Call URLSession downloadTask sequentially

URLSessionOperationPlayground.swift

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import Foundation


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class URLSessionOperation : Operation {
    
    var task : URLSessionDownloadTask?
    
    enum OperationState : Int {
        case ready
        case executing
        case finished
    }
    
    var state : OperationState = .ready
    
    override var isReady: Bool { return state == .ready }
    override var isExecuting: Bool { return state == .executing }
    override var isFinished: Bool { return state == .finished }
    
    init(session: URLSession, downloadTaskURL: URL, completionHandler: ((URL?, URLResponse?, Error?) -> Void)?) {
        super.init()
        task = session.downloadTask(with: downloadTaskURL, completionHandler: { [weak self] (localURL, response, error) in
            
            if let self = self {
                
                if let completionHandler = completionHandler {
                    completionHandler(localURL, response, error)
                }
                
                self.willChangeValue(forKey: "isFinished")
                self.willChangeValue(forKey: "isExecuting")
                
                self.state = .finished
                if let error = error {
                    print("error fetching \(downloadTaskURL.absoluteString), \(error.localizedDescription)")
                } else {
                    print("finished fetching \(downloadTaskURL.absoluteString)")
                }
                
                self.didChangeValue(forKey: "isFinished")
                self.didChangeValue(forKey: "isExecuting")
            }
            
        })
    }
    
    override func start() {
        if(self.isCancelled) {
            self.willChangeValue(forKey: "isFinished")
            state = .finished
            self.didChangeValue(forKey: "isFinished")
            return
        }
        
        self.willChangeValue(forKey: "isExecuting")
        self.task?.resume()
        state = .executing
        self.didChangeValue(forKey: "isExecuting")
    }
    
    override func cancel() {
        super.cancel()
        self.task?.cancel()
    }
}

let queue = OperationQueue()
queue.maxConcurrentOperationCount = 1

var urls = [
    URL(string: "https://github.com/fluffyes/AppStoreCard/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/DispatchQueue/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/telegrammy/archive/master.zip")!
]

for url in urls {
    let operation = URLSessionOperation(session: URLSession.shared, downloadTaskURL: url, completionHandler: { (localURL, urlResponse, error) in
        
    })
    
    queue.addOperation(operation)
}

queue.cancelAllOperations()
queue.waitUntilAllOperationsAreFinished()

print("finished fetching all")