KeyValue Observing in Swift

KeyValueObserver.playground

import Foundation

public final class Observer: NSObject {

    public let subject: NSObject

    public let keyPath: String

    public let didChange: ([NSKeyValueChangeKey : Any]) -> Void

    /*
    private static var context = 0

    private var context: UnsafeMutableRawPointer { 
        return UnsafeMutableRawPointer(&Observer.context) 
    }
    */
    
    public init(_ subject: NSObject,
        keyPath: String,
        didChange: @escaping ([NSKeyValueChangeKey : Any]) -> Void) {
        
        self.subject = subject
        self.keyPath = keyPath
        self.didChange = didChange

        super.init()

        subject.addObserver(self,
            forKeyPath: keyPath,
            options: [.initial, .new],
            context: nil)
    }
    
    override public func observeValue(
        forKeyPath keyPath: String?,
        of object: Any?,
        change: [NSKeyValueChangeKey : Any]?,
        context: UnsafeMutableRawPointer?) {
        
        /*
        if let change = change,
            context == self.context {
            didChange(change)
        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
        */
        if let change = change {
            didChange(change)
        }
    }
    
    deinit {
        subject.removeObserver(self, forKeyPath: keyPath, context: nil)
    }
}

let progress = Progress(totalUnitCount: 2)

let observer = Observer(progress, keyPath: #keyPath(Progress.fractionCompleted)) {
    change in
    if let newValue = change[.newKey] {
        print("newValue: \(newValue)")
    }
}
progress.completedUnitCount = 1