Angular2 passing function as component input is not working

Question

I've a component that takes function as input. I've passed this function from parent.

Though the function is called, the function is not able to access the dependencies of the instance on which this function is declared.

Here is the component

@Component({
  selector: 'custom-element',
  template: `
    {{val}}
  `
})
export class CustomElement {
  @Input() valFn: () => string;

  get val(): string {
    return this.valFn();
  }
}

Here is how the component is used

@Injectable()
export class CustomService {
  getVal(): string {
    return 'Hello world';
  }
}

@Component({
  selector: 'my-app',
  template: `
   <custom-element [valFn]="customVal"></custom-element>
  `,
})
export class App {
  constructor(private service: CustomService) {
  }
  customVal(): string {
    return this.service.getVal();
  }
}

When I run this app, I get an error in the console saying Cannot read property 'getVal' of undefined

Here is a plunker for the issue.

https://plnkr.co/edit/oQ229rXqOU9Zu1wQx18b?p=preview

Answer 1

You need to .bind(this) if you pass methods around:

<custom-element [valFn]="customVal.bind(this)"></custom-element>

or

export class App {
  constructor(private service: CustomService) {
  }
  customVal(): string {
    return this.service.getVal();
  }
  customValFn = this.customVal.bind(this);
}

with

<custom-element [valFn]="customValFn"></custom-element>

Answer 2

You can pass a get/set property instead of a function in a similar way like that:

Somewhere in your view:

<input type="text" [(ngModel)]="yourprop">

In your component file:

@Component({
  selector: 'myapp',
  templateUrl: './myapp.component.html',
  styleUrls: ['./myapp.component.scss']
})
export class App {
  constructor() { }

  yourprop: any;

  get yourprop(): any {
    return this.scheduleEndDate;
  };

  //set accessor including call the onchange callback
  set yourprop(v: any) {
    // TODO do something else
    // You can do whatever you want just like you have passed a function

    if (v !== this.scheduleEndDate) {
      this.scheduleEndDate = v;
    }
  }

}

more info @ https://almerosteyn.com/2016/04/linkup-custom-control-to-ngcontrol-ngmodel