1-StageProtocol.swift

import Foundation


protocol StageProtocol {
    func perform() throws -> Self?
    
    var queue: dispatch_queue_t { get }
    
    func run(completion: (() throws -> Self) -> ())
}

extension StageProtocol {
    func run(completion: (() throws -> Self) -> ()) {
        dispatch_async(queue) {
            do {
                if let result = try self.perform() {
                    // Pass to next stage
                    result.run(completion)
                }
                else {
                    // Completed!
                    completion({ self })
                }
            }
            catch let error {
                // Experienced an error
                completion({ throw error })
            }
        }
    }
}

2-FileOpenStage.swift

enum FileOpenStage: StageProtocol {
    case read(fileURL: NSURL)
    case parse(data: NSData)
    case finish(object: AnyObject)
    
    func perform() throws -> FileOpenStage? {
        switch self {
        case let .read(fileURL):
            return try .parse(
                data: NSData(contentsOfURL: fileURL, options: .DataReadingMappedIfSafe)
            )
        case let .parse(data):
            return try .finish(
                object: NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions())
            )
        case .finish:
            return nil
        }
    }
    
    var queue: dispatch_queue_t {
        switch self {
        case .read, .parse:
            return dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0)
        case .finish:
            return dispatch_get_main_queue()
        }
    }
}

func openFile(resource: String, `extension`: String) {
    let fileURL = NSBundle.mainBundle().URLForResource(resource, withExtension: `extension`)!
    
    FileOpenStage.read(fileURL: fileURL).run { useOutput in
        do {
            let output = try useOutput()
            if case let .finish(object) = output {
                print("OPENED: \(object)")
            }
            else {
                fatalError("Unexpected output \(output)")
            }
        }
        catch {
            print("ERROR! \(error)")
        }
    }
}


openFile("example", `extension`: "json")


import XCPlayground
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true