feat: add seating plan drag and quick assessment

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

@@ -1,4 +1,7 @@
 using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Threading;
 using LehrerApp.Core.Models;
 using LehrerApp.Desktop.ViewModels.Groups;
 using LehrerApp.Desktop.Views.Shared;
@@ -7,6 +10,15 @@ namespace LehrerApp.Desktop.Views.Groups;
 
 public partial class SeatingPlanTabView : UserControl
 {
+    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;
+
     public SeatingPlanTabView() => InitializeComponent();
 
     protected override void OnDataContextChanged(EventArgs e)
@@ -16,9 +28,128 @@ public partial class SeatingPlanTabView : UserControl
         {
             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 { IsEditable: 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;
+        _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)
+    {
+        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 { IsEditable: 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)
+    {
+        if (_draggedSeat is not null)
+        {
+            _pendingDropTarget = null;
+            _pendingClearSeat = true;
+            e.DragEffects = DragDropEffects.Move;
+        }
+    }
+
+    private async void OnSeatTapped(object? sender, TappedEventArgs e)
+    {
+        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;