SwiftUI: observe @Environment property changes

Question

I was trying to use the SwiftUI @Environment property wrapper, but I can't manage to make it work as I expected. Please, help me understanding what I'm doing wrong.

As an example I have an object that produces an integer once per second:

class IntGenerator: ObservableObject {
    @Published var newValue = 0 {
        didSet {
            print(newValue)
        }
    }

    private var toCanc: AnyCancellable?

    init() {
        toCanc = Timer.TimerPublisher(interval: 1, runLoop: .main, mode: .default)
            .autoconnect()
            .map { _ in Int.random(in: 0..<1000) }
            .assign(to: \.newValue, on: self)
    }
}

This object works as expected since I can see all the integers generated on the console log. Now, let's say we want this object to be an environment object accessible from all over the app and from whoever. Let's create the related environment key:

struct IntGeneratorKey: EnvironmentKey {
    static let defaultValue = IntGenerator()
}

extension EnvironmentValues {
    var intGenerator: IntGenerator {
        get {
            return self[IntGeneratorKey.self]
        }
        set {
            self[IntGeneratorKey.self] = newValue
        }
    }
}

Now I can access this object like this (for example from a view):

struct TestView: View {
    @Environment(\.intGenerator) var intGenerator: IntGenerator

    var body: some View {
        Text("\(intGenerator.newValue)")
    }
}

Unfortunately, despite the newValue being a @Published property I'm not receiving any update on that property and the Text always shows 0. I'm sure I'm missing something here, what's going on? Thanks.

Answer 1

Environment gives you access to what is stored under EnvironmentKey but does not generate observer for its internals (ie. you would be notified if value of EnvironmentKey changed itself, but in your case it is instance and its reference stored under key is not changed). So it needs to do observing manually, is you have publisher there, like below

@Environment(\.intGenerator) var intGenerator: IntGenerator

@State private var value = 0
var body: some View {
    Text("\(value)")
        .onReceive(intGenerator.$newValue) { self.value = $0 }
}

and all works... tested with Xcode 11.2 / iOS 13.2