Swift UI - How to access a @State variable externally?

Question

I have ContentView.swift

struct ContentView: View {
    @State var seconds: String = "60"

    init(_ appDelegate: AppDelegate) {
        launchTimer()
    }
    
    func launchTimer() {
        let timer = DispatchTimeInterval.seconds(5)
        let currentDispatchWorkItem = DispatchWorkItem {
            print("in timer", "seconds", seconds) // Always `"60"`
            print("in timer", "$seconds.wrappedValue", $seconds.wrappedValue) // Always `"60"`

            launchTimer()
        }
        
        DispatchQueue.main.asyncAfter(deadline: .now() + timer, execute: currentDispatchWorkItem)
    }

    var body: some View {
        TextField("60", text: $seconds)
    }
    
    func getSeconds() -> String {
        return $seconds.wrappedValue;
    }
}

And AppDelegate.swift

class AppDelegate: NSObject, NSApplicationDelegate {
    var contentView = ContentView()

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        launchTimer()
    }

    
    func launchTimer() {
        print(contentView.getSeconds()) // Always `"60"`

        let timer = DispatchTimeInterval.seconds(Int(contentView.getSeconds()) ?? 0)
        
        DispatchQueue.main.asyncAfter(deadline: .now() + timer) {
            self.launchTimer()
        }
    }
    

    @objc func showPreferenceWindow(_ sender: Any?) {
        if(window != nil) {
            window.close()
        }
        
        window = NSWindow(
            contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
            styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
            backing: .buffered, defer: false)
        window.isReleasedWhenClosed = false
        window.contentView = NSHostingView(rootView: contentView)
        
        window.center()
        window.makeKeyAndOrderFront(nil)
        NSApplication.shared.activate(ignoringOtherApps: true)
    }
}

contentView.getSeconds() always returns 60, even if I modify the value of the TexField in my content view (by manually typing in the text field).

How does one get the wrapped value/real value of a state variable from the app delegate?

Answer 1

@State/@StateObject is tricky business, what happens is that SwiftUI connects the state values to a certain view instance from the UI hierarchy.

In your case, the @State var seconds: String = "60" is connected to the view below (simplified scheme):

NSWindow
  -> NSHostingView
    -> ContentView <----- this is where the @State usage is valid

And as others said, ContentView being a struct, it's a value type, meaning that its contents are copied in all usage sites, so instead of having a unique instance, like a class has, you end up with multiple instances.

And only one of those instances, the copy SwiftUI makes when it adds the view to the UI tree, is the one that is connected to the text field, and gets updated.

To make things even funnier, the State property wrapper is also a struct, meaning that it "suffers" from the same symptoms as the view.

That's one of the perks of using SwiftUI, in contrast with UIKit, you don't care at all about the view instances.

Now, if you want to use the value of seconds from AppDelegate, or any other place, you will have to circulate the data storage instead of the view.

For starters, change the view to receive a @Binding instead

struct ContentView: View {
    @Binding var seconds: String = "60"

    // the rest of the view code remains the same

Then create an ObservableObject that stores the data, and inject its $seconds binding when creating the view:

class AppData: ObservableObject {
    @Published var seconds: String = "60"
}

class AppDelegate {
    let appData = AppData()

    // ... other code left out for clarity

    @objc func showPreferenceWindow(_ sender: Any?) {
        // ... 
        let contentView = ContentView(seconds: appData.$seconds
        window.contentView = NSHostingView(rootView: contentView)
        // ... 
    }
}

SwiftUI is more about the data than the view itself, so if you want to access the same data in multiple places, make sure you circulate the storage instead of the view the data is attached to. And also make sure that the source of truth of that data is a class, as object references are guaranteed to point to the same memory location (assuming of course the reference doesn't change).

Answer 2

Your ContentView (as well as any other SwiftUI view) is a struct, ie. value type, so in every place where you use it - you use a new copy of initially created value, ie.:

class AppDelegate: NSObject, NSApplicationDelegate {
    var contentView = ContentView()       // << 1st copy

    ...
    
    func launchTimer() {
        print(contentView.getSeconds())   // << 2nd copy
        let timer = DispatchTimeInterval.seconds(Int(contentView.getSeconds()) ?? 0)  // << 3d copy
        
    ...

    }
    
    ...

    @objc func showPreferenceWindow(_ sender: Any?) {

    ...
        
        window.contentView = NSHostingView(rootView: contentView)   // << 4th copy
        
    }
}

You should not use any @State externally, because it is valid only inside view's body.

If you need something to share/access in different places then use class confirming to ObservableObject as view model type, and as instances of such class is a reference type then passing it here and there you can use/access same object from different places.

Update:

why the value stays to "60" even inside the ContentView

You call launchTimer in init, but in that place State is not constructed yet, so binding to it is not valid. As was already written above state is valid in body, so you have to set your timer also in body when binding will be ready, say in .onAppear, like below

var body: some View {
    TextField("60", text: $seconds)
         .onAppear {
              launchTimer()
         }
}

Prepared & tested with Xcode 13 / iOS 15

Answer 3

add a variable to appDelegate

var seconds : Int? 

then do this in your contentView :

    .onChange(of: seconds) { seconds in
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {return}
        appDelegate.seconds = seconds
    }

Answer 4

Ended up instantiating the ContentView with the AppDelegate as a param. I added a seconds attribute to the AppDelegate to be modified by the ContentView.

AppDelegate.swift

import SwiftUI

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
    var contentView: ContentView?
    var seconds = 60
    
    func applicationDidFinishLaunching(_ aNotification: Notification) {
        self.contentView = ContentView(self)        

        launchTimer()
    }
    
    func launchTimer() {
        let timer = DispatchTimeInterval.seconds(seconds)
        
        print(timer)
        
        DispatchQueue.main.asyncAfter(deadline: .now() + timer) {
            self.launchTimer()
        }
    }
}

ContentView.swift

import SwiftUI

struct ContentView: View {
    @State var seconds: String = "60"
    var appDelegate: AppDelegate
    
    init(_ appDelegate: AppDelegate) {
        self.appDelegate = appDelegate
    }

    var body: some View {
        TextField("60", text: $seconds, onCommit: {
            self.appDelegate.seconds = Int($seconds.wrappedValue) ?? 0
         })
    }
}

This code hurts my brain and surely there is a better way of doing this . But I'm only so far in my Swift journey, so that will do for now lol. Please post if you have a better solution.