Avoid app termination without calling the `endBackgroundTask(_:)`.

BackgroundTask.swift

import UIKit

final class BackgroundTask {
    private var taskID = UIBackgroundTaskIdentifier.invalid

    func begin(withName taskName: String = #function) {
        taskID = UIApplication.shared.beginBackgroundTask(withName: taskName) {
            UIApplication.shared.endBackgroundTask(self.taskID)
            self.taskID = UIBackgroundTaskIdentifier.invalid
        }
    }

    func end() {
        guard taskID != UIBackgroundTaskIdentifier.invalid else { return }
        UIApplication.shared.endBackgroundTask(taskID)
    }

    deinit {
        end()
    }
}

Usage

func sendDataToServer(data: NSData) {
    // Perform the task on a background queue.
    DispatchQueue.global().async {
        let task = BackgroundTask()
        task.begin()

        // Send the data synchronously.
        self.sendAppDataToServer( data: data)

        // End the task assertion.
        task.end()
    }
}

// Prints if timeout "[BackgroundTask] Background Task 16 ("sendDataToServer(data:)"), was created over 30 seconds ago. In applications running in the background, this creates a risk of termination. Remember to call UIApplication.endBackgroundTask(_:) for your task in a timely manner to avoid this."