todos/App/Previews/Modifiers/EmptyContainerModifier.swift

import SwiftData
import SwiftUI

/// A preview modifier that provides an empty SwiftData model container for SwiftUI previews.
///
/// This modifier creates an in-memory SwiftData model container with no data, useful for testing and previewing views in their empty or initial state.
/// It's particularly helpful for testing empty state UI, loading states, or views that should gracefully handle no data.
///
/// ## Usage
/// Use the `.emptyContainer` trait on your preview:
///
/// ```swift
/// #Preview(traits: .emptyContainer) {
///     TaskGroupListView()
/// }
/// ```
///
/// This automatically provides an empty model container to the preview, allowing SwiftData queries to return empty results.
struct EmptyContainerModifier: PreviewModifier {
    
    // MARK: Functions
    
    /// Creates and configures a shared in-memory model container with no data.
    ///
    /// This method is called once when the preview is loaded and creates an in-memory
    /// SwiftData container with no persisted entities. The container is configured
    /// to store data only in memory, ensuring previews don't affect any persistent storage.
    ///
    /// - Returns: An empty `ModelContainer` configured for in-memory storage.
    /// - Throws: An error if the model container cannot be created.
    static func makeSharedContext() async throws -> ModelContainer {
        try ModelContainer(
            for: Todo.self,
            configurations: .init(isStoredInMemoryOnly: true)
        )
    }
    
    /// Applies the empty model container to the preview content.
    ///
    /// This method is called by SwiftUI to apply the modifier to the preview.
    /// It injects the empty model container into the SwiftUI environment, making it
    /// available to any views that use `@Query` or `@Environment(\.modelContext)`.
    /// Queries will return empty results, allowing you to test empty state behavior.
    ///
    /// - Parameters:
    ///   - content: The preview content to modify.
    ///   - context: The empty model container.
    /// - Returns: The content with the empty model container applied.
    func body(
        content: Content,
        context: ModelContainer
    ) -> some View {
        content.modelContainer(context)
    }
    
}

// MARK: - PreviewTrait

extension PreviewTrait where T == Preview.ViewTraits {
    
    /// A preview trait that provides an empty model container.
    ///
    /// Use this trait to test views in their empty state without any data.
    /// This is particularly useful for testing empty state UI, placeholder content,
    /// and ensuring views handle the absence of data gracefully.
    ///
    /// ## Example
    /// ```swift
    /// #Preview("Empty State", traits: .emptyContainer) {
    ///     TaskGroupListView()
    /// }
    /// ```
    ///
    /// The view will receive an empty SwiftData container, causing any `@Query`
    /// properties to return empty collections.
    static var emptyContainer: Self = .modifier(EmptyContainerModifier())
    
}