AttachingObserver (Weak KVO)

AttachingObserver.swift

// Based on https://gist.github.com/joerick/a0b87607e69071fa6a3c
class AttachingObserver: NSObject {
	typealias ChangeHandler = (NSObject, [NSKeyValueChangeKey: Any]?) -> Void
	weak var observedObject: NSObject?
	let keyPath: String
	let changeHandler: ChangeHandler
	
	init(observe object: NSObject, keyPath: String, changeHandler: @escaping ChangeHandler) {
		self.observedObject = object
		self.keyPath = keyPath
		self.changeHandler = changeHandler
		super.init()
		
		object.addObserver(self, forKeyPath: keyPath, options: [.initial, .new], context: nil)
		objc_setAssociatedObject(object, "observer", self, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
	}
	
	deinit {
		observedObject?.removeObserver(self, forKeyPath: keyPath)
	}
	
	override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
		if keyPath == self.keyPath {
			changeHandler(object as! NSObject, change)
		}
	}
}