todos/App/Sources/Views/TaskList/TaskListContent.swift

import SwiftData
import SwiftUI

extension TaskListView {

    /// Internal content view that manages the dynamic SwiftData query for tasks.
    ///
    /// This view is responsible for querying and displaying the list of tasks based on the specified
    /// group and completion status filter. It exists as a separate view to enable dynamic query
    /// updates when filter parameters change.
    ///
    /// Task reordering is enabled based on the ``TodoGroup/canReorder`` property.
    /// Groups that support reordering (``TodoGroup/today``, ``TodoGroup/upcoming``,
    /// ``TodoGroup/overdue``) allow users to drag and drop tasks to change their order.
    /// The ``TodoGroup/completed`` group does not support reordering as tasks are
    /// displayed in chronological order by completion date.
    struct Content: View {

        // MARK: Type aliases

        /// Closure type for handling task deletion operations.
        ///
        /// - Parameters:
        ///   - todos: The current array of todo tasks.
        ///   - offsets: The index set of tasks to delete.
        typealias DeleteClosure = ([Todo], IndexSet) -> Void

        /// Closure type for handling task reorder operations.
        ///
        /// - Parameters:
        ///   - todos: The current array of todo tasks.
        ///   - source: The index set of items being moved.
        ///   - destination: The destination index for the moved items.
        typealias ReorderClosure = ([Todo], IndexSet, Int) -> Void

        // MARK: Properties

        /// Query to fetch todo tasks filtered by group and completion status.
        ///
        /// This query is initialized with a todo fetch descriptor that applies predicates
        /// based on the group type and completion filter. SwiftData automatically
        /// monitors changes and updates the view when the underlying data changes.
        ///
        /// Tasks are sorted by their `order` property first, then by date-based criteria
        /// specific to each group, enabling both manual and automatic ordering.
        @Query
        private var todos: [Todo]

        /// Indicates whether tasks in this view can be reordered through drag and drop.
        ///
        /// This value is derived from the ``TodoGroup/canReorder`` property and
        /// determines whether the `.onMove` modifier is applied to the task list.
        private let canReorder: Bool

        /// Closure called when tasks are deleted through swipe-to-delete.
        ///
        /// This closure is invoked by the `.onDelete` modifier and is responsible for
        /// removing the selected tasks from the model context.
        private let onDelete: DeleteClosure

        /// Closure called when tasks are reordered through drag and drop.
        ///
        /// This closure is invoked by the `.onMove` modifier and is responsible for
        /// updating the `order` property of affected tasks to reflect their new positions.
        private let onReorder: ReorderClosure

        // MARK: Initializers

        /// Creates a content view with a configured task query and reorder handler.
        ///
        /// This initializer sets up a query with a fetch descriptor that filters
        /// tasks based on the specified group and completion status. When these parameters
        /// change, SwiftUI recreates the view with new values, causing the query to update.
        ///
        /// The reordering capability is automatically determined based on the
        /// ``TodoGroup/canReorder`` property. For groups that support reordering,
        /// the provided closure will be called when users drag and drop tasks to new positions.
        ///
        /// - Parameters:
        ///   - group: The todo group category to filter by (today, upcoming, overdue, or completed).
        ///   - showCompleted: Whether to include completed tasks in the results. Only applies to groups other than `.completed`.
        ///   - onReorder: A closure called when tasks are reordered.
        ///   - onDelete: A closure called when tasks are deleted.
        init(
            group: TodoGroup,
            showCompleted: Bool,
            onDelete: @escaping DeleteClosure,
            onReorder: @escaping ReorderClosure
        ) {
            self._todos = .init(
                FetchDescriptor<Todo>(
                    group: group,
                    showCompleted: showCompleted
                )
            )

            self.canReorder = group.canReorder
            self.onDelete = onDelete
            self.onReorder = onReorder
        }

        // MARK: Body

        /// The content and behavior of the view.
        ///
        /// Displays either a content unavailable state or a list of tasks
        /// based on whether any tasks match the current filters.
        ///
        /// When tasks are present, displays them in a grouped list. The list supports
        /// drag and drop reordering for groups where `canReorder` is true, allowing
        /// users to manually prioritize tasks within the group. For groups where
        /// reordering is disabled (such as completed tasks), the list is displayed
        /// in read-only mode with chronological sorting.
        var body: some View {
            Group {
                if todos.isEmpty {
                    ContentUnavailableView(
                        "view.task-list-content.content-unavailable.title",
                        systemImage: "checkmark.circle"
                    )
                }
                else {
                    List {
                        items
                    }
                    .listStyle(.grouped)
                    .onResignFocus()
                }
            }
        }
    }

}

// MARK: - Helpers

private extension TaskListView.Content {

    // MARK: Computed

    /// A view that renders the task items with appropriate modifiers.
    @ViewBuilder
    var items: some View {
        let items = ForEach(todos) {
            TaskListView.Item($0)
        }
        .onDelete { offsets in
            onDelete(todos, offsets)
        }

        if canReorder {
            items.onMove { source, destination in
                onReorder(todos, source, destination)
            }
        }
        else {
            items
        }
    }

}

// MARK: - Previews

#Preview(
    "Task list content when there are no tasks",
    traits: .emptyContainer
) {
    TaskListView.Content(
        group: .today,
        showCompleted: .random()
    ) { _, _ in
        // On delete closure
    } onReorder: { _, _, _ in
        // On reorder closure
    }
}

#Preview(
    "Task list content when completed tasks not shown",
    traits: .sampleTasks
) {
    TaskListView.Content(
        group: .today,
        showCompleted: false
    ) { _, _ in
        // On delete closure
    } onReorder: { _, _, _ in
        // On reorder closure
    }
}

#Preview(
    "Task list content when completed tasks shown",
    traits: .sampleTasks
) {
    TaskListView.Content(
        group: .today,
        showCompleted: true
    ) { _, _ in
        // On delete closure
    } onReorder: { _, _, _ in
        // On reorder closure
    }
}

#Preview(
    "Task list content when only completed tasks shown",
    traits: .sampleTasks
) {
    TaskListView.Content(
        group: .completed,
        showCompleted: .random()
    ) { _, _ in
        // On delete closure
    } onReorder: { _, _, _ in
        // On reorder closure
    }
}