LehrerApp/LehrerApp.Desktop/Views/Groups/SeatingPlanTabView.axaml.cs

using Avalonia.Controls;
using Avalonia.Controls.Primitives;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.Threading;
using Avalonia.VisualTree;
using LehrerApp.Core.Models;
using LehrerApp.Desktop.ViewModels.Groups;
using LehrerApp.Desktop.Views.Shared;

namespace LehrerApp.Desktop.Views.Groups;

public partial class SeatingPlanTabView : UserControl
{
    private const double AutoScrollEdgeSize = 72;
    private const double AutoScrollStep = 14;

    private object? _dragCandidate;
    private PointerPressedEventArgs? _dragTrigger;
    private Avalonia.Point _pressPosition;
    private SeatCellViewModel? _draggedSeat;
    private StudentSeatOption? _draggedStudent;
    private SeatCellViewModel? _pendingDropTarget;
    private bool _pendingClearSeat;
    private DateTime _ignoreTapUntil;
    private readonly DispatcherTimer _autoScrollTimer;
    private Avalonia.Vector _autoScrollDirection;

    public SeatingPlanTabView()
    {
        InitializeComponent();
        _autoScrollTimer = new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(30) };
        _autoScrollTimer.Tick += OnAutoScrollTick;
    }

    protected override void OnDataContextChanged(EventArgs e)
    {
        base.OnDataContextChanged(e);
        if (DataContext is SeatingPlanTabViewModel vm)
        {
            vm.OnEditPlan = ShowPlanDialog;
            vm.OnConfirmDelete = ShowDeleteConfirmDialog;
            vm.OnAssessStudent = ShowAssessmentDialog;
        }
    }

    private void OnDragSourcePressed(object? sender, PointerPressedEventArgs e)
    {
        if (sender is not Control control ||
            !e.GetCurrentPoint(control).Properties.IsLeftButtonPressed) return;
        var candidate = control.DataContext;
        if (candidate is SeatCellViewModel { IsOccupied: false } or null) return;
        if (DataContext is not SeatingPlanTabViewModel { CanEditLayout: true }) return;
        _dragCandidate = candidate;
        _dragTrigger = e;
        _pressPosition = e.GetPosition(this);
    }

    private async void OnDragSourceMoved(object? sender, PointerEventArgs e)
    {
        if (_dragCandidate is null || _dragTrigger is null ||
            !e.GetCurrentPoint(this).Properties.IsLeftButtonPressed) return;
        var current = e.GetPosition(this);
        if (Math.Abs(current.X - _pressPosition.X) < 6 && Math.Abs(current.Y - _pressPosition.Y) < 6) return;

        _draggedSeat = _dragCandidate as SeatCellViewModel;
        _draggedStudent = _dragCandidate as StudentSeatOption;
        _pendingDropTarget = null;
        _pendingClearSeat = false;
        _dragCandidate = null;
        var trigger = _dragTrigger;
        _dragTrigger = null;
        var data = new DataTransfer();
        data.Add(DataTransferItem.CreateText("LehrerApp-Sitzplatz"));
        var effect = await DragDrop.DoDragDropAsync(trigger, data, DragDropEffects.Move);
        var draggedSeat = _draggedSeat;
        var draggedStudent = _draggedStudent;
        var dropTarget = _pendingDropTarget;
        var clearSeat = _pendingClearSeat;
        _draggedSeat = null;
        _draggedStudent = null;
        _pendingDropTarget = null;
        _pendingClearSeat = false;
        StopAutoScroll();
        _ignoreTapUntil = DateTime.UtcNow.AddMilliseconds(250);

        // Never mutate an ItemsControl while Avalonia is still processing its native
        // drop event. Assigning an unassigned student removes the dragged source item.
        if ((effect & DragDropEffects.Move) == 0 || DataContext is not SeatingPlanTabViewModel vm) return;
        Dispatcher.UIThread.Post(() =>
        {
            if (clearSeat && draggedSeat is not null) vm.ClearSeat(draggedSeat);
            else if (dropTarget is not null && draggedSeat is not null) vm.MoveSeat(draggedSeat, dropTarget);
            else if (dropTarget is not null && draggedStudent is not null) vm.AssignStudent(draggedStudent, dropTarget);
        }, DispatcherPriority.Background);
    }

    private void OnDragSourceReleased(object? sender, PointerReleasedEventArgs e)
    {
        _dragCandidate = null;
        _dragTrigger = null;
    }

    private void OnSeatDragEnter(object? sender, DragEventArgs e)
    {
        if (sender is Border { DataContext: SeatCellViewModel target } && CanDropOn(target))
            target.IsDropTarget = true;
    }

    private void OnSeatDragLeave(object? sender, DragEventArgs e)
    {
        if (sender is Border { DataContext: SeatCellViewModel target }) target.IsDropTarget = false;
    }

    private void OnSeatDragOver(object? sender, DragEventArgs e)
    {
        e.DragEffects = sender is Border { DataContext: SeatCellViewModel target } && CanDropOn(target)
            ? DragDropEffects.Move : DragDropEffects.None;
    }

