Call child component method from parent class - Angular

I think most easy way is using Subject. In below example code, the child will be notified each time 'tellChild()' is called.

Parent.component.ts

import {Subject} from 'rxjs/Subject';
...
export class ParentComp {
  changingValue: Subject<boolean> = new Subject();
        
  tellChild() {
    this.changingValue.next(true);
  }
}

Parent.component.html

<my-comp [changing]="changingValue"></my-comp>

Child.component.ts

...
export class ChildComp implements OnInit{
  @Input() changing: Subject<boolean>;
  
  ngOnInit(){
    this.changing.subscribe(v => { 
      console.log('value is changing', v);
    });
  }
}

Working sample on Stackblitz

You can do this by using @ViewChild for more info check this link

With type selector

child component

@Component({
  selector: 'child-cmp',
  template: '<p>child</p>'
})
class ChildCmp {
  doSomething() {}
}

parent component

@Component({
  selector: 'some-cmp',
  template: '<child-cmp></child-cmp>',
  directives: [ChildCmp]
})
class SomeCmp {

  @ViewChild(ChildCmp) child:ChildCmp;

  ngAfterViewInit() {
    // child is set
    this.child.doSomething();
  }
}

With string selector

child component

@Component({
  selector: 'child-cmp',
  template: '<p>child</p>'
})
class ChildCmp {
  doSomething() {}
}

parent component

@Component({
  selector: 'some-cmp',
  template: '<child-cmp #child></child-cmp>',
  directives: [ChildCmp]
})
class SomeCmp {

  @ViewChild('child') child:ChildCmp;

  ngAfterViewInit() {
    // child is set
    this.child.doSomething();
  }
}