An Angular2 Application Spinner Utilizing Observables

I’m currently working on a project using Angular2, and, as with many applications, I needed a loading indicator. After doing some research, I decided to use Spin.js, a JavaScript library that can be used to render a ‘spinner’ without using external images or CSS. I then started thinking about how to do this with Angular2; do I need a component that I put in every page?  Is there a way to share a single root level component with my app? etc.  I read through some blogs and other documentation and learned how to set up an observable subscription in order to control a single component using a service that you pass around your application.  I think this can be a really useful pattern of development for a number of purposes, but here’s a look at how I created an Angular2 spinner component with Spin.js.

The Spinner Component

First of all, I created the component that I’ll put an instance of in my root application view.

@Component({

selector: 'ng2-spinner',

templateUrl: './app/spinner/spinner.component.html'

})
export class SpinnerComponent implements OnInit, OnDestroy {

In the component I exposed the Spin.js options as Angular2 component inputs

@Input() lines: number = 12; // The number of lines to draw
@Input() length: number = 20; // The length of each line
@Input() width: number = 12; // The line thickness
@Input() radius: number = 50; // The radius of the inner circle
@Input() scale: number = 1.0; // Scales overall size of the spinner
...

Then on the component’s ngOnInit callback I initialized the component with a setup of inputs from the view specifying spin.js options. It’s important to do so in the ngOnInit callback because doing so in the constructor would be too early. The inputs would not yet be passed from the parent view element.

ngOnInit() {

this.initSpinner();
this.createServiceSubscription();

}

private initSpinner() {

let options = {

lines: this.lines,
length: this.length,
width: this.width,
radius: this.radius,
scale: this.scale,
corners: this.corners,
color: this.color,
opacity: this.opacity,
rotate: this.rotate,
direction: this.direction,
speed: this.speed,
trail: this.trail,
fps: this.fps,
zIndex: 2e9, // Artificially high z-index to keep on top
className: this.className,
top: this.top,
left: this.left,
shadow: this.shadow,
hwaccel: this.hwaccel,
position: this.position

};
console.log('Creating spinner with options:');
console.log(JSON.stringify((options)));
this.spinner = new Spinner(options);

}

Now there is an instance of the spin.js object created that can be shown and hidden as needed. Next comes the magic, you need to create a service that holds a shared observable in order to control the spinner component from other components within the application. Note: it’s important to import the Rxjs ‘share’ operator in order to create the shared observable, I did so in a file called rxjs-operators.ts that I’ve imported into my main application component.

@Injectable()
export class SpinnerService {

private spinnerObserver: Observer;
public spinnerObservable: Observable;

constructor() {

this.spinnerObservable = new Observable(observer => {

this.spinnerObserver = observer;

}

).share();

}

}

The spinner component can then subscribe to this observable. It will be able to receive notifications from the service telling it whether it should show or hide the spinner. I’ve also called this subscription code from within the ngOnInit callback, though it’d be fine to do so within the component constructor as well.

private createServiceSubscription() {

this.subscription =

this.spinnerService.spinnerObservable.subscribe(show => {

if (show) {

this.startSpinner();

} else {

this.stopSpinner();

}

});

}

The service can use the observable in order to trigger the show and hide states in the component.

show() {

if (this.spinnerObserver) {

this.spinnerObserver.next(true);

}

}

hide() {

if (this.spinnerObserver) {

this.spinnerObserver.next(false);

}

}

Now I can drop an instance of my spinner component in my main app component specifying its view options:

<ng2-spinner [radius]="25" [lines]="15" [width]="8"
[length]="15" [opacity]="0.1" [shadow]="true"></ng2-spinner>

And pass around the spinner service to control showing/hiding it everywhere else in my application

constructor(private spinnerService: SpinnerService) {
}

public spin(event: MouseEvent): void {

event.preventDefault();
this.spinnerService.show();
setTimeout(() => {

this.spinnerService.hide();

}, 1000);

}

I think this is a really useful pattern and can imagine it’ll come in handy on future projects.
You can check out my demo project with the full code listings at:
https://github.com/seanlmcgill/ng2spin