Can't access singleton instance from Swift framework

Question

Using Xcode 6.3, I created a very simple, contrived command line tool in Swift. It contains three modules.

The main module:

import Foundation

let displayer = ValueDisplayer()

displayer.displayValue()

A ValueDisplayer module:

import Foundation

class ValueDisplayer {
    func displayValue() {
        println("The value is \(ValueProvider.instance.value)")
    }
}

and a ValueProvider module:

import Foundation

public class ValueProvider {
    class var instance: ValueProvider {
        struct Static {
            static let instance = ValueProvider()
        }

        return Static.instance
    }

    var value: Int {
        return Int(arc4random())
    }
}

This all compiled and ran successfully. However, I then decided to convert the ValueProvider to a framework. I created a Cocoa Framework target, and moved the ValueProvider module into it instead of in the command line target. I modified the ValueDisplayer module to this:

import Foundation
import ValueProvider

class ValueDisplayer {
    func displayValue() {
        println("The value is \(ValueProvider.instance.value)")
    }
}

and configured the command line tool to link against the framework. Now I get the following compilation error in the ValueDisplayer module related to the println statement:

Module 'ValueProvider' has no member named 'instance'

I'm confused as to why the code doesn't work anymore. I'm wondering if the ValueProvider class is not being qualified correctly anymore, only it is unclear how that should be done.

What is needed to allow the command line tool to compile by linking against the framework?

Answer 1

public is "opt-in". It is not applied magically to members of a public entity; you must apply it explicitly to a member of a public entity if you want that member to be public as well. You made the class ValueProvider public but you did not make its class var instance public, so it is not visible from another module.

So, you would need to rewrite like this:

public class ValueProvider {
    public class var instance: ValueProvider {
        // ...
    }
    public var value: Int {
        // ...
    }
}

[And by the way, a file is not a module. A framework is a module. That is why you have to change your code when you move it from merely being in a separate file to being in a different framework.]

Answer 2

I got program to build and run. What I had to do was go to the Build Phases for the framework and remove the generated header file from the Headers section. I don’t know why this makes things work. My understanding is that header is needed to help with Objective-C interaction. If the program had needed to interact with Objective-C as well as Swift, I’m wondering how anyone could get things to work properly.