Inconsistency between NSURLSessionDataTask and NSURLSessionDownloadTask

Bizarre.swift

//
//  ViewController.swift
//  RaceCondition
//
//  Created by Oliver Foggin on 17/07/2016.
//  Copyright © 2016 Oliver Foggin. All rights reserved.
//

import Foundation
import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

class FooBar: NSObject {
    
    func startDownload() {
        let url = NSURL(string: "http://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_month.geojson")!
//        let task = NSURLSession.sharedSession().dataTaskWithURL(url) {
//            (data, response, error) in
//            print("completion handler started")
//            self.downloadFinishe()
//        }
        
        let task = NSURLSession.sharedSession().downloadTaskWithURL(url) {
            url, response, error in
            print("completion handler started")
            self.downloadFinishe()
        }
        
        task.addObserver(self, forKeyPath: "state", options: [], context: nil)
        
        task.resume()
        print("download started")
    }
    
    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if let task = object as? NSURLSessionTask where keyPath == "state" && task.state == .Completed {
            task.removeObserver(self, forKeyPath: "state")
            print("Task completed")
        }
    }
    
    func downloadFinishe() {
        print("download finished")
        
        var numbers = [Int]()
        
        for _ in 0...1000 {
            numbers.append(Int(arc4random_uniform(10)))
        }
        
        numbers = numbers.sort()
        print(numbers[500])
        
        print("completion processing finished")
    }
}


let foobar = FooBar()

foobar.startDownload()