Introduction

Observe design pattern Wiki
I use the Observer design pattern when I need to inform the user about changes made to an object in the background thread.

Observable variable class definition

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class Observable<T> {
    init(value: T) {
        self.value = value
    }
    var value: T {
        didSet {
            DispatchQueue.main.async {
                self.valueChanged?(self.value)
            }
        }
    }

    private var valueChanged: ((T) -> Void)?
    func addObserver(_ onChange: ((T) -> Void)?) {
        valueChanged = onChange
    }

    func removeObserver() {
        valueChanged = nil
    }
}

Create observable variable

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var hasInternet = Observable<Bool>(value: true)

Set observable variable value

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hasInternet.value = false

Observe the variable for value change

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hasInternet.addObserver( {value in
    if !value{
        //handle observable variable value changed to false
    }else{
        //handle observable variable value changed to true
    }
})

Sample

For example, I want to monitor the internet connection. If internet is disconnected, pop up a notification message.

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func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    ...

    self.monitorNetworkConnection()
}
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class CommonModel{
    ...
    static var hasInternet = Observable<Bool>(value: true)
}
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import Network

let internetMonitor = NWPathMonitor()
func monitorNetworkConnection(){
    internetMonitor.pathUpdateHandler = { pathUpdateHandler in
        if pathUpdateHandler.status == .satisfied {
            print("Internet connection is on.")
            
            if !CommonModel.hasInternet.value{
                CommonModel.hasInternet.value = true
            }
            
        } else {
            print("There's no internet connection.")
            
            if CommonModel.hasInternet.value{
                CommonModel.hasInternet.value = false
            }
        }
    }
    internetMonitor.start(queue: DispatchQueue.global())
}
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class TestViewController: UIViewController {

    override func viewWillAppear(_ animated: Bool) {
        
        if !CommonModel.hasInternet.value{
            //Pop-up no internet message here
        }

        CommonModel.hasInternet.addObserver( {value in
            if !value{
                //Pop-up no internet message here
            }
        })
    }

    override func viewWillDisappear(_ animated: Bool) {
        ...
        CommonModel.hasInternet.removeObserver()
    }
}