Upload progress reporting NSProgress for NSURLSessionTask

gistfile1.txt

import Foundation

public class UploadProgress: NSProgress
{
    var sessionTask: NSURLSessionTask!
    
    required public init(sessionTask: NSURLSessionTask)
    {
        super.init(parent: nil, userInfo: nil)
        
        self.sessionTask = sessionTask
        
        self.cancellationHandler = { [weak self] in
            self?.sessionTask?.cancel()
        }
        
        addObserver()
        updateValues()
    }
    
    deinit
    {
        removeObserver()
    }
    
    func addObserver()
    {
        sessionTask.addObserver(self, forKeyPath: "countOfBytesExpectedToSend", options: NSKeyValueObservingOptions.New, context: nil)
        sessionTask.addObserver(self, forKeyPath: "countOfBytesSent", options: NSKeyValueObservingOptions.New, context: nil)
        sessionTask.addObserver(self, forKeyPath: "state", options: NSKeyValueObservingOptions.New, context: nil)
    }
    
    func removeObserver()
    {
        if sessionTask != nil
        {
            sessionTask.removeObserver(self, forKeyPath: "countOfBytesExpectedToSend", context: nil)
            sessionTask.removeObserver(self, forKeyPath: "countOfBytesSent", context: nil)
            sessionTask.removeObserver(self, forKeyPath: "state", context: nil)
            
            // we can also give up the strong reference
            sessionTask = nil
        }
    }
    
    func updateValues()
    {
        self.totalUnitCount = sessionTask.countOfBytesExpectedToSend
        self.completedUnitCount = sessionTask.countOfBytesSent
    }
    
    override public func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>)
    {
        if keyPath == "state"
        {
            if sessionTask.state == .Canceling || sessionTask.state == .Completed
            {
                // we are don with the observing
                removeObserver()
            }
        }
        else
        {
            updateValues()
        }
    }
}

import ObjectiveC

// Declare a global var to produce a unique address as the assoc object handle
var UploadProgressAssociatedObjectHandle: UInt8 = 0

public extension NSURLSessionTask
{
    var uploadProgress: UploadProgress
    {
        get
        {
            if let existingProgress = objc_getAssociatedObject(self, &UploadProgressAssociatedObjectHandle) as? UploadProgress
            {
                return existingProgress
            }
            
            let progress = UploadProgress(sessionTask: self)
            self.uploadProgress = progress
            
            return progress
        }
        
        set
        {
            objc_setAssociatedObject(self, &UploadProgressAssociatedObjectHandle, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}

usage: let progress = dataTask.uploadProgress and observe fractionPercentage