[Angular] Write Compound Components with Angular’s ContentChild

Allow the user to control the view of the toggle component. Break the toggle component up into multiple composable components that can be rearranged by the app developer.

Compound component mainly for rendering flexibility. It hides the implements detial from users, but users can freely re-order the child component order or add new component into it.

In this post we are looking into how to apply "@ConetentChild" to do compound components. 

This is not a recommended way, just for learning!

So a highlevel compound component looks like:

<toggle (toggle)="onToggle($event)">
  <toggle-button></toggle-button>
  <toggle-on>On</toggle-on>
  <toggle-off>Off</toggle-off>
</toggle>

For the parent component 'toggle' component, the job for it is hide implement details from the consumers. It handles the internal state. Which means 'toggle' component needs to access the state of its Children components.

<toggle-button></toggle-button>
<toggle-on>On</toggle-on>
<toggle-off>Off</toggle-off>

The way to can accomplish it is using '@ContentChild':

@ContentChild(ToggleOnComponent) toggleOn: ToggleOnComponent;
@ContentChild(ToggleOffComponent) toggleOff: ToggleOffComponent;
@ContentChild(ToggleButtonComponent) toggleButton: ToggleButtonComponent;

Listen for Child component's ouput event:

toggle-button component has Output event call 'toggle':

@Component({
  selector: 'toggle-button',
  template: '<switch [on]="on" (click)="onClick()" ></switch>',
})
export class ToggleButtonComponent  {
  @Input() on: boolean;
  @Output() toggle: EventEmitter<boolean> = new EventEmitter();
  onClick() {
    this.on = !this.on;
    this.toggle.emit(this.on);
  }
}

Then we can listen the Output event in 'ngAfterContentInit()' lifecycle hooks.

 ngAfterContentInit() {
    this.toggleButton.toggle.subscribe(on => {
      this.on = on;
      this.toggle.emit(on);
      this.update();
    });
  }

Also 'toggle' component will take care to update Children components state:

  update() {
    this.toggleOn.on = this.on;
    this.toggleOff.on = this.on;
    this.toggleButton.on = this.on;
  }