[ios-background-execution] Add wrapper for iOS background tasks

src/androidMain/kotlin/ch/dreipol/dreimultiplatform/coroutines/BackgroundTask.kt

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+package ch.dreipol.dreimultiplatform.coroutines
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.launch
+
+actual fun CoroutineScope.launchBackgroundTask(block: suspend CoroutineScope.() -> Unit) : Job = launch(block = block)

src/commonMain/kotlin/ch/dreipol/dreimultiplatform/coroutines/BackgroundTask.kt

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+package ch.dreipol.dreimultiplatform.coroutines
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+
+/** A task that continues running while the app is in the background on iOS.
+ */
+expect fun CoroutineScope.launchBackgroundTask(block: suspend CoroutineScope.() -> Unit) : Job

src/iosMain/kotlin/ch/dreipol/dreimultiplatform/coroutines/BackgroundTask.kt

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+package ch.dreipol.dreimultiplatform.coroutines
+
+import co.touchlab.kermit.Logger
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.InternalCoroutinesApi
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.launch
+import platform.UIKit.UIApplication
+
+actual fun CoroutineScope.launchBackgroundTask(block: suspend CoroutineScope.() -> Unit) : Job = BackgroundTask(launch(block = block))
+
+private class BackgroundTask(val underlyingJob: Job) : Job by underlyingJob {
+    private val backgroundTaskIdentifier = UIApplication.sharedApplication.beginBackgroundTaskWithExpirationHandler {
+        Logger.w { "Cancelling long running background task!" }
+        underlyingJob.cancel("Background Task is about to expire!")
+    }
+
+    @OptIn(InternalCoroutinesApi::class)
+    private val completionHandle = underlyingJob.invokeOnCompletion(onCancelling = true) {
+        UIApplication.sharedApplication.endBackgroundTask(backgroundTaskIdentifier)
+    }
+}