Make 2 serial network requests with ReactiveCocoa in an Xcode Playground (with changes from @sebastiangrail)

rac_serial_requests.swift

import Result
import ReactiveCocoa
import Foundation
import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
    
let producer1 = SignalProducer<AnyObject, NSError> { observer, disposable in
    let task1 = NSURLSession.sharedSession().dataTaskWithURL(NSURL(string:"http://jsonplaceholder.typicode.com/posts/1")!, completionHandler: { (data, response, error) in
        if error != nil {
            observer.sendFailed(NSError(domain:"", code:5, userInfo:nil))
        } else {
            let json = try! NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions())
            observer.sendNext(json)
            observer.sendCompleted()
        }
    })
    task1.resume()
}

let producer2 = SignalProducer<AnyObject, NSError> { observer, disposable in
    let task2 = NSURLSession.sharedSession().dataTaskWithURL(NSURL(string:"http://jsonplaceholder.typicode.com/users/1")!, completionHandler: { (data, response, error) in
        if error != nil {
            observer.sendFailed(NSError(domain:"", code:5, userInfo:nil))
        } else {
            let json = try! NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions())
            observer.sendNext(json)
            observer.sendCompleted()
        }
    })
    task2.resume()
}

producer1.flatMap(.Latest) { result -> SignalProducer<AnyObject, NSError> in
    print("result from producer1 = \(result)")
    return producer2
}
.start { event in
    switch event {
    case let .Next(value):
        print("result from producer2 = \(value)")
    case let .Failed(error):
        print("result from producer2 = \(error)")
    case .Completed:
        print("Completed event")
    case .Interrupted:
        print("Interrupted event")
    }
}

/**

Console:

result from producer1 = {
    body = "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto";
    id = 1;
    title = "sunt aut facere repellat provident occaecati excepturi optio reprehenderit";
    userId = 1;
}
result from producer2 = {
    address =     {
        city = Gwenborough;
        geo =         {
            lat = "-37.3159";
            lng = "81.1496";
        };
        street = "Kulas Light";
        suite = "Apt. 556";
        zipcode = "92998-3874";
    };
    company =     {
        bs = "harness real-time e-markets";
        catchPhrase = "Multi-layered client-server neural-net";
        name = "Romaguera-Crona";
    };
    email = "Sincere@april.biz";
    id = 1;
    name = "Leanne Graham";
    phone = "1-770-736-8031 x56442";
    username = Bret;
    website = "hildegard.org";
}
Completed event
*/