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.
    struct Content: View {

        // MARK: Properties

        /// Query to fetch todos filtered by group and completion status.
        ///
        /// This query is initialized with a `FetchDescriptor` that applies predicates
        /// based on the group type and completion filter. SwiftData automatically
        /// monitors changes and updates the view when the underlying data changes.
        @Query
        private var tasks: [Todo]

        // MARK: Initializers

        /// Creates a content view with a configured task query.
        ///
        /// This initializer sets up the `@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.
        ///
        /// - 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`.
        init(
            group: TodoGroup,
            showCompleted: Bool
        ) {
            _tasks = .init(
                FetchDescriptor<Todo>(
                    group: group,
                    showCompleted: showCompleted
                )
            )
        }

        // 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.
        var body: some View {
            Group {
                if tasks.isEmpty {
                    ContentUnavailableView(
                        "view.task-list-content.content-unavailable.title",
                        systemImage: "checkmark.circle"
                    )
                }
                else {
                    List {
                        ForEach(tasks) { todo in
                            TaskListView.Item(todo)
                        }
                    }
                    .listStyle(.grouped)
                }
            }
        }
    }

}

// MARK: - Previews

#Preview(
    "Task list content when there are no tasks",
    traits: .emptyContainer
) {
    TaskListView.Content(
        group: .today,
        showCompleted: .random()
    )
}

#Preview(
    "Task list content when completed tasks not shown",
    traits: .sampleTasks
) {
    TaskListView.Content(
        group: .today,
        showCompleted: false
    )
}

#Preview(
    "Task list content when completed tasks shown",
    traits: .sampleTasks
) {
    TaskListView.Content(
        group: .today,
        showCompleted: true
    )
}