App/Sources/View Models/TaskListViewModel.swift

@@ -15,17 +15,12 @@ extension TaskListView {
     /// - Provides the localized title for the navigation bar
     /// - Determines whether the completion filter toggle should be displayed
     /// - Provides label and icon configuration for the completion toggle button
+    /// - Handles task reordering through drag and drop interactions
     @Observable
     final class ViewModel {
 
         // MARK: Properties
 
-        /// The task group to display tasks for.
-        ///
-        /// This property determines which tasks are fetched and displayed in the list.
-        /// It is used by the content view to initialize the SwiftData query with the appropriate predicates and sort descriptors for the group.
-        let group: TodoGroup
-
         /// Whether to show completed tasks in the list.
         ///
         /// This property controls the inclusion of completed tasks in the query results:
@@ -33,6 +28,13 @@ extension TaskListView {
         /// - `false`: Completed tasks are filtered out at the database level
         var showCompleted: Bool
 
+        /// The task group to display tasks for.
+        ///
+        /// This property determines which tasks are fetched and displayed in the list.
+        /// It is used by the content view to initialize the SwiftData query with the appropriate
+        /// predicates and sort descriptors for the group.
+        let group: TodoGroup
+
         // MARK: Initialization
 
         /// Initializes a new view model instance for a specific task group.
@@ -40,7 +42,7 @@ extension TaskListView {
         /// - Parameter group: The task group to display tasks for.
         init(group: TodoGroup) {
             self.group = group
-            showCompleted = false
+            self.showCompleted = false
         }
 
         // MARK: Computed Properties
@@ -82,6 +84,36 @@ extension TaskListView {
 
         // MARK: Methods
 
+        /// Reorders tasks after a drag and drop operation.
+        ///
+        /// This method is called when users drag and drop tasks to reorder them within a group.
+        /// It updates the `order` property of all tasks to reflect their new positions, ensuring
+        /// that the user's manual ordering preference is persisted.
+        ///
+        /// - Parameters:
+        ///   - tasks: The current array of tasks before reordering.
+        ///   - source: The index set of items being moved (typically a single index).
+        ///   - destination: The destination index where items should be inserted.
+        /// - Note: This method should only be called for groups where `canReorder` is `true`.
+        ///   The view layer is responsible for only enabling drag and drop for applicable groups.
+        /// - SeeAlso: ``TodoGroup/canReorder`` for determining which groups support reordering.
+        func reorderTasks(
+            _ tasks: [Todo],
+            from source: IndexSet,
+            to destination: Int
+        ) {
+            var updatedTasks = tasks
+
+            updatedTasks.move(
+                fromOffsets: source,
+                toOffset: destination
+            )
+
+            for (index, task) in updatedTasks.enumerated() {
+                task.order = index
+            }
+        }
+
         /// Toggles the visibility of completed tasks.
         ///
         /// This method flips the `showCompleted` state, which triggers SwiftUI to recreate the task list view and its children.

App/Sources/Views/TaskListView.swift

@@ -19,7 +19,13 @@ struct TaskListView: View {
 
     /// Creates a new task list view for a specific group.
     ///
+    /// Initializes the view with a view model configured for the specified group.
+    /// The view model manages the state for completion filtering and task reordering,
+    /// while the view handles the presentation layer including navigation and toolbar.
+    ///
     /// - Parameter group: The todo group category to display tasks for.
+    /// - Note: The group determines which tasks are fetched, whether
+    ///   completion filtering is available, and whether drag and drop reordering is enabled.
     init(group: TodoGroup) {
         _viewModel = .init(initialValue: ViewModel(group: group))
     }
@@ -34,7 +40,9 @@ struct TaskListView: View {
         Content(
             group: viewModel.group,
             showCompleted: viewModel.showCompleted
-        )
+        ) { tasks, source, destination in
+            viewModel.reorderTasks(tasks, from: source, to: destination)
+        }
         .navigationTitle(viewModel.navigationTitle)
         .toolbar {
             if viewModel.canShowCompleted {

UnitTests/Tests/View Models/TaskListViewModelTests.swift

@@ -46,7 +46,8 @@ struct TaskListViewModelTests {
         if showCompleted {
             #expect(title == "view.task-list.label.hide-completed")
             #expect(icon == "eye.slash")
-        } else {
+        }
+        else {
             #expect(title == "view.task-list.label.show-completed")
             #expect(icon == "eye")
         }
@@ -90,6 +91,56 @@ struct TaskListViewModelTests {
 
     // MARK: Method tests
 
+    @Test("reorderTasks updates order property when moving task forward")
+    func reorderTasksMovingForward() {
+        // GIVEN
+        let viewModel = TaskListView.ViewModel(group: .today)
+        let tasks: [Todo] = [
+            .init(title: "Task 1", due: .now, order: 0),
+            .init(title: "Task 2", due: .now, order: 1),
+            .init(title: "Task 3", due: .now, order: 2),
+            .init(title: "Task 4", due: .now, order: 3),
+        ]
+
+        // WHEN
+        viewModel.reorderTasks(
+            tasks,
+            from: IndexSet(integer: 0),
+            to: 3
+        )
+
+        // THEN
+        #expect(tasks[0].order == 2)
+        #expect(tasks[1].order == 0)
+        #expect(tasks[2].order == 1)
+        #expect(tasks[3].order == 3)
+    }
+
+    @Test("reorderTasks updates order property when moving task backward")
+    func reorderTasksMovingBackward() {
+        // GIVEN
+        let viewModel = TaskListView.ViewModel(group: .today)
+        let tasks: [Todo] = [
+            .init(title: "Task 1", due: .now, order: 0),
+            .init(title: "Task 2", due: .now, order: 1),
+            .init(title: "Task 3", due: .now, order: 2),
+            .init(title: "Task 4", due: .now, order: 3),
+        ]
+
+        // WHEN
+        viewModel.reorderTasks(
+            tasks,
+            from: IndexSet(integer: 3),
+            to: 1
+        )
+
+        // THEN
+        #expect(tasks[0].order == 0)
+        #expect(tasks[1].order == 2)
+        #expect(tasks[2].order == 3)
+        #expect(tasks[3].order == 1)
+    }
+
     @Test("toggleCompletedVisbility toggles the showCompleted property")
     func toggleCompletedVisibility() {
         // GIVEN