Using SwiftData in SwiftUI Previews

When building a SwiftUI app with SwiftData, previews often need a ModelContainer populated with sample data.

Instead of creating a container inside every #Preview, we can define a reusable preview container for the entire project.

Create a Preview Container

Suppose we have a simple SwiftData model:

import SwiftData

@Model
final class Item {
    var title: String
    var createdAt: Date

    init(title: String, createdAt: Date = .now) {
        self.title = title
        self.createdAt = createdAt
    }
}

We can add a preview container as an extension of ModelContainer:

extension ModelContainer {
    @MainActor
    static var preview: ModelContainer {
        let schema = Schema([
            Item.self
        ])

        let configuration = ModelConfiguration(
            schema: schema,
            isStoredInMemoryOnly: true
        )

        do {
            let container = try ModelContainer(
                for: schema,
                configurations: [configuration]
            )

            let context = container.mainContext

            context.insert(
                Item(
                    title: "Read Swift book"
                )
            )
            context.insert(
                Item(
                    title: "Build iOS app"
                )
            )
            context.insert(
                Item(
                    title: "Learn SwiftData"
                )
            )

            return container
        } catch {
            fatalError(
                "Failed to create preview container: \(error)"
            )
        }
    }
}

The important part is:

isStoredInMemoryOnly: true

This keeps the preview database entirely in memory. Preview data does not affect the app’s real persistent store, and every new container starts with a clean database.

Use It in #Preview

Now a SwiftUI preview only needs:

#Preview {
    ContentView()
        .modelContainer(.preview)
}

This keeps previews small and moves all sample data into one place.

For larger projects, I usually put this code in a dedicated file:

Preview/
└── PreviewContainer.swift

As more SwiftData models are added, they can be included in the same schema and populated with representative sample data.

It is a small abstraction, but it makes SwiftUI previews much easier to maintain.