A safer block-based NSNotification observer API. This method automatically uses an unowned reference to the observing object, and automatically unregisters the observer when it's deallocated. This implementation is kind of a toy, but a useful toy.

NSNotificationCenter+block.swift

// Note: You could just as easily write this in Objective-C, but I didn't want to. So nyeh.
//
// Typical usage:
//    NSNotificationCenter.defaultCenter().addObserver(self, name: SomeNotification, object: someObject) { self_, note in
//        ...do something with self_...
//    }
// 
// It would benefit from being allowed to shadow `self` in a block parameter list, but Swift 
// forbids that. (http://www.openradar.me/18550356)

import Foundation

public extension NSNotificationCenter {
    func addObserver<T: AnyObject>(observer: T, name aName: String?, object anObject: AnyObject?, queue: NSOperationQueue? = NSOperationQueue.mainQueue(), handler: (observer: T, notification: NSNotification) -> Void) -> AnyObject {
        let observation = addObserverForName(aName, object: anObject, queue: queue) { [unowned observer] note in
            handler(observer: observer, notification: note)
        }
        
        ObservationRemover(observation).makeRetainedBy(observer)
        
        return observation
    }
}

private class ObservationRemover: NSObject {
    let observation: NSObjectProtocol
    
    init(_ obs: NSObjectProtocol) {
        observation = obs
        super.init()
    }
    
    func makeRetainedBy(owner: AnyObject) {
        observationRemoversForObject(owner).addObject(self)
    }
    
    deinit {
        NSNotificationCenter.defaultCenter().removeObserver(observation)
    }
}

private var ObservationRemoverKey: UnsafePointer<Void> = nil

private func observationRemoversForObject(object: AnyObject) -> NSMutableArray {
    if ObservationRemoverKey == nil {
        withUnsafePointer(&ObservationRemoverKey) { pointer in
            ObservationRemoverKey = UnsafePointer<Void>(pointer)
        }
    }
    
    var retainedRemovers = objc_getAssociatedObject(object, ObservationRemoverKey) as NSMutableArray?
    if retainedRemovers == nil {
        retainedRemovers = NSMutableArray()
        objc_setAssociatedObject(object, ObservationRemoverKey, retainedRemovers, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))
    }
    return retainedRemovers!
}

Hi, tried to implement this code and i got two errors:
Line 51:
var retainedRemovers = objc_getAssociatedObject(object, ObservationRemoverKey) as NSMutableArray?
and line 54:
objc_setAssociatedObject(object, ObservationRemoverKey, retainedRemovers, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))

I fixed them by changing line 51 to:
var retainedRemovers = objc_getAssociatedObject(object, ObservationRemoverKey) as? NSMutableArray
and line 54 to be:
objc_setAssociatedObject(object, ObservationRemoverKey, retainedRemovers, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)