diff --git a/ObserverSet.swift b/ObserverSet.swift
index fca5b05..cc36a70 100644
--- a/ObserverSet.swift
+++ b/ObserverSet.swift
@@ -26,122 +26,198 @@
 import Dispatch
 
 /// A reference to an entry in the list of observers. Use this to remove an observer.
-open class ObserverSetEntry<Parameters> {
- 
-  public typealias ObserverCallback = (Any) -> (Parameters) -> Void
-  
-  fileprivate weak var observer: AnyObject?
-  fileprivate let callBack: ObserverCallback
-  
-  fileprivate init(observer: AnyObject?, callBack: @escaping ObserverCallback) {
-    self.observer = observer
-    self.callBack = callBack
-  }
-  
-}
+public class ObserverSetEntry<Parameters> {
 
-/// A set of observers that can be notified of certain actions. A more Swift-like version of NSNotificationCenter.
-open class ObserverSet<Parameters> {
-  
-  // MARK: - Private properties
-  
-  private var entries: [ObserverSetEntry<Parameters>] = []
-  
-  // MARK: - Initialisers
-  
-  /**
-   Creates a new instance of an observer set.
-   
-   - returns: A new instance of an observer set.
-   */
-  public init() {}
-  
-  // MARK: - Public functions
-  
-  /**
-   Adds a new observer to the set.
-   
-   - parameter observer: The object that is to be notified.
-   - parameter callBack: The function to call on the observer when the notification is to be delivered.
-   
-   - returns: An entry in the list of observers, which can be used later to remove the observer.
-   */
-  @discardableResult
-  open func add<ObserverType: AnyObject>(_ observer: ObserverType, _ callBack: @escaping (ObserverType) -> (Parameters) -> Void) -> ObserverSetEntry<Parameters> {
-    let entry = ObserverSetEntry<Parameters>(observer: observer) { observer in callBack(observer as! ObserverType) }
-    synchronized {
-      self.entries.append(entry)
-    }
-    return entry
-  }
-  
-  /**
-   Adds a new function to the list of functions to invoke when a notification is to be delivered.
-   
-   - parameter callBack: The function to call when the notification is to be delivered.
-   
-   - returns: An entry in the list of observers, which can be used later to remove the observer.
-   */
-  @discardableResult
-  open func add(_ callBack: @escaping (Parameters) -> Void) -> ObserverSetEntry<Parameters> {
-    return self.add(self) { ignored in callBack }
-  }
-  
-  /**
-   Removes an observer from the list, using the entry which was returned when adding.
-   
-   - parameter entry: An entry returned when adding a new observer.
-   */
-  open func remove(_ entry: ObserverSetEntry<Parameters>) {
-    synchronized {
-      self.entries = self.entries.filter{ $0 !== entry }
-    }
-  }
-  
+	fileprivate var isExpired: Bool { false }
 
-  /**
-   Removes an observer from the list.
+	fileprivate func call(_ parameters: Parameters) {
+		// overridden
+	}
+}
 
-   - parameter observer: An observer to remove from the list of observers.
-   */
-  open func removeObserver(_ observer: AnyObject) {
-    synchronized {
-      self.entries = self.entries.filter{ $0.observer !== observer }
-    }
-  }
+private class ObserverSetEntryImpl<ObserverType: AnyObject, Parameters>: ObserverSetEntry<Parameters> {
+	
+	typealias ObserverCallback = (ObserverType) -> (Parameters) -> Void
+	
+	private(set) weak var observer: ObserverType?
+	private let callBack: ObserverCallback
+	private let queue: DispatchQueue?
+	
+	override var isExpired: Bool { observer == nil }
+	
+	init(queue: DispatchQueue?, observer: ObserverType, callBack: @escaping ObserverCallback) {
+		self.queue = queue
+		self.observer = observer
+		self.callBack = callBack
+	}
+	
+	override func call(_ parameters: Parameters) {
+		if let observer = observer {
+			let callBack = self.callBack(observer)
+			if let queue = queue {
+				queue.async {
+					callBack(parameters)
+				}
+			} else {
+				callBack(parameters)
+			}
+		}
+	}
+	
+}
 
