An NSObject extension which allows you to use enums as key paths for common KVO tasks.

NSObject+RawRepresentableKVO.swift

extension NSObject {
    func addObserver <RawType: RawRepresentable where RawType.RawValue == String>(observer: NSObject, forKeyPath keyPath: RawType, options: NSKeyValueObservingOptions, context: UnsafeMutablePointer<Void>) {
        addObserver(observer, forKeyPath: keyPath.rawValue, options: options, context: context)
    }

    func removeObserver <RawType: RawRepresentable where RawType.RawValue == String>(observer: NSObject, forKeyPath keyPath: RawType) {
        removeObserver(self, forKeyPath: keyPath.rawValue)
    }

    func removeObserver <RawType: RawRepresentable where RawType.RawValue == String>(observer: NSObject, forKeyPath keyPath: RawType, context: UnsafeMutablePointer<Void>) {
        removeObserver(observer, forKeyPath: keyPath.rawValue, context: context)
    }

    func willChangeValueForKey <RawType: RawRepresentable where RawType.RawValue == String>(key: RawType) {
        willChangeValueForKey(key.rawValue)
    }

    func didChangeValueForKey <RawType: RawRepresentable where RawType.RawValue == String>(key: RawType) {
        didChangeValueForKey(key.rawValue)
    }
}

Example:

enum OperationState: String {
    case isExecuting, isFinished, isReady, isCancelled
}

thingToBeObserved?.addObserver(self,
    forKeyPath: OperationState.isFinished,
    options: [.Initial, .New],
    context: someContext
)

thingToBeObserved?.removeObserver(self,
    forKeyPath: OperationState.isFinished,
    context: someContext
)