Upload protocol

Upload.swift

/// An object that will be transfered to or from S3
public protocol S3Transferable: class {
    //A unique identifier for this object
    var id: UUID { get }
    //The key for the object in S3
    var objectCloudKey: String { get }
    //A CurrentValueSubject that will publish the progress as a Double and never fail
    var progress: CurrentValueSubject<Double, Never> { get }
    //The object returned by AWS that will give us access to some functionality and status.
    var task: AWSS3TransferUtilityTask? { get set }
    //The status of the transfer published by a CurrentValueSubject that will never fail.
    var status: CurrentValueSubject<AWSS3TransferUtilityTransferStatusType, Never> { get }
    //We will handle some combine stuff internally so I know I will need these publishers
    var taskStatusKVOSubscription: AnyCancellable? { get set }
    var taskStatusSubscription: AnyCancellable? { get set}
    //Function to add a task
    func add(task: AWSS3TransferUtilityTask)
}

extension S3Transferable {
    //Default implementation
    public func add(task: AWSS3TransferUtilityTask) {
        //Store the task to to local variable
        self.task = task
        //Subscribe to the status property of the task using a combine publisher
        self.taskStatusKVOSubscription = task.publisher(for: \.status).sink(receiveValue: { [weak self] (newStatus) in
            //Send the new status value over the publisher
            self?.status.send(newStatus)
        })
        //Subscribe to the status publisher
        self.taskStatusSubscription = self.status.sink(receiveValue: { [weak self] (newStatus) in
            switch newStatus {
            case .cancelled, .error, .completed:
                //So that when the upload is complete I can remove things and clean up.
                self?.taskStatusKVOSubscription?.cancel()
                self?.taskStatusKVOSubscription = nil
                self?.taskStatusSubscription?.cancel()
                self?.taskStatusSubscription = nil
                self?.task = nil
            default:
                break
            }
        })
    }
}