Call a method of the child component

I have a nested child component like this:

<app-main>
    <child-component />
</app-main>

My appMain component needs to invoke a method on child-component.

How to invoke a method on the child-component?

You can get a reference to an element using

@ViewChild('childComponent') child;

where childComponent is a template variable <some-elem #childComponent>` or

@ViewChild(ComponentType) child;

where ComponentType is the type of a component or directive and then in ngAfterViewInit or an event handlers call child.someFunc().

ngAfterViewInit() {
  console.log(this.child);
}

See also get hold of an element in the template

Parent and child can communicate via data binding.

Example:

@Component({
    selector: 'child-component',
    inputs: ['bar'],
    template: `"{{ bar }}" in child, counter  {{ n }}`
})
class ChildComponent{
    constructor () {
        this.n = 0;
    }
    inc () {
        this.n++;
    }
}

@Component({
    selector: 'my-app',
    template: `
        <child-component #f [bar]="bar"></child-component><br>
        <button (click)="f.inc()">call child func</button>
        <button (click)="bar = 'different'">change parent var</button>
    `,
    directives: [ChildComponent]
})
class AppComponent {
    constructor () {
        this.bar = 'parent var';
    }
}

bootstrap(AppComponent);  

Demo

#f creates a reference to the child component and can be used in template or passed to function. Data from parent can be passed by [ ] binding.

Being a son component

@Component({
    // configuration
    template: `{{data}}`,
    // more configuration
})
export class Son {

  data: number = 3;
  
  constructor() { }

  updateData(data:number) {
    this.data = data;
  }

}

Having a father component

@Component({
    // configuration
})
export class Parent {
  @ViewChild(Son) mySon: Son;

  incrementSonBy5() {
    this.mySon.updateData(this.mySon.data + 5);
  }
}

In the father's template

<son></son>
<button (click)="incrementSonBy5()">Increment son by 5</button>

This solution only works for one <son></son>instance in the parent template. If you have more than one instance only will work in the first one of the template.

The best way to access a child component is @ViewChild.

Let's say you have AppMainComponent with a nested ChildComponent from your example.

// app-main.component.ts
import { Component } from '@angular/core';

@Component({
    selector: 'app-main',
    template: `
        <child-component />
    `
})

export class AppMainComponent {}

You want to invoke a clear method from your ChildComponent.

// child.component.ts
import { Component } from '@angular/core';

@Component({
    selector: 'child-component',
    template: '{{ greeting }}'
})

class ChildComponent {
    greeting: String = 'Hello World!';

    clear() {
        this.greeting = null;
    }
}

You can accomplish it by importing the ChildComponent class, ViewChild decorator and pass component's class in it as a query. This way, you would have access to the ChildComponent's interface stored in the custom variable. Here is an example:

// app-main.component.ts
import { Component, ViewChild } from '@angular/core';
import { ChildComponent } from './components/child/child.component';

@Component({
    selector: 'app-main',
    template: `
        <child-component />
    `
})

class ChildComponent {
    @ViewChild(ChildComponent)
    child: ChildComponent;

    clearChild() {
        this.child.clear();
    }
}

Notice! Child view becomes available only after ngAfterViewInit.

> Respond after Angular initializes the component's views and child views. Called once after the first ngAfterContentChecked(). A component-only hook.

If you want to execute method automatically, you need to do it inside this lifecycle hook.

You can also get a QueryList of child components via ViewChildren decorator.

import { Component, ViewChildren, QueryList } from '@angular/core';
import { ChildComponent } from './components/child/child.component';

...

@ViewChildren(ChildComponent)
children: QueryList<ChildComponent>;

QueryList might be very useful, e.g. you can subscribe for children changes.

It's also possible to create template reference variables and get access to them via the ViewChild decorator.