How to set addObserver in SwiftUI?

Question

How do I add NotificationCenter.default.addObserve in SwiftUI?

When I tried adding observer I get below error

Argument of '#selector' refers to instance method 'VPNDidChangeStatus' that is not exposed to Objective-C

But when I add @objc in front of func I get below error

@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes

Here is my code

let NC = NotificationCenter.default

var body: some View {
     VStack() {

     }.onAppear {

           self.NC.addObserver(self, selector: #selector(self.VPNDidChangeStatus),
                              name: .NEVPNStatusDidChange, object: nil)

     }
} 

@objc func VPNDidChangeStatus(_ notification: Notification) {
    //    print("VPNDidChangeStatus", VPNManager.shared.status)
}

Take a tour on this it might helps -https://stackoverflow.com/questions/38980887/protocol-extension-on-an-objc-protocol — Abhishek, Nov 12 '19 at 12:39

score 87 · Answer 1 · answered Apr 10 '20 at 03:05

87

The accepted answer may work but is not really how you're supposed to do this. In SwiftUI you don't need to add an observer in that way.

You add a publisher and it still can listen to NSNotification events triggered from non-SwiftUI parts of the app and without needing combine.

Here as an example, a list will update when it appears and when it receives a notification, from a completed network request on another view / controller or something similar etc.

If you need to then trigger an @objc func for some reason, you will need to create a Coordinator with UIViewControllerRepresentable

struct YourSwiftUIView: View {

    let pub = NotificationCenter.default
            .publisher(for: NSNotification.Name("YourNameHere"))


    var body: some View {
        List() {
            ForEach(userData.viewModels) { viewModel in
                SomeRow(viewModel: viewModel)
            }
        }
        .onAppear(perform: loadData)
        .onReceive(pub) { (output) in
            self.loadData()
        }
    }

    func loadData() {
        // do stuff
    }
}

answered Apr 10 '20 at 03:05

MadeByDouglas

2,509
1
18
22

5

this works. you can then just post a notification like this -> self.nc.post(name: Notification.Name("RemoteContatcsReceived"), object: nil) With xCode 11.5 I did not have to use @objc yay! – Dave Kozikowski Jun 09 '20 at 21:19
If you move it into a state object then it won't screw up your previews because state objects aren't init when previewing. – malhal Nov 10 '20 at 20:18
1

`NotificationCenter.publisher(for:object:)` *does* use `Combine`. You can *call* an Objective C function from any code you want. Presumably you're referring to *defining* an objective C function. Even that has absolutely nothing to do with `UIViewControllerRepresentable` or coordinators. You just need to define the function in a class. – Peter Schorn Jun 13 '21 at 13:16
struct YourSwiftUIView: View { @State updateYourSwiftUIView: Bool = false ... } func loadData() { // do stuff updateYourSwiftUIView.toggle() } } – Neph Muw Aug 16 '21 at 13:18

Rohit Makwana · Answer 2 · 2021-07-16T05:27:06.477

I have one approach for NotificationCenter usage in SwiftUI.

For more information Apple Documentation

Notification extension

extension NSNotification {
    static let ImageClick = Notification.Name.init("ImageClick")
}

ContentView

struct ContentView: View {
    var body: some View {
        VStack {
            DetailView()
        }
        .onReceive(NotificationCenter.default.publisher(for: NSNotification.ImageClick))
        { obj in
           // Change key as per your "userInfo"
            if let userInfo = obj.userInfo, let info = userInfo["info"] {
              print(info)
           }
        }
    }
}

DetailView

struct DetailView: View {
    var body: some View {
        Image(systemName: "wifi")
            .frame(width: 30,height: 30, alignment: .center)
            .foregroundColor(.black)
            .onTapGesture {
                NotificationCenter.default.post(name: NSNotification.ImageClick, 
                                                object: nil, userInfo: ["info": "Test"])
        }
    }
}

Great answer! One thing: use Notification instead of NSNotification in Swift. — Tamás Sengel, Aug 27 '20 at 04:52

tadija · Answer 3 · 2022-11-29T14:33:30.330

I use this extension so it's a bit nicer on the call site:

/// Extension

extension View {
    func onReceive(
        _ name: Notification.Name,
        center: NotificationCenter = .default,
        object: AnyObject? = nil,
        perform action: @escaping (Notification) -> Void
    ) -> some View {
        onReceive(
            center.publisher(for: name, object: object), 
            perform: action
        )
    }
}

/// Usage

struct MyView: View {
    var body: some View {
        Color.orange
            .onReceive(.myNotification) { _ in
                print(#function)
            }
    }
}

extension Notification.Name {
    static let myNotification = Notification.Name("myNotification")
}

score 7 · Answer 4 · edited Aug 23 '22 at 10:08

7

It is not SwiftUI-native approach, which is declarative & reactive. Instead you should use NSNotificationCenter.publisher(for:object:) from Combine.

See more details in Apple Documentation

edited Aug 23 '22 at 10:08

Martijn Pieters

1,048,767
296
4,058
3,343

answered Nov 12 '19 at 13:32

Asperi

228,894
20
464
690

score 6 · Accepted Answer · answered Nov 12 '19 at 15:19

This worked for me

   let NC = NotificationCenter.default



   self.NC.addObserver(forName: .NEVPNStatusDidChange, object: nil, queue: nil, 
                       using: self.VPNDidChangeStatus)


   func VPNDidChangeStatus(_ notification: Notification) {


    }

score 1 · Answer 6 · answered Nov 12 '19 at 13:27

exchange this

self.NC.addObserver(self, selector: #selector(self.VPNDidChangeStatus),
                          name: .NEVPNStatusDidChange, object: nil)

to

self.NC.addObserver(self, selector: #selector(VPNDidChangeStatus(_:)),
                          name: .NEVPNStatusDidChange, object: nil)

How to set addObserver in SwiftUI?

6 Answers6

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