Make 2 serial network requests with ReactiveCocoa in an Xcode Playground

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")
    }
}