Is there a SwiftUI equivalent for viewWillDisappear(_:) or detect when a view is about to be removed?

问题

In SwiftUI, I'm trying to find a way to detect that a view is about to be removed only when using the default navigationBackButton. Then perform some action.

Using onDisappear(perform:) acts like viewDidDisappear(_:), and the action performs after another view appears.

Or, I was thinking the above problem might be solved by detecting when the default navigationBarBackButton is pressed. But I've found no way to detect that.

Is there any solution to perform some action before another view appears?

(I already know it is possible to do that by creating a custom navigation back button to dismiss a view)

回答1:

Here is approach that works for me, it is not pure-SwiftUI but I assume worth posting

Usage:

   SomeView()
   .onDisappear {
        print("x Default disappear")
    }
   .onWillDisappear { // << order does NOT matter
        print(">>> going to disappear")
    }

Code:

struct WillDisappearHandler: UIViewControllerRepresentable {
    func makeCoordinator() -> WillDisappearHandler.Coordinator {
        Coordinator(onWillDisappear: onWillDisappear)
    }

    let onWillDisappear: () -> Void

    func makeUIViewController(context: UIViewControllerRepresentableContext<WillDisappearHandler>) -> UIViewController {
        context.coordinator
    }

    func updateUIViewController(_ uiViewController: UIViewController, context: UIViewControllerRepresentableContext<WillDisappearHandler>) {
    }

    typealias UIViewControllerType = UIViewController

    class Coordinator: UIViewController {
        let onWillDisappear: () -> Void

        init(onWillDisappear: @escaping () -> Void) {
            self.onWillDisappear = onWillDisappear
            super.init(nibName: nil, bundle: nil)
        }

        required init?(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }

        override func viewWillDisappear(_ animated: Bool) {
            super.viewWillDisappear(animated)
            onWillDisappear()
        }
    }
}

struct WillDisappearModifier: ViewModifier {
    let callback: () -> Void

    func body(content: Content) -> some View {
        content
            .background(WillDisappearHandler(onWillDisappear: callback))
    }
}

extension View {
    func onWillDisappear(_ perform: @escaping () -> Void) -> some View {
        self.modifier(WillDisappearModifier(callback: perform))
    }
}

来源：https://stackoverflow.com/questions/59745663/is-there-a-swiftui-equivalent-for-viewwilldisappear-or-detect-when-a-view-is