Angular6 - ngIf does not destroy object

Question

I have a parent component that will change the child's form that is passed as @Input() when a button is clicked. I use ngIf to render the child component, but when I click to change the form, the child component does not get destroyed and recreated.

parent.component.ts

form: FormGroup;
showChildForm: boolean;
num: number;

ngOnInit(){ 
   this.showChildForm = false; 
   this.num = 1;
   this.form = new FormGroup({group:{'name', new FormControl('name'+this.num,[])}})
}

changeForm(){
   this.num += 1;
   this.showChildForm = true; 
   this.form = new FormGroup({group:{'name', new FormControl('name'+this.num,[])}})
}

parent.component.html

<button (click)="changeForm()"></button>
<child *ngIf="showChildForm" [form]="form"></child>

child.component.ts

@Input() form: FormGroup;

child.component.html

<form [formGroup]="form">
   <input type="text" [formControl]="form.get('name')"/>
</form>

Answer 1

Inside changeForm you are not setting this.showChildForm to false again.

Try doing this:

changeForm(){
  this.num += 1;
  this.showChildForm = false; 
  setTimeout(() => {
    this.showChildForm = true; 
    this.form = new FormGroup({group:{'name', new FormControl('name'+this.num,[])}})
  })
}

toggling it off and then back on again in the next tick cycle (using setTimeout) will cause the component to be destroyed and recreated.

Answer 2

I had similar problem. In my opinion using changeDetector is more elegant than setTimeout

constructor(private changeDetector: ChangeDetectorRef){}

changeForm() {
    this.num += 1;
    this.showChildForm = false;

    // now notify angular to check for updates
    this.changeDetector.detectChanges();    
    // change detection should remove the component now

    // then we can enable it again to create a new instance
    this.showChildForm = true; 
    this.form = new FormGroup({group:{'name', new FormControl('name'+this.num,[])}})
}