todos/App/Previews/Modifiers/SampleTaskDataModifier.swift

import SwiftData
import SwiftUI

/// A preview modifier that provides sample task data for SwiftUI previews.
///
/// This modifier creates an in-memory SwiftData model container populated with representative sample todos across all task groups
/// (today, upcoming, overdue, completed).
/// It's designed to be used with SwiftUI previews to display realistic data without
/// requiring a persistent database.
///
/// ## Usage
/// Use the `.sampleTasks` trait on your preview:
///
/// ```swift
/// #Preview(traits: .sampleTasks) {
///     TaskGroupListView()
/// }
/// ```
///
/// This automatically provides a model container with sample data to the preview.
struct SampleTaskDataModifier: PreviewModifier {

    // MARK: Functions

    /// Creates and configures a shared in-memory model container with sample task data.
    ///
    /// This method is called once when the preview is loaded and creates an in-memory
    /// SwiftData container populated with 8 representative todo tasks spanning all
    /// task groups and various configurations.
    ///
    /// - Returns: A `ModelContainer` configured for in-memory storage with sample data.
    /// - Throws: An error if the model container cannot be created.
    static func makeSharedContext() async throws -> ModelContainer {
        let modelContainer = try ModelContainer(
            for: Todo.self,
            configurations: .init(isStoredInMemoryOnly: true)
        )

        let calendar: Calendar = .current

        let tasks: [Todo] = [
            .init(
                title: "Today task #1",
                due: .now
            ),
            .init(
                title: "Today task #2",
                note: "This is some random note attached to the task.",
                due: .now
            ),
            .init(
                title: "Tomorrow task",
                due: calendar.date(byAdding: .day, value: 1, to: .now) ?? .now
            ),
            .init(
                title: "Next week task",
                note: "This is some random note attached to the task.",
                due: calendar.date(byAdding: .day, value: 7, to: .now) ?? .now
            ),
            .init(
                title: "Yesterday task",
                due: calendar.date(byAdding: .day, value: -1, to: .now) ?? .now
            ),
            .init(
                title: "Last week task",
                note: "This is some random note attached to the task.",
                due: calendar.date(byAdding: .day, value: -7, to: .now) ?? .now
            ),
            .init(
                title: "Completed task #1",
                due: calendar.date(byAdding: .day, value: 1, to: .now) ?? .now,
                completed: .now
            ),
            .init(
                title: "Completed task #2",
                note: "This is some random note attached to the task.",
                due: calendar.date(byAdding: .day, value: -1, to: .now) ?? .now,
                completed: .now
            ),
        ]

        for task in tasks {
            modelContainer.mainContext.insert(task)
        }

        return modelContainer
    }

    /// Applies the model container to the preview content.
    ///
    /// This method is called by SwiftUI to apply the modifier to the preview.
    /// It injects the model container into the SwiftUI environment, making it
    /// available to any views that use `@Query` or `@Environment(\.modelContext)`.
    ///
    /// - Parameters:
    ///   - content: The preview content to modify.
    ///   - context: The model container with sample data.
    /// - Returns: The content with the 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 sample task data.
    ///
    /// Use this trait to automatically populate previews with realistic sample todos
    /// without manually creating a model container and inserting data.
    ///
    /// ## Example
    /// ```swift
    /// #Preview(traits: .sampleTasks) {
    ///     TaskGroupListView()
    /// }
    /// ```
    ///
    /// This is equivalent to manually creating a container with sample data but
    /// much more concise and reusable across multiple previews.
    static var sampleTasks: Self = .modifier(SampleTaskDataModifier())

}