SwiftUI #Preview test data

Question

The #Preview macro is frustrating me! Test data that looks obvious to me fails to build. Here is a stripped down example:-

    import SwiftUI

    struct TestView: View {
        @State private var intValue = 0

        var body: some View {
            TestMenuView(intValue: $intValue)
        }
    }

    #Preview {
        TestView()
    }

    struct TestMenuView: View {
        @Binding var intValue: Int

        var body: some View {
            Text("Value passed in was \($intValue)")
        }
    }

    #Preview {
        TestMenuView(intValue: 1)
    }

But the #Preview of TestMenuView gives the error:- Cannot convert value of type 'Int' to expected argument type 'Binding'

TestMenuView(intValue: .constant(1))

Joakim Danielson
– Joakim Danielson

2024-05-01 07:59:42 +00:00
Commented May 1, 2024 at 7:59 — Joakim Danielson
– Joakim Danielson, Commented May 1, 2024 at 7:59
Thank you, Joakim, that gets my #Preview working

GraemeC
– GraemeC

2024-05-02 03:48:37 +00:00
Commented May 2, 2024 at 3:48 — GraemeC
– GraemeC, Commented May 2, 2024 at 3:48

bjorn.lau · Accepted Answer · 2024-05-01 12:35:01Z

0

The compiler is telling you that you are trying to initialize the view with an Int type, instead of the expected type Binding<Int>.

You can get around this in a couple of ways.

Initialize it with a constant value like so: TestMenuView(intValue: .constant(1))
Or if you want the preview to be able to display a dynamic value, you can wrap your preview in another view that passes the value from a state.

Replace your #Preview { } with this:

struct TestMenuView_Previews: PreviewProvider {
    struct TestMenuViewContainer: View {
        @State var intValue = 1

        var body: some View {
            TestMenuView(intValue: $intValue)
        }
    }

    static var previews: some View {
        TestMenuViewContainer()
    }
}

edited May 1, 2024 at 12:35

answered May 1, 2024 at 9:16

bjorn.lau

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1 Comment

GraemeC Over a year ago

Thank you, Bjorn, both of those answers work. Now that I am starting understand #Preview better, I think I'll be able to deal with the real code for my app.

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