app.module.ts

import { NgModule, APP_INITIALIZER } from "@angular/core";
import { NativeScriptModule } from "nativescript-angular/nativescript.module";

import {
    on as applicationOn,
    suspendEvent,
    ApplicationEventData,
} from "application";

import { AppComponent } from "./app.component";
import { CountDownTimerService } from "./countdown-timer.service";

export const handleSuspend = (countDownTimerService: CountDownTimerService) => 
    () => applicationOn(suspendEvent, (args: ApplicationEventData) => {
        if (args.android) {
            countDownTimerService.startCountDown(1);
        } else if (args.ios) {
            console.log("UIApplication: " + args.ios);
        }
    });

@NgModule({
    bootstrap: [ AppComponent ],
    imports: [ NativeScriptModule, ],
    declarations: [ AppComponent, ],
    providers: [
        CountDownTimerService,
        {
            provide: APP_INITIALIZER,
            useFactory: handleSuspend,
            deps: [CountDownTimerService],
            multi: true,
        },
    ],
})
export class AppModule { }