : ['', [Validators.required]],
        });
    }
}

And a component B componentB.component.ts

@Component({
    selector: 'common-b',
    template: `
    <div [formGroup]="_metadataIdentifier">
        <div class="form-group">
        <label>Common B</label>
        <div>
            <input type="text" formControlName="valueB">
            <small>Description</small>
        </div>
    </div>
    `
})


export class ComponentB implements OnInit{

    @Input('group')
    public myForm: FormGroup;

    constructor(private _fb: FormBuilder) {
    }

    ngOnInit() {
        this.myForm= this._fb.group({
            valueB : ['', [Validators.required]]
        });
    }
}

My question is how can I compose a form using this two sub components without moving the control of the inputs to the main component. For example a main.component.ts

@Component({
    selector: 'main',
    template: `
    <form [formGroup]="myForm" (ngSubmit)="onSubmit(myForm.value)">
        <div>
            <common-a [group]="formA"></common-a>
            <common-b [group]="formB"></common-b>
            <div>
                <button>Register!</button>
            </div>
        </div>
    </form>
    `
})


export class Main implements OnInit{

    @Input('group')
    public myForm: FormGroup;

    public formA : FormGroup;

    public formB : FormGroup;

    constructor(private _fb: FormBuilder) {
    }

    ngOnInit() {
        this.myForm = this._fb.group({
            //how can I compose these two sub forms here
            //leaving the form control names agnostic to this component
        });
    }
}

The concept behind this idea is to build many complex forms that share some of their building blocks.

That is, I don't want my Main component to know the names of the formControlNames [valueA1,valueA2,valueB] but automagically insert them and update/validate on the top level form group.

Any ideas or points to the right direction would be helpfull.

Answer 1

This can be accomplish by passing in our top level FormGroup to the child component and having the child component add itself into the higher level FormGroup using formGroupName that way the upper level FormGroup needs to know essentially nothing about the lower levels:

main.component.ts

template: `<...>
    <common-a [parentForm]="myForm"></common-a>
    <...>

We will also get rid of the formA, formB declarations as they are no longer used.

component-a.component.ts [formGroup] is our parent group, formGroupName is how we will identify and attach the component's controls as a group to work in the larger whole (they will nest inside the parent group).

@Component({<...>
template: `
<div [formGroup]="parentForm">
    <div class="form-group">
    <label>Common A[1]</label>
    <div formGroupName="componentAForm">
        <input type="text" formControlName="valueA1">
        <small>Description 1</small>
    </div>
    <div class="form-group">
    <label>Common A[2]</label>
    <div formGroupName="componentAForm">
        <input type="text" formControlName="valueA2">
        <small>Description 2</small>
    </div>
</div>`
})

export class ComponentA implements OnInit {
     @Input() parentForm: FormGroup;
     componentAForm: FormGroup;

     constructor(private _fb: FormBuilder) {}

     ngOnInit() {
         this.componentAForm = this._fb.group({
             valueA1: ['', Validators.required],
             valueA2: ['', Validators.required]
         });

         this.parentForm.addControl("componentAForm", this.componentAForm);
     }
}

Here's a plunker (note that I made component B a little more dynamic here just to see if it could be done, but the implementation above for is equally applicable to B): http://plnkr.co/edit/fYP10D45pCqiCDPnZm0u?p=preview