swiftui Could not cast value of type 'SwiftUI.Text' (0x2211789e8) to 'SwiftUI._ViewModifier_Content<User.PopUpViewModifier>

I'm trying to create a modifier that builds a sheet with content on any View.

struct ContentView: View {
    @State private var open: Bool = false
    
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundColor(.accentColor)
            
            Button("Open sheet") {
                open.toggle()
            }
        }
        .padding()
        .popUpModifier(isPresented: $open) {
            Text("Hello, world!")
        }
    }
}

Here's the modifier:

struct PopUpViewModifier: ViewModifier {
    @Binding var isPresented: Bool
    var content: Content
    
    func body(content: Content) -> some View {
        content
            .sheet(isPresented: $isPresented, onDismiss: {UIImpactFeedbackGenerator(style: .medium).impactOccurred()}) {
                content
            }
    }
}

And here's the View's extension:

extension View {
    func popUpModifier<Content: View>(isPresented: Binding<Bool>, content: @escaping () -> Content) -> some View {
        self
            .modifier(PopUpViewModifier(isPresented: isPresented, content: content() as! PopUpViewModifier.Content))
    }
}

Whenever I run it I get a fatal error:

Could not cast value of type 'SwiftUI.Text' (0x2211789e8) to 'SwiftUI._ViewModifier_Content<User.PopUpViewModifier>

It happens no matter which content I include in the modifier content brackets.
Also - without content() as! PopUpViewModifier.Content) XCode won't compile and raise alert

> Cannot convert value of type 'Content' to expected argument type 'PopUpViewModifier.Content' (aka '_ViewModifier_Content<PopUpViewModifier>')

Any suggestions?

You're mixing the content that a ViewModifier normally takes (ie the content that it modifies) with the content that you're attempting to show in your popup.

Right now, you're trying to use one variable for both, presenting a sheet with that content from the content -- basically, recursion.

Setting up a separate variable for the sheet content solves this:

struct ContentView: View {
    @State private var open: Bool = false
    
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundColor(.accentColor)
            
            Button("Open sheet") {
                open.toggle()
            }
        }
        .padding()
        .popUpModifier(isPresented: $open) {
            Text("Hello, world!")
        }
    }
}

struct PopUpViewModifier<T: View>: ViewModifier {
    @Binding var isPresented: Bool
    var popupContent: () -> T
    
    func body(content: Content) -> some View {
        content
            .sheet(isPresented: $isPresented, onDismiss: {UIImpactFeedbackGenerator(style: .medium).impactOccurred()}) {
                popupContent()
            }
    }
}

extension View {
    func popUpModifier<T: View>(isPresented: Binding<Bool>, popupContent: @escaping () -> T) -> some View {
        self
            .modifier(PopUpViewModifier(isPresented: isPresented, popupContent: popupContent))
    }
}