Bindings.swift

// From objc.io's app architecture book
// https://github.com/objcio/app-architecture

import Foundation

extension NSObjectProtocol where Self: NSObject {
    func observe<Value>(_ keyPath: KeyPath<Self, Value>, onChange: @escaping (Value) -> ()) -> Disposable {
        let observation = observe(keyPath, options: [.initial, .new]) { _, change in
            // See: https://bugs.swift.org/browse/SR-6066
            let newValue = change.newValue ?? (nil as Any? as! Value)
            onChange(newValue)
        }
        return Disposable { observation.invalidate() }
    }
    
    func bind<Value, Target>(_ sourceKeyPath: KeyPath<Self, Value>, to target: Target, at targetKeyPath: ReferenceWritableKeyPath<Target, Value>) -> Disposable {
        return observe(sourceKeyPath) { target[keyPath: targetKeyPath] = $0 }
    }
}

final class Disposable {
    let dispose: () -> ()
    init(_ dispose: @escaping () -> ()) {
        self.dispose = dispose
    }
    deinit {
        dispose()
    }
}