KeyValueObservable.swift

extension KeyValueObservable {
    @discardableResult func addObserver<Observer: AnyObject, Value>(_ observer: Observer, keyPath: KeyPath<Self, Value>, options: Observation.Options, handler: @escaping (Observer, Value) -> Void) -> Observation {
        let observation = Observation()
        let observationHandler: (PartialKeyPath<Self>) -> Bool = { [weak self, weak observer] changedKeyPath in
            guard let self = self else { return false }
            guard let observer = observer else { return false }
            guard changedKeyPath == keyPath else { return true }
            handler(observer, self[keyPath: keyPath])
            return true
        }
        observationStore.observationInfos[observation] = observationHandler
        
        if options.contains(.initial) {
            handler(observer, self[keyPath: keyPath])
        }
        
        return observation
    }
    
    func removeObservation(_ observation: Observation) {
        observationStore.observationInfos.removeValue(forKey: observation)
    }
    
    func didChangeValue(for keyPath: PartialKeyPath<Self>) {
        observationStore.observationInfos = observationStore.observationInfos.filter({ (_, handler) -> Bool in
            return handler(keyPath)
        })
    }
}