Handy NSObject extension to propagate a value through bindings. Yep, juste like any Cocoa control. OSX only (no bindings on iOS)

NSObjectBindingExtension.swift

import Cocoa

fileprivate func nullableToOptionnal(value: Any?) -> Any? {
    return value is NSNull ? nil : value
}

public extension NSObject {
    public func propagateBoundValue(value: AnyObject?, forBinding key: String) {
        guard let info = self.infoForBinding(key) else { return }
        guard let boundObject = nullableToOptionnal(value: info[NSObservedObjectKey]) as? NSObject, 
            let boundKeyPath = nullableToOptionnal(value: info[NSObservedKeyPathKey]) as? String
            else {
                NSException().raise()
                return
        }
        
        if let opt = info[NSOptionsKey] as? [String: AnyObject], let transformer = nullableToOptionnal(value: opt[NSValueTransformerBindingOption]) as? ValueTransformer ??
            (opt[NSValueTransformerNameBindingOption] as? String).flatMap({
                ValueTransformer(forName: NSValueTransformerName(rawValue: $0))
            }), NSClassFromString(transformer.className)!.allowsReverseTransformation() {
            boundObject.setValue(transformer.reverseTransformedValue(value), forKeyPath: boundKeyPath)
        } else {
            boundObject.setValue(value, forKeyPath: boundKeyPath)
        }
    }
}