NotificationCenter+Deferred.swift

import UIKit
import ObjectiveC

fileprivate struct Keys {
    static var notificationActive = "lp_notificationActive"
    static var notificationDeffered = "lp_notificationDeferred"
}

fileprivate extension NotificationCenter {
    var deferredNotifications:[NotificationType] {
        get {
            if let value = objc_getAssociatedObject(self, &Keys.notificationDeffered) as? [NotificationType] {
                return value
            } else {
                return []
            }
        }
        
        set {
            objc_setAssociatedObject(newValue, &Keys.notificationDeffered, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
    
    func fireNotifications() {
        while let notification = deferredNotifications.popLast() {
            post(notification)
        }
    }
}

extension NotificationCenter {
    var active:Bool {
        get {
            if let value = objc_getAssociatedObject(self, &Keys.notificationActive) as? NSNumber {
                return value.boolValue
            } else {
                return false
            }
        }
        
        set {
            if newValue { fireNotifications() }
            
            let value = NSNumber(booleanLiteral: newValue)
            objc_setAssociatedObject(self, &Keys.notificationActive, value, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
    
    func post(deferred notification: NotificationType) {
        if active {
            post(notification)
        } else {
            deferredNotifications.insert(notification, at: 0)
        }
    }
}