HSObserver

Summary

Better Notification & Key Value Observers for Swift.

• Simpler API with sensible defaults
• Easier to avoid ‘dangling’ observers
• Delivers on main thread by default (avoid unexpected concurrency bugs)
• Easy activation/deactivation
• Simple integration with view controller lifecycles

Installation

HSObserver is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod 'HSObserver'

Or Install as a swift package

Observers are Released Automatically

class Watcher {
    struct Notif {
        static let wave = NSNotification.Name.init("waveNotification")
    }
    

    var waveObserver:HSObserver
    init() {
        waveObserver = HSObserver.init(forName: Watcher.Notif.wave,
                                           activate:true,
                                           using: { (notif) in
            //Do Something
        })
    }
}

Unlike a standard observer, waveObserver is fully released when Watcher is released.

(Posting a wave notification will not call the //Do Something code once Watcher is released)

Delivery happens on the main thread

It’s easy to get bitten by notifications unexpectedly arriving on a background thread. In almost all cases - you don’t want that! .

(you can change this for a given observer if you want to - but you probably shouldn’t)

If you use the initialiser without specifying a queue (the default), then the calling block is marked as @MainActor - so should play nicely with swift async code

Observers can be Activated and Deactivated

    var waveObserver:HSObserver
    init() {
        waveObserver = HSObserver.init(forName: Watcher.Notif.wave,
                                           using: { (notif) in
            //Do Something
        })

        //activate
        waveObserver.activate()

        //deactivate
        waveObserver.deactivate()
    }

N.B. Remember that you have to activate your observer for it to work.

• either specify activate:true in the initialiser
• or call myObserver.activate()`
• or chain on the initialiser HSObserver.init(....).activate()

HSHasObservers integrates well with View Controller lifecycle

A common pattern for a view controller is to activate observers in viewWillAppear, and de-activate them in viewDidDisppear

Adding the HSHasObservers protocol to any class allows you to add a group of observers, and activate or deactivate them easily.

Observers can be added manually, or by chaining .add(to:self) to an HSObserver

class ViewController: NSViewController, HSHasObservers {

    override func viewDidLoad() {
        super.viewDidLoad()

        //Add manually
        let waveObserver = HSObserver.init(forName: Watcher.Notif.wave,
                                           using: { (notif) in
                                            //Do Something
        })
        self.add(observer: waveObserver)

        //Or by chaining
        HSObserver.init(forName: Watcher.Notif.wave,
                            using: { (notif) in
                                //Do Something
        }).add(to: self)
    }
}

this works well with the view lifecycle

    override func viewWillAppear() {
        super.viewWillAppear()

        activateObservers()
    }

    override func viewDidDisappear() {
        super.viewDidDisappear()

        deactivateObservers()
    }

Add Multiple Observers

    let manyThingsObserver = HSObserver.init(forNames: [Watcher.Notif.wave,Watcher.Notif.hello] ,
                                                     activate:true,
                                           using: { (notif) in
                                            //Do Something
        })

Specify centre, queue, etc

    /// Create observer
    ///
    /// - parameter name:  notification name
    /// - parameter obj:   object to observe (default nil)
    /// - parameter queue: queue to run the block on (default main)
    /// - parameter center: notification center (default NotificationCenter.default)
    /// - parameter block: block to run (beware of retain cycles!)
    ///
    /// - returns: unactivated manager. Call activate() to start
    convenience init(forName name: NSNotification.Name,
                     object obj: Any? = nil,
                     queue: OperationQueue? = .main,
                     center newCenter: NotificationCenter = NotificationCenter.default,
                     activate: Bool = false,
                     using block: @escaping (Notification) -> Swift.Void)

HSObservers lets you skip the defaults. We assume

• object = nil
• queue = .main
• center = NotificationCenter.default
• activate = false

you can override each of these in the initialiser

Note that Apple’s default is to call your block on the same queue as the sender. If you want to do this, then just use centre = nil

I find that I typically want to use notifications to update the UI - so my default is to use .main

Brent Simmons has a great article on why you should almost always be using .main

Convenience functions on NSNotification.Name

Post a notification directly

class Watcher {
  struct Notif {
      static let wave = NSNotification.Name.init("waveNotification")
  }
    

  func doPosting() {
      Watcher.Notif.wave.post()
      //or
      Watcher.Notif.wave.post(object:self,userInfo:["Foo":"Bar"])
  }
}

Assume the default notification centre and default options when posting directly from NotificationCenter (I strongly reccomend that you structure your notifications within a Notif struct of the relevant object. It makes things really easy to read)

NotificationCenter.post(Watcher.Notif.wave)
//is equivalent to
NotificationCenter.default.post(Watcher.wave,object:nil)

Now with Key Value Notifications

for example, to observe the duration of an AVPlayerItem

durationObserver = HSKeyPathObserver.init(forKeyPath: "duration",
                                        of: item,
                                        activate:true) {
                                          [weak self](_) in
                                          self?.generateImages()
}

(again, remember to keep a reference to durationObserver or it will disappear)

Author

ConfusedVorlon, rob@hobbyistsoftware.com

License

HSNotification is available under the MIT license. See the LICENSE file for more info.