From 24639081c09a46258a33c0a1a4f50e1afd880e28 Mon Sep 17 00:00:00 2001
From: Javier Cicchelli <javier@rock-n-code.com>
Date: Tue, 10 Feb 2026 01:32:31 +0100
Subject: [PATCH] Implemented the EmptyContainerModifier preview modifier in
 the app target.

---
 .../Modifiers/EmptyContainerModifier.swift    | 79 +++++++++++++++++++
 App/Sources/Views/NewTaskView.swift           |  6 +-
 Todos.xcodeproj/project.pbxproj               |  4 +-
 3 files changed, 85 insertions(+), 4 deletions(-)
 create mode 100644 App/Previews/Modifiers/EmptyContainerModifier.swift

diff --git a/App/Previews/Modifiers/EmptyContainerModifier.swift b/App/Previews/Modifiers/EmptyContainerModifier.swift
new file mode 100644
index 0000000..9f9cd28
--- /dev/null
+++ b/App/Previews/Modifiers/EmptyContainerModifier.swift
@@ -0,0 +1,79 @@
+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())
+    
+}
diff --git a/App/Sources/Views/NewTaskView.swift b/App/Sources/Views/NewTaskView.swift
index c25b455..7b95d0b 100644
--- a/App/Sources/Views/NewTaskView.swift
+++ b/App/Sources/Views/NewTaskView.swift
@@ -99,7 +99,9 @@ struct NewTaskView: View {
 
 // MARK: - Previews
 
-#Preview("New todo task") {
+#Preview(
+    "New todo task",
+    traits: .emptyContainer
+) {
     NewTaskView()
-        .modelContainer(for: Todo.self, inMemory: true)
 }
diff --git a/Todos.xcodeproj/project.pbxproj b/Todos.xcodeproj/project.pbxproj
index 3688d17..7747608 100644
--- a/Todos.xcodeproj/project.pbxproj
+++ b/Todos.xcodeproj/project.pbxproj
@@ -378,7 +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_ASSET_PATHS = "App/Previews/Modifiers/SampleTaskDataModifier.swift App/Previews/Modifiers/EmptyContainerModifier.swift";
 				DEVELOPMENT_TEAM = 7FMNM89WKG;
 				ENABLE_APP_SANDBOX = YES;
 				ENABLE_HARDENED_RUNTIME = YES;
@@ -424,7 +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_ASSET_PATHS = "App/Previews/Modifiers/SampleTaskDataModifier.swift App/Previews/Modifiers/EmptyContainerModifier.swift";
 				DEVELOPMENT_TEAM = 7FMNM89WKG;
 				ENABLE_APP_SANDBOX = YES;
 				ENABLE_HARDENED_RUNTIME = YES;