Nativescript: Continue code execution when app goes to background - ios

Question

(edited to provide updated info)
I have a nativescript app that performs various tasks that I would like to continue going if the phone goes into background mode or is locked.
Focused on iOS, with Nativescript Angular. I am also new to using obj C code in Nativescript.
As an easy example, let's say I want to print to the console every 5 seconds after a user hits a button, so I have the following code in my component ts file:
coolComponent.ts:
#Component({...})
Export class coolComponent {
...
whenButtonClicked(){
setInterval(function(){
console.log('button has been clicked. show every 5 seconds!');
}, 5000);
}
Without further code, when the user hits the button, it will print to console every 5 seconds, but then stop when the app is in the background or phone is locked. How do I get the function to continue executing even when app is in the background or locked?
In seeing different sources, like here (NS docs on background execution) and here (docs on app delegate) , it looks like the first step is to create a custom-app-delegate, get that to work, and then identify the background task in info.plist.
I have gotten things generally to be functional, like this:
app/custom-app-delegate.ts:
import { ios, run as applicationRun } from "tns-core-modules/application";
export class CustomAppDelegate extends UIResponder implements
UIApplicationDelegate {
public static ObjCProtocols = [UIApplicationDelegate];
public applicationDidEnterBackground(application: UIApplication) {
console.log('in background mode!')
}
}
main.ts:
import { platformNativeScriptDynamic } from "nativescript-angular/platform";
import { AppModule } from "./app.module";
import * as application from "tns-core-modules/application";
import { CustomAppDelegate } from "./custom-app-delegate";
application.ios.delegate = CustomAppDelegate;
platformNativeScriptDynamic().bootstrapModule(AppModule);
app/app.module.ts:
import { CustomAppDelegate } from "./custom-app-delegate";
app/App_Resources/iOS/info.plist:
...
<key>UIBackgroundModes</key>
<array>
<string>fetch</string>
</array>
edit: create reference.d.ts:
/// <reference path="./node_modules/tns-platform-declarations/ios.d.ts" />
/// <reference path="./node_modules/tns-platform-declarations/android.d.ts" />
Edit: FYI, to get the custom-app-delegate to work, I also had to download "tns-platform-declerations", with the command:
$ npm i tns-platform-declarations --save-dev
With this, the app properly reads "in background mode!" when the app goes to the background. So the custom-app-delegate is functional.
However, the examples online assume that the code in the custom-app-delegate is independent of the rest of the app, so they assume there are new tasks to do when the app goes into background mode.
That is not the case here. I have a task that is being performed from the coolComponent function, and when the app goes into background or is locked I want that to continue.
This probably requires that coolComponent.ts communicate with custom-app-delegate, but I don't know how to do this.
Just repeating the code in both files--having the setInterval function appear in both coolComponent.ts and custom-app-delegate--does not work, because this would not result in the custom-app-delegate continuing on the same timing that began in coolComponent.ts after the user hit the button.
So how can I have the code start in coolComponent.ts and continue after the app is in background mode?

Answer 1

Technically you can't force your app to be active when user no longer wants it to be (by locking the phone / minimising the app). If you like to run anything in background, you will have to use background fetch in iOS.

Answer 2

iOS allows you to run code in the background only for certain situations. For instance (but not limited to):
Background location updates.
Audio and video playback (PiP in iPad)
Remote Push Notifications handling
among others...
If your app does not fit any of the available categories, the best you can do is to request to iOS more time to run in the background (by default is 10 seconds). This will allow you to run for 3 more minutes. Just keep running the task in an infinite loop and gracefully terminate your app before the granted 180 seconds.
Regarding background fetch, this mechanism allows apps to update its contents in the background. iOS will execute apps that declare background fetch at least once a day, so you, in your delegate can perform an update from the server. This mechanism is not suitable for what you are looking for.