How can I pass the FormGroup of a parent component to its child component using the current Form API

Question

I would like to pass the parent component's FormGroup to its child for the purpose of displaying an error-message using the child.

Given the following parent:

parent.component.ts

import { Component, OnInit } from '@angular/core'
import {
  REACTIVE_FORM_DIRECTIVES, AbstractControl, FormBuilder, FormControl, FormGroup, Validators
} from '@angular/forms'

@Component({
  moduleId: module.id,
  selector: 'parent-cmp',
  templateUrl: 'language.component.html',
  styleUrls: ['language.component.css'],
  directives: [ErrorMessagesComponent]
})
export class ParentCmp implements OnInit {
  form: FormGroup;
  first: AbstractControl;
  second: AbstractControl;
  
  constructor(private _fb: FormBuilder) {
    this.first = new FormControl('');
    this.second = new FormControl('')
  }
  
  ngOnInit() {
    this.form = this._fb.group({
      'first': this.first,
      'second': this.second
    });
  }
}

I would now like to pass the form:FormGroup variable above to the child component below:

error-message.component.ts

import { Component, OnInit, Input } from '@angular/core'
import { NgIf } from '@angular/common'
import {REACTIVE_FORM_DIRECTIVES, FormGroup } from '@angular/forms'

@Component({
  moduleId: module.id,
  selector: 'epimss-error-messages',
  template: `<span class="error" *ngIf="errorMessage !== null">{{errorMessage}}</span>`,
  styles: [],
  directives: [REACTIVE_FORM_DIRECTIVES, NgIf]  
})
export class ErrorMessagesComponent implements OnInit {
  @Input() ctrlName: string

  constructor(private _form: FormGroup) { }

  ngOnInit() { }

  get errorMessage() {
    // Find the control in the Host (Parent) form
    let ctrl = this._form.find(this.ctrlName);
    console.log('ctrl| ', ctrl);

//    for (let propertyName of ctrl.errors) {
//      // If control has a error
//      if (ctrl.errors.hasOwnProperty(propertyName) && ctrl.touched) {
//        // Return the appropriate error message from the Validation Service
//        return CustomValidators.getValidatorErrorMessage(propertyName);
//      }
//    }

    return null;
  }

The constructor formGroup represents the FormGroup of the parent - in its present form it does not work.

I am trying to follow this obsolete example at http://iterity.io/2016/05/01/angular/angular-2-forms-and-advanced-custom-validation/

Answer 1

In the parent component do this:

<div [formGroup]="form">
  <div>Your parent controls here</div>
  <your-child-component [formGroup]="form"></your-child-component>
</div>

And then in your child component you can get hold of that reference like so:

export class YourChildComponent implements OnInit {
  public form: FormGroup;

  // Let Angular inject the control container
  constructor(private controlContainer: ControlContainer) { }

  ngOnInit() {
    // Set our form property to the parent control
    // (i.e. FormGroup) that was passed to us, so that our
    // view can data bind to it
    this.form = <FormGroup>this.controlContainer.control;
  }
}

You can even ensure either formGroupName or [formGroup] is specified on your component by changing its selector like so:

selector: '[formGroup] epimss-error-messages,[formGroupName] epimss-error-messages'

This answer should be sufficient for your needs, but if you want to know more I've written a blog entry here:

https://mrpmorris.blogspot.co.uk/2017/08/angular-composite-controls-formgroup-formgroupname-reactiveforms.html