[SWIFT] A story that did not work when trying to handle events in Notification Center

Introduction

I tried to process notifications between classes using NotificationCenter, but it didn't work, so I'll leave a note of it. Since it is made to move by copy and paste, you can paste it on Playground etc. and check it.

If it doesn't work

Introducing the case that does not work first. Roughly speaking, the Sender class posts the notification keyword test when it is instantiated. The Receiver class will receive the notification keyword test when instantiated, and will execute test () when it does.

import Foundation
class Sender {
    let notificationCenter = NotificationCenter()
    init() {
        notificationCenter.post(name: NSNotification.Name(rawValue: "test"),
                                        object: nil)
    }
}

class Receiver {
    let notificationCenter = NotificationCenter()
    init() {
        notificationCenter.addObserver(forName: .init(rawValue: "test"),
                                               object: nil,
                                               queue: nil,
                                               using: { [unowned self] notification in
                                                test()
                                               })
        }
    func test() {
        print("notification")
    }
}

let receiver = Receiver()
let sender = Sender()

If it works

Here's when it works. The difference from if it didn't work is that you are using an instantiation of NotificationCenter. notificationCenter →NotificationCenter.default

import Foundation
class Sender {
    init() {
        NotificationCenter.default.post(name: NSNotification.Name(rawValue: "test"),
                                        object: nil)
    }
}

class Receiver {
    init() {
        NotificationCenter.default.addObserver(forName: .init(rawValue: "test"),
                                               object: nil,
                                               queue: nil,
                                               using: { [unowned self] notification in
                                                test()
                                               })
        }
    func test() {
        print("notification")
    }
}

let receiver = Receiver()
let sender = Sender()

Cause

The cause was that each class instantiated and used NotificationCenter. It's natural that you can't do it because you don't use it across classes.

bonus

Still want to use the instantiated one! I will describe what to do in that case. It may be a rare case, but I think that this idea can be used in AVAudioPlayer etc. Maybe this is more common.

import Foundation
class Sender {
    let notificationCenter = NotificationCenter()
    func send() {
        notificationCenter.post(name: NSNotification.Name(rawValue: "test"),
                                        object: nil)
    }
}

class Receiver {
    let notificationCenter: NotificationCenter
    init(initNC : NotificationCenter) {
        self.notificationCenter = initNC
        notificationCenter.addObserver(forName: .init(rawValue: "test"),
                                               object: nil,
                                               queue: nil,
                                               using: { [unowned self] notification in
                                                test()
                                               })
        }
    func test() {
        print("notification")
    }
}

let sender = Sender()
let receiver = Receiver(initNC: sender.notificationCenter)
sender.send()//notification

I changed a part of the source code when it didn't work as follows.

--Changed to send the notification of Sender class bysend ()instead ofinit (). --Defined a variable of type NotificationCenter in the Receiver class.

By doing this, you can use a variable of type NotificationCenter that is common between classes.