//
//  BackgroundHandler.swift
//  AudioPlayer
//
//  Created by Kevin DELANNOY on 22/03/16.
//  Copyright © 2016 Kevin Delannoy. All rights reserved.
//

#if os(OSX)
    import Foundation
#else
    import UIKit

    /// A `BackgroundTaskCreator` serves the purpose of creating background tasks.
    protocol BackgroundTaskCreator: class {
        /// Marks the beginning of a new long-running background task.
        ///
        /// - Parameter handler: A handler to be called shortly before the app’s remaining background time reaches 0.
        ///     You should use this handler to clean up and mark the end of the background task. Failure to end the task
        ///     explicitly will result in the termination of the app. The handler is called synchronously on the main
        ///     thread, blocking the app’s suspension momentarily while the app is notified.
        /// - Returns: A unique identifier for the new background task. You must pass this value to the
        ///     `endBackgroundTask:` method to mark the end of this task. This method returns `UIBackgroundTaskInvalid`
        ///     if running in the background is not possible.
        func beginBackgroundTask(expirationHandler handler: (() -> Void)?) -> UIBackgroundTaskIdentifier

        /// Marks the end of a specific long-running background task.
        ///
        /// You must call this method to end a task that was started using the `beginBackgroundTask(expirationHandler:)`
        /// method. If you do not, the system may kill your app.
        ///
        /// This method can be safely called on a non-main thread.
        ///
        /// - Parameter identifier: An identifier returned by the `beginBackgroundTask(expirationHandler:)` method.
        func endBackgroundTask(_ identifier: UIBackgroundTaskIdentifier)
    }

    extension UIApplication: BackgroundTaskCreator {}
#endif

/// A `BackgroundHandler` handles background.
class BackgroundHandler: NSObject {
    #if !os(OSX)
    /// The background task creator
    var backgroundTaskCreator: BackgroundTaskCreator = UIApplication.shared
    #endif

    /// The backround task identifier if a background task started. Nil if not.
    private var taskIdentifier: Int?

    /// The number of background request received. When this counter hits 0, the background task, if any, will be
    /// terminated.
    private var counter = 0

    /// Ends background task if any on deinitialization.
    deinit {
        endBackgroundTask()
    }

    /// Starts a background task if there isn't already one.
    ///
    /// - Returns: A boolean value indicating whether a background task was created or not.
    @discardableResult
    func beginBackgroundTask() -> Bool {
        #if os(OSX)
            return false
        #else
            counter += 1

            guard taskIdentifier == nil else {
                return false
            }

            taskIdentifier = backgroundTaskCreator.beginBackgroundTask { [weak self] in
                if let taskIdentifier = self?.taskIdentifier {
                    self?.backgroundTaskCreator.endBackgroundTask(taskIdentifier)
                }
                self?.taskIdentifier = nil
            }
            return true
        #endif
    }

    /// Ends the background task if there is one.
    ///
    /// - Returns: A boolean value indicating whether a background task was ended or not.
    @discardableResult
    func endBackgroundTask() -> Bool {
        #if os(OSX)
            return false
        #else
            guard let taskIdentifier = taskIdentifier else {
                return false
            }

            counter -= 1

            guard counter == 0 else {
                return false
            }

            if taskIdentifier != UIBackgroundTaskInvalid {
                backgroundTaskCreator.endBackgroundTask(taskIdentifier)
            }
            self.taskIdentifier = nil
            return true
        #endif
    }
}