I'm unsure of what' the best way to dispatch requests in RAC 3. Whether or not using actions or methods is preferred and why. This code shows each way that I've implemented so far.

RAC3_API.swift

typealias RequestSignalProducer = SignalProducer<(NSData, NSURLResponse), NSError>

class ApiClient: ApiConnectable {
    let apiURL = NSURL(string: "http://yardclub.github.io/mobile-interview/api")!
    var getCategories: Action<Void, [Category], NSError>!
    
    init() {
        getCategories = Action {
            let categoriesURL = self.apiURL.URLByAppendingPathComponent("catalog.json")
            return self.getRequest(categoriesURL) |> map { parseJsonArray($0.0) }
        }
    }

    func getSubcategoriesForCategory(category: Category) -> SignalProducer<[Subcategory], NSError> {
        let subcategoriesURL = self.apiURL.URLByAppendingPathComponent("catalog/\(category.id).json")
        return self.getRequest(subcategoriesURL) |> map { parseJsonArray($0.0) }
    }

    func getRequest(url: NSURL) -> RequestSignalProducer {
        let request = NSURLRequest(URL: url)
        return NSURLSession.sharedSession().rac_dataWithRequest(request)
    }
}


// This class uses the action approach
class ChooseCategoryController {
    let apiClient: ApiConnectable
    var categories: MutableProperty<[Category]> = MutableProperty([])

    init(apiClient: ApiConnectable) {
        self.apiClient = apiClient
        self.categories <~ self.apiClient.getCategories.values
    }

    func requestCategories() {
        self.apiClient.getCategories.apply().start()
    }
    
    // With actions I get niceties like this for free.
    var refreshing: SignalProducer<Bool, NoError> {
        return apiClient.getCategories.executing.producer
    }

    var refreshBegan: SignalProducer<Bool, NoError> {
        return refreshing |> filter({ $0 == true })
    }

    var refreshEnded: SignalProducer<Bool, NoError> {
        return refreshing |> filter({ $0 == false })
    }
}

// This class uses the method approach
class ChooseSubcategoryController {
    let apiClient: ApiConnectable
    let category: Category
    var subcategories: MutableProperty<[Subcategory]> = MutableProperty([])
    let request: SignalProducer<[Subcategory], NSError>

    init(apiClient: ApiConnectable, category: Category) {
        self.apiClient = apiClient
        self.category = category
        self.request = apiClient.getSubcategoriesForCategory(category)
    }

    func requestSubcategories() {
        // You can only use the <~ operator with a SignalProducer that produces
        // an error of type NoError
        let noErrorRequest = request |> catch { _ in SignalProducer<[Subcategory], NoError>(value: []) }
        subcategories <~ noErrorRequest
    }
}