    private void OnSeatDrop(object? sender, DragEventArgs e)
    {
        StopAutoScroll();
        if (sender is not Border { DataContext: SeatCellViewModel target }) return;
        target.IsDropTarget = false;
        if (!CanDropOn(target))
        {
            e.DragEffects = DragDropEffects.None;
            return;
        }
        _pendingDropTarget = target;
        _pendingClearSeat = false;
        e.DragEffects = DragDropEffects.Move;
    }

    private bool CanDropOn(SeatCellViewModel target) =>
        DataContext is SeatingPlanTabViewModel { CanEditLayout: true }
        && ((_draggedSeat is not null && _draggedSeat != target) || _draggedStudent?.StudentId is not null);

    private void OnUnassignedDragOver(object? sender, DragEventArgs e) =>
        e.DragEffects = _draggedSeat is not null ? DragDropEffects.Move : DragDropEffects.None;

    private void OnUnassignedDrop(object? sender, DragEventArgs e)
    {
        StopAutoScroll();
        if (_draggedSeat is not null)
        {
            _pendingDropTarget = null;
            _pendingClearSeat = true;
            e.DragEffects = DragDropEffects.Move;
        }
    }

    private void OnRoomDragOver(object? sender, DragEventArgs e)
    {
        if (_draggedSeat is null && _draggedStudent is null)
        {
            StopAutoScroll();
            return;
        }

        var position = e.GetPosition(RoomScrollViewer);
        _autoScrollDirection = new Avalonia.Vector(
            GetAutoScrollDirection(position.X, RoomScrollViewer.Bounds.Width),
            GetAutoScrollDirection(position.Y, RoomScrollViewer.Bounds.Height));

        if (_autoScrollDirection == default)
            StopAutoScroll();
        else if (!_autoScrollTimer.IsEnabled)
            _autoScrollTimer.Start();
    }

    private void OnRoomDragLeave(object? sender, DragEventArgs e) => StopAutoScroll();

    private void OnAutoScrollTick(object? sender, EventArgs e)
    {
        var maxX = Math.Max(0, RoomScrollViewer.Extent.Width - RoomScrollViewer.Viewport.Width);
        var maxY = Math.Max(0, RoomScrollViewer.Extent.Height - RoomScrollViewer.Viewport.Height);
        var offset = RoomScrollViewer.Offset;
        RoomScrollViewer.Offset = new Avalonia.Vector(
            Math.Clamp(offset.X + _autoScrollDirection.X * AutoScrollStep, 0, maxX),
            Math.Clamp(offset.Y + _autoScrollDirection.Y * AutoScrollStep, 0, maxY));
    }

    private static double GetAutoScrollDirection(double position, double viewportSize)
    {
        if (position < AutoScrollEdgeSize) return -1;
        if (position > viewportSize - AutoScrollEdgeSize) return 1;
        return 0;
    }

    private void StopAutoScroll()
    {
        _autoScrollDirection = default;
        _autoScrollTimer.Stop();
    }

    private async void OnSeatTapped(object? sender, TappedEventArgs e)
    {
        if (e.Source is Control source &&
            (source is Button or Expander || source.FindAncestorOfType<Button>() is not null ||
             source.FindAncestorOfType<Expander>() is not null)) return;
        if (DateTime.UtcNow < _ignoreTapUntil || sender is not Border { DataContext: SeatCellViewModel seat }
            || !seat.IsOccupied || DataContext is not SeatingPlanTabViewModel vm) return;
        await vm.AssessStudent(seat);
    }

    private async Task ShowAssessmentDialog(SeatAssessmentViewModel vm)
    {
        var dialog = new SeatAssessmentDialog { DataContext = vm };
        var owner = TopLevel.GetTopLevel(this) as Window;
        if (owner is not null) await dialog.ShowDialog(owner);
    }

    private async Task<SeatingPlan?> ShowPlanDialog(SeatingPlan? plan)
    {
        if (DataContext is not SeatingPlanTabViewModel vm) return null;
        var dialogVm = vm.CreateDialogViewModel(plan);
        var dialog = new SeatingPlanDialog { DataContext = dialogVm };
        var owner = TopLevel.GetTopLevel(this) as Window;
        if (owner is null || !await dialog.ShowDialog<bool>(owner)) return null;
        return dialogVm.Result;
    }

    private async Task<bool> ShowDeleteConfirmDialog(SeatingPlanSummary plan)
    {
        var dialog = new ConfirmDialog
        {
            DataContext = new ConfirmDialogInfo
            {
                Title = "Sitzplan löschen?",
                Message = $"Der Sitzplan „{plan.Name}“ und seine Zuordnungen werden gelöscht.",
                ConfirmText = "Löschen",
            },
        };
        var owner = TopLevel.GetTopLevel(this) as Window;
        return owner is not null && await dialog.ShowDialog<bool>(owner);
    }
}