App/Previews/Modifiers/SampleTaskDataModifier.swift

@@ -0,0 +1,131 @@
+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 today: Date = .now
+        let tomorrow: Date = calendar.date(byAdding: .day, value: 1, to: today) ?? today
+        let yesterday = calendar.date(byAdding: .day, value: -1, to: today) ?? today
+
+        let tasks: [Todo] = [
+            .init(
+                title: "Today task 1",
+                due: today
+            ),
+            .init(
+                title: "Today task 2",
+                note: "This is some random note attached to the task.",
+                due: today
+            ),
+            .init(
+                title: "Tomorrow task",
+                due: tomorrow
+            ),
+            .init(
+                title: "Next week task",
+                note: "This is some random note attached to the task.",
+                due: calendar.date(byAdding: .day, value: 7, to: today) ?? today
+            ),
+            .init(
+                title: "Yesterday task",
+                due: yesterday
+            ),
+            .init(
+                title: "Last week task",
+                note: "This is some random note attached to the task.",
+                due: calendar.date(byAdding: .day, value: -7, to: today) ?? today
+            ),
+            .init(
+                title: "Completed task 1",
+                due: today,
+                completed: today
+            ),
+            .init(
+                title: "Completed task 2",
+                note: "This is some random note attached to the task.",
+                due: yesterday,
+                completed: today
+            ),
+        ]
+
+        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())
+
+}

Todos.xcodeproj/project.pbxproj

@@ -378,6 +378,7 @@
 				ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 1;
+				DEVELOPMENT_ASSET_PATHS = App/Previews/Modifiers/SampleTaskDataModifier.swift;
 				DEVELOPMENT_TEAM = 7FMNM89WKG;
 				ENABLE_APP_SANDBOX = YES;
 				ENABLE_HARDENED_RUNTIME = YES;
@@ -423,6 +424,7 @@
 				ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 1;
+				DEVELOPMENT_ASSET_PATHS = App/Previews/Modifiers/SampleTaskDataModifier.swift;
 				DEVELOPMENT_TEAM = 7FMNM89WKG;
 				ENABLE_APP_SANDBOX = YES;
 				ENABLE_HARDENED_RUNTIME = YES;