Correct way to propagate computed value in Angular2

I want to use computed @Input

properties value

But the initial spread value doesn't work.

https://plnkr.co/edit/1MMpOYOKIouwnNc3uIuy

I create a App

(root component with a template driven form) and a NumComponent

(child component that just stores the value entered).

When I pass the attribute NumComponent

as [useThree]="true"

, then I want to set the default '3' toNumComponent

But I cannot find a way without using setTimeout

Is there a way to propagate the initial value without setTimeout?

Edited 5/5

Application component

@Component({
  selector: 'my-app',
  template: `
    <div>
      <form novalidate #form="ngForm">
        <app-num name="num" ngModel [useThree]="true"></app-num>
      </form>
      <pre>{{form.value | json}}</pre>
    </div>
  `
})
export class App {}

      

        
        
        
      

    

NumComponent

export const NumValueAccessor = {
  provide: NG_VALUE_ACCESSOR,
  useExisting: forwardRef(() => NumComponent),
  multi: true
};

@Component({
  selector: 'app-num',
  template: `<input [(ngModel)]="num" type="text" (ngModelChange)="updateValue()" />`,
  providers: [NumValueAccessor]
})
export class NumComponent implements ControlValueAccessor {
  num = 0;

  // I want set literal number 3 to `num` property
  // when `useThree` is true.
  @Input() useThree = false;

  onChange = (_: any) => {};

  updateValue(num = this.num) {
    this.onChange(String(num));
  }

  writeValue(value: string): void {
    if (this.useThree) {
      /**********
       * ISSUE
       **********/
      // this code is not work. after code ran, `NumComponent` has
      // value 3 but AppComponent internal FormComponent value
      // is '' (empty string)
      // this.num = 3;
      // this.updateValue(3);

      // ran code with `setTimeout` solve this problem. but
      // I don't want using setTimeout for this feature.
      // setTimeout(() => {
      //   this.num = 3;
      //   this.updateValue(3);
      // }, 0);

      // Is there any way to propagate computed initial value?
      this.num = 3;
      this.updateValue(3);

      /**********
       * ISSUE
       **********/

      this.useThree = false;
      return;
    }
    this.num = Number(value);
  }

  registerOnChange(fn: any): void {
    this.onChange = fn;
  }

  registerOnTouched(fn: any): void {}

  setDisabledState(isDisabled: boolean): void {}
}

      

        
        
        
      

    

It seems that the parent component does not implement the common meaning during the initialization lifecycle.