todos/App/Sources/View Models/TaskListViewModel.swift

import SwiftData
import SwiftUI

extension TaskListView {

    /// View model for managing the state and business logic of the task list view.
    ///
    /// 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
    ///
    /// - 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
    /// - Handles task reordering through drag and drop interactions
    @Observable
    final class ViewModel {

        // MARK: Properties

        /// 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

        /// 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.
        ///
        /// - Parameter group: The task group to display tasks for.
        init(group: TodoGroup) {
            self.group = group
            self.showCompleted = false
        }

        // MARK: Computed Properties

        /// The localized button title and icon for the show/hide completed todo tasks.
        ///
        /// This computed property provides the appropriate label and icon based on
        /// the current `showCompleted` state.
        ///
        /// - 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

        /// 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.
        /// The toggle has no effect when `canShowCompleted` is `false`.
        func toggleCompletedVisibility() {
            showCompleted.toggle()
        }

    }
}