-  /**
-   Call this method to notify all observers.
-   
-   - parameter parameters: The parameters that are required parameters specified using generics when the instance is created.
-   */
-  open func notify(_ parameters: Parameters) {
-    var callBackList: [(Parameters) -> Void] = []
-    synchronized {
-      for entry in self.entries {
-        if let observer = entry.observer {
-          callBackList.append(entry.callBack(observer))
-        }
-      }
-      self.entries = self.entries.filter{ $0.observer != nil }
-    }
-    for callBack in callBackList {
-      callBack(parameters)
-    }
-  }
-  
-  // MARK: - Private functions
-  // MARK: Locking support
-  
-  private var queue = DispatchQueue(label: "com.theappbusiness.ObserverSet", attributes: [])
-  
-  private func synchronized(_ f: () -> Void) {
-    queue.sync(execute: f)
-  }
+/// A set of observers that can be notified of certain actions. A more Swift-like version of NSNotificationCenter.
+open class ObserverSet<Parameters> {
+	
+	// MARK: - Private properties
+	
+	private var entries: [ObserverSetEntry<Parameters>] = []
+	
+	// MARK: - Initialisers
+	
+	/**
+	Creates a new instance of an observer set.
+	
+	- returns: A new instance of an observer set.
+	*/
+	public init() {}
+	
+	// MARK: - Public functions
+	
+	/**
+	Adds a new observer to the set.
+	
+	- parameter queue: The queue to call the function on when the notification is delivered. If nil (the default) it will be delivered on the same queue the notification was sent.
+	- parameter observer: The object that is to be notified.
+	- parameter callBack: The function to call on the observer when the notification is to be delivered.
+	
+	- returns: An entry in the list of observers, which can be used later to remove the observer.
+	*/
+	@discardableResult
+	open func add<ObserverType: AnyObject>(queue: DispatchQueue? = nil, _ observer: ObserverType, _ callBack: @escaping (ObserverType) -> (Parameters) -> Void) -> ObserverSetEntry<Parameters> {
+		let entry = ObserverSetEntryImpl<ObserverType, Parameters>(queue: queue, observer: observer, callBack: callBack)
+		synchronized {
+			self.entries.append(entry)
+		}
+		return entry
+	}
+	
+	/**
+	Adds a new function (or closure) to call when a notification is to be delivered.
+	This callback will be notified until it is removed by passing the return value to the `remove(_:)` method
+	
+	The return value is not discardable as the callback should be removed manually. To attach it to the life cycle of an object, use the `add` method that takes a `lifeCycleObserver` parameter.
+	
+	- parameter queue: The queue to call the function on when the notification is delivered. If nil (the default) it will be delivered on the same queue the notification was sent.
+	- parameter callBack: The function to call when the notification is to be delivered.
+	
+	- returns: An entry in the list of observers, which should be used later to remove the observer.
+	*/
+	open func add(queue: DispatchQueue? = nil, _ callBack: @escaping (Parameters) -> Void) -> ObserverSetEntry<Parameters> {
+		add(queue: queue, self) { _ in callBack }
+	}
+	
+	/**
+	Adds a new function (or closure) to call when a notification is to be delivered.
+	The callback will be removed when the `lifeCycleObserver` object is deallocated
+	
+	- parameter queue: The queue to call the function on when the notification is delivered. If nil (the default) it will be delivered on the same queue the notification was sent.
+	- parameter callBack: The function to call when the notification is to be delivered.
+	
+	- returns: An entry in the list of observers, which can be used later to remove the observer.
+	*/
+	@discardableResult
+	open func add<ObserverType: AnyObject>(queue: DispatchQueue? = nil, _ lifeCycleObserver: ObserverType, _ callBack: @escaping (Parameters) -> Void) -> ObserverSetEntry<Parameters> {
+		add(queue: queue, lifeCycleObserver) { _ in callBack }
+	}
+	
+	/**
+	Sets the keyPath on an observer to the value received when a notification is delivered.
+	
+	- parameter queue: The queue to call the function on when the notification is delivered. If nil (the default) it will be delivered on the same queue the notification was sent.
+	- parameter keyPath: The keyPath to set when the notification is to be delivered.
+	
+	- returns: An entry in the list of observers, which can be used later to remove the observer.
+	*/
+	@discardableResult
+	open func add<ObserverType: AnyObject>(queue: DispatchQueue? = nil, _ observer: ObserverType, _ keyPath: ReferenceWritableKeyPath<ObserverType, Parameters>) -> ObserverSetEntry<Parameters> {
+		add(queue: queue, observer) { observer in { observer[keyPath: keyPath] = $0 } }
+	}
+	
+	/**
+	Removes an observer from the list, using the entry which was returned when adding.
+	
+	- parameter entry: An entry returned when adding a new observer.
+	*/
+	open func remove(_ entry: ObserverSetEntry<Parameters>) {
+		synchronized {
+			self.entries = self.entries.filter{ $0 !== entry }
+		}
+	}
+	
+	
+	/**
+	Removes an observer from the list.
+	
+	- parameter observer: An observer to remove from the list of observers.
+	*/
+	open func removeObserver<ObserverType: AnyObject>(_ observer: ObserverType) {
+		synchronized {
+			self.entries.removeAll(where: { ($0 as? ObserverSetEntryImpl<ObserverType, Parameters>)?.observer === observer })
+		}
+	}
+	
+	/**
+	Call this method to notify all observers.
+	
+	- parameter parameters: The parameters that are required parameters specified using generics when the instance is created.
+	*/
+	open func notify(_ parameters: Parameters) {
+		let callBackList = synchronized { () -> [ObserverSetEntry<Parameters>] in
+			self.entries.removeAll(where: { $0.isExpired })
+			return self.entries
+		}
+		for callBack in callBackList {
+			callBack.call(parameters)
+		}
+	}
+	
+	// MARK: - Private functions
+	// MARK: Locking support
+	
+	private var queue = DispatchQueue(label: "com.theappbusiness.ObserverSet", attributes: [])
+	
+	private func synchronized<T>(_ f: () -> T) -> T {
+		queue.sync(execute: f)
+	}
 }
 
 public extension ObserverSet where Parameters == Void {
-  public func notify() {
-    notify(())
-  }
+	
+	/**
+	Call this method to notify all observers.
+	*/
+	func notify() {
+		notify(())
+	}
+	/**
+	Adds a new observer to the set.
+	
+	Convenience method so that the callback function can be `func callback() { ... }` instead of `func callback(_: Void) { ... }`
+
+	- parameter queue: The queue to call the function on when the notification is delivered. If nil (the default) it will be delivered on the same queue the notification was sent.
+	- parameter observer: The object that is to be notified.
+	- parameter callBack: The function to call on the observer when the notification is to be delivered.
+	
+	- returns: An entry in the list of observers, which can be used later to remove the observer.
+	*/
+	@discardableResult
+	func add<ObserverType: AnyObject>(queue: DispatchQueue? = nil, _ observer: ObserverType, _ callBack: @escaping (ObserverType) -> () -> Void) -> ObserverSetEntry<Parameters> {
+		add(queue: queue, observer) { observer in
+			{ _ in
+				callBack(observer)()
+			}
+		}
+	}
+	
 }