Add ViewBuilder capacity to preview variables in the test template

[vince4]

Short description 📝

The generated tests does not support #Preview with multiple views. Fix issue #146

#Preview
{
    NXBadgeView(item: NXBadgeViewItem(icon: "generic_small_star_10", text: "4.0", color: .pink))
    NXBadgeView(item: NXBadgeViewItem(icon: nil, text: "Top award", color: .purple))
    NXBadgeView(item: NXBadgeViewItem(icon: nil, text: "Top award", color: .yellow, isLarge: true))
}

^ This preview generate a test that doesn't build.

Solution 📦

Because #Preview is of type @ViewBuilder, the preview variables, used in the generated tests, should also be @ViewBuilder.

Test 🧪

I did not find any existing test for the PreviewTestsTemplate, but it would be nice to add some. @BarredEwe : Do you have some suggestions of tests I could add ?

[BarredEwe]

Hi! Thanks for the fix. Regarding tests, I think what we have in PrefireExample is enough here.

[vince4]

Hi! Thanks for the fix. Regarding tests, I think what we have in PrefireExample is enough here.

I see. So I can add a new Preview inside PrefireExample that contains multiple views.
I will add one here : https://github.com/BarredEwe/Prefire/blob/main/Example/Shared/Examples/TestView.swift#L89-L94

[vince4]

I added the Preview macro with multiple views to the Example project, but then I realized that my initial implementation was too naive. It broke support for UIView and UIViewController.

The current solution still isn’t perfect, because it’s technically possible to build a Preview with a single UIViewController without using a return statement. This works only because there is a specific (but private) result builder for them, called PreviewMacroBodyBuilder:

@available(iOS 17.0, macOS 14.0, tvOS 17.0, *)
@freestanding(declaration)
public macro Preview(
    _ name: String? = nil,
    traits: PreviewTrait<Preview.ViewTraits>...,
    @PreviewMacroBodyBuilder<UIViewController> body: @escaping @MainActor () -> UIViewController
) = #externalMacro(module: "PreviewsMacros", type: "KitViewMacro")

Initially, I tried to create my own result builder that supports both SwiftUI.View and UIViewController, but it didn’t work in the example. As soon as the closure includes a return statement, it disables the effect of the result builder.

So the current implementation works correctly for SwiftUI views using @ViewBuilder Previews, but it has a known limitation: this type of preview is not supported:

#Preview {
    UIViewController()
}

[BarredEwe]

A working solution, but I think we can make it a bit simpler using generic functions.

public init(@ViewBuilder _ view: @escaping @MainActor () -> Content, name: String, isScreen: Bool, device: DeviceConfig, traits: UITraitCollection = .init()) {

I quickly sketched out an implementation here - take a look. If you like it, we can merge it.

Prefire-15-26-16.patch

[vince4]

@BarredEwe : Your solution is much nicer, directly modifying the PrefireSnapshot init. However, it doesn't compile in my project, because in some strange cases it still needs the @_disfavoredOverload to avoid this error:

TestViewTests.generated.swift:112:31 Ambiguous use of 'init(_:name:isScreen:device:traits:)'
TestViewTests.generated.swift:128:26 Generic parameter 'Content' could not be inferred

I managed to reproduce the error with this #Preview example:

#Preview(traits: .sizeThatFitsLayout) {
    let userStory: PreviewModel.UserStory = .testStory
    VStack
    {
        TestView(isLoading: true)
            .previewUserStory(userStory)
            .snapshot(delay: 0.1, precision: 0.9)
            .previewUserStory(.auth)
    }.padding()
}

I’ve updated the merge request.

[BarredEwe]

Great! I’ve checked it, it works well. Thanks for contributing!