Utilizing PreviewModifier to construct a previewing setting – Donny Wals

Xcode 16 and iOS 18 include a function that enables us to construct elaborate preview environments utilizing a brand new PreviewModifier protocol. This protocol permits us to outline objects that may create a single context or setting that’s cached and used throughout your SwiftUI previews.

That is helpful as a result of it signifies that you possibly can, for instance, populate a database with a bunch of mock knowledge that’s then utilized in your previews.

You can even use PreviewModifier to use particular styling to your previews, to wrap all of them in a particular wrapper, and extra.

Basically, they’re a instrument that lets you configure your previews constantly throughout the board.

Adorning views utilizing PreviewModifier

The PreviewModifier protocol specifies two strategies which you can implement:

• A static makeSharedContext methodology
• An occasion methodology known as physique

The physique occasion strategies is handed the view that’s being previewed and a Context. This context can both be an object that you just created in makeSharedContext or Void in the event you don’t implement makeSharedContext.

For this instance, let’s assume that you just went forward and didn’t implement makeSharedContext. In a state of affairs like that, we will use PreviewModifier to brighten a view for our previews. For instance, we may wrap it in one other view or apply some styling to it.

I’m fairly certain that you just’re extra inventive than me so I’m simply going to go forward and present you the way you’d apply a orange background to your previewed view. Yeah, I do know… very inventive. The purpose is to indicate you the way to do that with the intention to do one thing a lot smarter than what I’m describing right here.

struct OrangeBackground: PreviewModifier {
    func physique(content material: Content material, context: Void) -> some View {
        content material
            .padding()
            .background {
                RoundedRectangle(cornerRadius: 16)
                    .fill(.orange)
            }
    }
}

#Preview(traits: .modifier(OrangeBackground())) {
    Textual content("Hi there, world!")
}

Let’s take a look at the PreviewModifier first, after which I’ll clarify how I utilized it to my preview.

The modifier is outlined as a struct and I solely applied the physique operate.

This operate is padded Content material which is no matter view the #Preview macro is getting used on (on this case, Textual content), and it receives a context. On this case Void as a result of I didn’t make a context.

The content material argument might be styled, modified, and wrapped nonetheless you want. It’s a view so you are able to do issues like give it a background, remodel it, regulate its setting, and far way more. Something you are able to do with a view within a View physique you are able to do right here.

The primary distinction is that you just’re receiving a completely instantiated occasion of your view. Meaning you may’t inject new state or bindings into it or in any other case modify it. You possibly can solely apply view modifiers to it.

This brings us to our subsequent function of PreviewModifier making a context to offer mocked knowledge and extra.

Utilizing PreviewModifier to inject mock knowledge

To inject mock knowledge into your previews by means of PreviewModifier all it’s essential to do is implement the makeSharedContext methodology from the PreviewModifier protocol. This methodology is static and is named as soon as for all of your previews. Which means the context that you just create on this methodology is reused for your whole previews.

In apply that is good as a result of it means you get constant mock knowledge on your previews with out the overhead of recreating this knowledge steadily.

Right here’s what a pattern implementation for makeSharedContext seems to be like:

struct MockDataSource {
    // ...
}

struct OrangeBackground: PreviewModifier {
    static func makeSharedContext() async throws -> MockDataSource {
        return MockDataSource()
    }
}

On this case, I’m creating an occasion of some knowledge supply in my makeSharedContext methodology. This MockDataSource would maintain all mocks and all knowledge for my views which is nice.

Nonetheless, the one manner for us to essentially use that mock knowledge in our view is by including our knowledge supply (or the mocked knowledge) to our previewed view’s setting.

struct OrangeBackground: PreviewModifier {
    static func makeSharedContext() async throws -> MockDataSource {
        return MockDataSource()
    }

    func physique(content material: Content material, context: MockDataSource) -> some View {
        content material
            .setting(.dataSource, context)
    }
}

Since we will’t make a brand new occasion of our content material, we will’t inject our mock knowledge supply instantly into the view by means of its initializer. The one manner we will get the info supply to the view is by including it to the setting.

This isn’t preferrred for my part, however the design is sensible.

I’m additionally fairly certain that Apple designed this API with mocking SwiftData databases in thoughts and it might work nice for that.

On prime of getting to make use of the setting, the PreviewModifier solely works in initiatives that focus on iOS 18 or later. Not an enormous downside however it might have been good if utilizing Xcode 16 was adequate for us to have the ability to use this helpful new API.