isStable examples and caveats

Note two important points about isStable, demonstrated in the examples below:

• the application will never be stable if you start any kind of recurrent asynchronous task when the application starts (for example for a polling process, started with a setInterval, a setTimeout or using RxJS operators like interval);
• the isStable Observable runs outside of the Angular zone.

Let's imagine that you start a recurrent task (here incrementing a counter, using RxJS interval), and at the same time subscribe to isStable.

constructor(appRef: ApplicationRef) {  appRef.isStable.pipe(     filter(stable => stable)  ).subscribe(() => console.log('App is stable now');  interval(1000).subscribe(counter => console.log(counter));}

In this example, isStable will never emit true, and the trace "App is stable now" will never get logged.

If you want to execute something when the app is stable, you have to wait for the application to be stable before starting your polling process.

constructor(appRef: ApplicationRef) {  appRef.isStable.pipe(    first(stable => stable),    tap(stable => console.log('App is stable now')),    switchMap(() => interval(1000))  ).subscribe(counter => console.log(counter));}

In this example, the trace "App is stable now" will be logged and then the counter starts incrementing every second.

Note also that this Observable runs outside of the Angular zone, which means that the code in the subscription to this Observable will not trigger the change detection.

Let's imagine that instead of logging the counter value, you update a field of your component and display it in its template.

constructor(appRef: ApplicationRef) {  appRef.isStable.pipe(    first(stable => stable),    switchMap(() => interval(1000))  ).subscribe(counter => this.value = counter);}

As the isStable Observable runs outside the zone, the value field will be updated properly, but the template will not be refreshed!

You'll have to manually trigger the change detection to update the template.

constructor(appRef: ApplicationRef, cd: ChangeDetectorRef) {  appRef.isStable.pipe(    first(stable => stable),    switchMap(() => interval(1000))  ).subscribe(counter => {    this.value = counter;    cd.detectChanges();  });}

Or make the subscription callback run inside the zone.

constructor(appRef: ApplicationRef, zone: NgZone) {  appRef.isStable.pipe(    first(stable => stable),    switchMap(() => interval(1000))  ).subscribe(counter => zone.run(() => this.value = counter));}