Where's my KVO?

I love my KVO (Key-Value Observing) and I have used it like crazy in the past in Objective-C, lately through ReactiveCocoa. And it was the first thing that I missed when I started working with Swift.

One solution is to mark your class with @objc. NSObject is no longer works it magic behind pure Swift classes, and Swift attributes/properties no longer take part on Key-Value coding. In Objective-C you could rely on synthesized @property's or even override + (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key.

The best solution I have for now, to keep classes purely in Swift, is the following snippet. Use it as a template to make Swift properties that generate KVO notifications when properties are set.

class MyClass: NSObject {
    
    // Private variable not to be KVO observed.
    private var _foo = HyperionAuthenticationStep.Credentials
    
    // Public variable to be KVO observed.
    var foo: SomeType {
        get {
            return _foo
        }
        set(newValue) {
            willChangeValueForKey("foo")
            _foo = newValue
            didChangeValueForKey("foo")
        }
    }
}

Admittedly, there is a lot of repetition, and I hate the "foo" strings lying around, but I still haven't found a good way of using preprocessor macros or reflection in Swift. I'm sure it will come around... eventually!