/*
Copyright (C) 2015 Apple Inc. All Rights Reserved.
See LICENSE.txt for this sample’s licensing information

Abstract:
Contains the code related to automatic background tasks
*/

import UIKit

/**
    `BackgroundObserver` is an `EarthquakeOperationObserver` that will
    automatically begin and end a background task if the application transitions to the
    background. This would be useful if you had a vital `EarthquakeOperation`
    whose execution *must* complete, regardless of the activation state of the app.
    Some kinds network connections may fall in to this category, for example.
*/
class BackgroundObserver: NSObject, EarthquakeOperationObserver {
    // MARK: Properties

    private var identifier = UIBackgroundTaskIdentifier.invalid
    private var isInBackground = false
    
    override init() {
        super.init()
        
        // We need to know when the application moves to/from the background.
        NotificationCenter.default.addObserver(self, selector: #selector(BackgroundObserver.didEnterBackground(notification:)), name: UIApplication.didEnterBackgroundNotification, object: nil)

        NotificationCenter.default.addObserver(self, selector: #selector(BackgroundObserver.didEnterForeground(notification:)), name: UIApplication.didBecomeActiveNotification, object: nil)
        
        isInBackground = UIApplication.shared.applicationState == .background
        
        // If we're in the background already, immediately begin the background task.
        if isInBackground {
            startBackgroundTask()
        }
    }
    
    deinit {
        NotificationCenter.default.removeObserver(self)
    }
    
    @objc func didEnterBackground(notification: NSNotification) {
        if !isInBackground {
            isInBackground = true
            startBackgroundTask()
        }
    }
    
    @objc func didEnterForeground(notification: NSNotification) {
        if isInBackground {
            isInBackground = false
            endBackgroundTask()
        }
    }
    
    private func startBackgroundTask() {
        if identifier == UIBackgroundTaskIdentifier.invalid {
            identifier = UIApplication.shared.beginBackgroundTask(withName: "BackgroundObserver", expirationHandler: {
                self.endBackgroundTask()
            })
        }
    }
    
    private func endBackgroundTask() {
        if identifier != UIBackgroundTaskIdentifier.invalid {
            UIApplication.shared.endBackgroundTask(convertToUIBackgroundTaskIdentifier(identifier.rawValue))
            identifier = UIBackgroundTaskIdentifier.invalid
        }
    }
    
    // MARK: Operation Observer
    
    func operationDidStart(operation: EarthquakeOperation) { }
    
    func operation(operation: EarthquakeOperation, didProduceOperation newOperation: Operation) { }

    func operationDidFinish(operation: EarthquakeOperation, errors: [NSError]) {
        endBackgroundTask()
    }
}

// Helper function inserted by Swift 4.2 migrator.
fileprivate func convertToUIBackgroundTaskIdentifier(_ input: Int) -> UIBackgroundTaskIdentifier {
	return UIBackgroundTaskIdentifier(rawValue: input)
}