Separate interfaces and implementations, including private implementations, in Swift.

Foo.swift

import Cocoa

class Foo {
    /// Public interface
    var message: String?
    let logMessage: () -> Void
    
    init(message: String) {
        // define a placeholder for `self`, for use within functions in this initializer
        // in advance of `self` being fully initialized
        var _self: Foo?
        
        /// Private implementation
        let _name = "Jeff"
        func name() -> String {
            return _name
        }
        
        /// Public implementation
        self.message = message
        self.logMessage = { () in
            println("\(name()): \(_self!.message)")
        }
        
        // `self` is fully initialized at this point
        _self = self
    }
}

var foo = Foo(message: "Hi")
foo.logMessage()    // prints "Jeff: Hi"

I can't figure out how to do subclassing, though. :\ logMessage can't be overridden by a subclass 'cause it's not a method, and it doesn't seem that it can be changed from within a subclass' initializer.

protocol Vehicle {
    init(numberOfWheels: Int)
    var wheelCount: NSInteger { get }
}

func createTruck(numberOfWheels: Int = 4) -> Vehicle {
    class Truck : Vehicle {
        var fourWheelDrive: Bool?

        init(numberOfWheels: Int) {
            fourWheelDrive = true
            wheelCount = numberOfWheels
        }
        var wheelCount: NSInteger
    }

    return Truck(numberOfWheels: numberOfWheels)
}

var t = createTruck(numberOfWheels: 3)

Has the same subclassing problem mentioned by @wearhere