Show completed task feature (#5)

This PR contains the work done to add support for showing or hiding completed todo tasks to the *Task list* view.

Reviewed-on: rock-n-code/todos-app#5
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

App/Sources/View Models/TaskListViewModel.swift

@@ -3,13 +3,18 @@ import SwiftUI
 
 extension TaskListView {
 
-    /// View model for managing the logic for the task list view.
+    /// View model for managing the state and business logic of the task list view.
     ///
-    /// This observable class encapsulates the state and business logic for the task list view.
-    /// It follows the MVVM pattern to separate presentation logic from the view layer.
+    /// This observable class encapsulates the presentation logic for the task list view, following the MVVM pattern to
+    /// separate concerns between the view and business logic layers.
     ///
     /// ## Responsibilities
-    /// - Providing the navigation title for the current group
+    ///
+    /// - Maintains the task group category (today, upcoming, overdue, completed)
+    /// - Manages the visibility preference for completed tasks
+    /// - 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
     @Observable
     final class ViewModel {
 
@@ -18,7 +23,15 @@ extension TaskListView {
         /// The task group to display tasks for.
         ///
         /// This property determines which tasks are fetched and displayed in the list.
-        private let group: TodoGroup
+        /// 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:
+        /// - `true`: Completed tasks are included in the fetch results
+        /// - `false`: Completed tasks are filtered out at the database level
+        var showCompleted: Bool
 
         // MARK: Initialization
 
@@ -27,17 +40,55 @@ extension TaskListView {
         /// - Parameter group: The task group to display tasks for.
         init(group: TodoGroup) {
             self.group = group
+            showCompleted = false
         }
 
-        // MARK: Computed
+        // MARK: Computed Properties
 
-        /// The navigation title for the current task group.
+        /// The localized button title and icon for the show/hide completed todo tasks.
         ///
-        /// This computed property provides the localized title for the current group,
-        /// suitable for display in a navigation bar.
+        /// This computed property provides the appropriate label and icon based on
+        /// the current `showCompleted` state.
         ///
-        /// - Returns: The localized title key for the current group.
-        var navigationTitle: LocalizedStringKey { group.title }
+        /// - Returns: A tuple containing the localized title key and SF Symbol icon name.
+        var buttonCompleted: (title: LocalizedStringKey, icon: String) {
+            let title: LocalizedStringKey = showCompleted
+                ? "view.task-list.label.hide-completed"
+                : "view.task-list.label.show-completed"
+            let icon = showCompleted
+                ? "eye.slash"
+                : "eye"
+
+            return (title, icon)
+        }
+
+        /// Whether the completed todo tasks should be displayed.
+        ///
+        /// This property delegates to the group's `canShowCompleted` property to determine if completed todo tasks should be shown.
+        ///
+        /// - Returns: `true` if the completed todo tasks should be displayed, `false` otherwise.
+        var canShowCompleted: Bool {
+            group.canShowCompleted
+        }
+
+        /// The navigation title for the task list view.
+        ///
+        /// This computed property provides the localized title for the task list view, suitable for display in a navigation bar.
+        ///
+        /// - Returns: The localized title key for the task list view.
+        var navigationTitle: LocalizedStringKey {
+            group.title
+        }
+
+        // MARK: Methods
+
+        /// Toggles the visibility of completed tasks.
+        ///
+        /// This method flips the `showCompleted` state, which triggers SwiftUI to recreate the task list view and its children.
+        /// The toggle has no effect when `canShowCompleted` is `false`.
+        func toggleCompletedVisibility() {
+            showCompleted.toggle()
+        }
 
     }
 }