How to create a SwiftUI #Preview in Xcode 15 for a view with a @Binding

Question

If I wanted to create a preview for a SwiftUI view that contains a @Binding I would have previously written something like this:

struct SpecialButton_Preview: PreviewProvider {
    static var previews: some View {
        @State var value: Bool = true
        SpecialButton(isOn: $value)
    }
}

However Xcode 15 now comes with a new syntax (#Preview) but when I try to add my example state property, it does not work:

#Preview {  // Error: Ambiguous use of 'Preview(_:traits:body:)'
    @State var value: Bool = true
    SpecialButton(isOn: $value)
}

How can I make this work?

score 5 · Accepted Answer · answered Jun 13 '23 at 20:20

5

You need to return the View to preview. I'm not exactly sure how this works, it has to do with how Swift macros work.

#Preview {
    @State var value: Bool = true
    return SpecialButton(isOn: $value)
}

From the WWDC Slack: "The new #Previews macro simply takes a closure that returns the thing to be previewed. So you can declare local variables or do other setup necessary in there just like you would in any other closure."

answered Jun 13 '23 at 20:20

Mikaela Caron

134
7

This gives me the following error: `Cannot use explicit 'return' statement in the body of result builder 'ViewBuilder'` – Ribena Jun 15 '23 at 07:53
@Ribena what did your whole code snippet look like? I put this in Xcode 15 beta 1 and it worked (also note this is subject to change because this is a beta Xcode right now) – Mikaela Caron Jun 19 '23 at 20:08

score 3 · Answer 2 · answered Jun 14 '23 at 21:07

3

This is what I ended up doing in order to have a mutable value:


#Preview {
    struct PreviewWrapper: View {
        @State var value: Bool = true
        
        var body: some View {
            SpecialButton(isOn: $value)
        }
    }
    return PreviewWrapper()
}

answered Jun 14 '23 at 21:07

de.

7,068
3
40
69

What does doing this accomplish? in this case, you could use what is mentioned here: https://stackoverflow.com/a/76468419/17309983 – Mikaela Caron Jun 19 '23 at 20:11

score 2 · Answer 3 · answered Jun 13 '23 at 20:13

2

What you could do before and still can do is:

SpecialButton(isOn: .constant(true))

answered Jun 13 '23 at 20:13

ChrisR

9,523
1
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score 0 · Answer 4 · answered Jun 15 '23 at 07:57

0

Do you need to keep it as a binding var in your preview? Otherwise try this, as it seems to work for me:

#Preview {
    SpecialButton(isOn: true)
}

I'd elaborate on the 'why' but that's still unclear!

answered Jun 15 '23 at 07:57

Ribena

1,086
1
11
20

Yes, the whole point is to preview "a SwiftUI view that contains a @Binding". – de. Jun 15 '23 at 08:21
@de. ...which my solution does, in a way? In my seemingly identical case, it allows my View containing a @Binding to be previewed whereas none of the other currently stated solutions work for me. Are you not getting `Cannot use explicit 'return' statement in the body of result builder 'ViewBuilder'` when you try the accepted solution, as I did when I tried it with the code you've provided? – Ribena Jun 15 '23 at 09:22

How to create a SwiftUI #Preview in Xcode 15 for a view with a @Binding

4 Answers4