canto/Assets/Scripts/UI/Elements/Recycler.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using KitsuneCafe.UI.MVVM;
using Unity.Properties;
using UnityEngine;
using UnityEngine.UIElements;

namespace KitsuneCafe.UI
{
  public class RecyclerViewModel
  {
    public record RotateEvent(Direction Direction, int FirstIndex, object CurrentItem);

    public enum Direction
    {
      None = 0,
      Clockwise = -1,
      CounterClockwise = 1
    }

    private int displayCount;
    public int DisplayCount
    {
      get => displayCount;
      set
      {
        if (displayCount != value)
        {
          displayCount = value;
        }
      }
    }

    private IList itemSource;
    public IList ItemSource
    {
      get => itemSource;
      set
      {
        if (itemSource != value)
        {
          itemSource = value;
        }
      }
    }

    public int Count => Math.Max(displayCount, itemSource.Count);

    private int currentIndex = 0;
    public int SelectedIndex => currentIndex;

    public readonly ICommand<Direction> RotateCommand;

    public event EventHandler<RotateEvent> Rotated = delegate { };

    public RecyclerViewModel()
    {
      RotateCommand = new RelayCommand<Direction>(CanRotate, Rotate);
    }

    private bool CanRotate(Direction direction)
    {
      return direction == Direction.Clockwise || direction == Direction.CounterClockwise;
    }

    public static int WrapIndex(int index, int count)
    {
      return (index + count) % count;
    }

    public int WrapIndex(int index)
    {
      return WrapIndex(index, Count);
    }

    public int GetOffset(int index)
    {
      return WrapIndex(currentIndex + index);
    }

    public static T GetItem<T>(IList<T> xs, int index)
    {
      return xs[WrapIndex(index, xs.Count)];
    }

    public static object GetItem(IList xs, int index)
    {
      return xs[WrapIndex(index, xs.Count)];
    }

    public object GetItem(int index)
    {
      if (0 > index || index >= itemSource.Count) { return null; }
      return GetItem(itemSource, index);
    }

    public static IEnumerable<T> GetSlice<T>(IList<T> xs, int first, int count)
    {
      var diff = count - xs.Count;
      var xs2 = new List<T>(xs);
      xs2.AddRange(Enumerable.Repeat<T>(default, diff));
      var len = first + count;
      for (int i = first; i < len; i++)
      {
        yield return GetItem<T>(xs2, i);
      }
    }

    public static IEnumerable GetSlice(IList xs, int first, int count)
    {
      var diff = count - xs.Count;
      var xs2 = xs.Cast<object>().ToList();
      xs2.AddRange(Enumerable.Repeat<object>(default, diff));
      var len = first + count;
      for (int i = first; i < len; i++)
      {
        yield return GetItem<object>(xs2, i);
      }
    }

    public IEnumerable GetSlice(int first, int count)
    {
      return GetSlice(itemSource, first, count);
    }

    public void Rotate(Direction direction)
    {
      var dir = direction switch
      {
        Direction.Clockwise => 1,
        Direction.CounterClockwise => -1,
        _ => 0
      };



      if (dir != 0)
      {
        currentIndex = WrapIndex(currentIndex + dir);
        Notify(direction);
      }
    }

    private void Notify(Direction direction)
    {
      Rotated?.Invoke(this, new RotateEvent(
        direction,
        currentIndex,
        GetItem(currentIndex)
      ));
    }
  }

  public class SelectEvent : EventBase<SelectEvent>
  {
    public int SelectedIndex;
    public object SelectedItem;

    protected override void Init()
    {
      base.Init();
      bubbles = true;
      tricklesDown = true;
    }
  }

  [UxmlElement]
  public partial class RecyclerView : VisualElement
  {
    public const string BaseClass = "kitsunecafe__recycler-view";

    [UxmlAttribute, CreateProperty]
    public int DisplayCount
    {
      get => viewModel.DisplayCount;
      set
      {
        viewModel.DisplayCount = value;
        CreateItems();
      }
    }

    [CreateProperty]
    public IList ItemSource
    {
      get => viewModel.ItemSource;
      set
      {
        if (ItemSource != value)
        {
          viewModel.ItemSource = value;
          CreateItems();
          Rebind();
        }
      }
    }

    private VisualTreeAsset template;

    [UxmlAttribute, CreateProperty]
    public VisualTreeAsset Template
    {
      get => template;
      set
      {
        if (template != value)
        {
          template = value;
          CreateItems();
          Rebind();
        }
      }
    }

    private readonly RecyclerViewModel viewModel = new();
    private readonly DragManipulator drag;

    public RecyclerView()
    {
      AddToClassList(BaseClass);

      drag = new DragManipulator(this);
      drag.Dragged += OnDrag;
      this.AddManipulator(drag);

      RegisterCallback<NavigationMoveEvent>(OnNavigationMove);
      viewModel.Rotated += OnRotated;
    }

    private void CreateItems()
    {
      Clear();
      if (DisplayCount == 0 || template == null) { return; }
      for (int i = 0; i < DisplayCount; i++)
      {
        var ve = template.CloneTree();
        ve.focusable = true;
        Add(ve);
      }

      FocusFirst();
    }

    public void Rebind()
    {
      if (viewModel == null || ItemSource == null || DisplayCount == 0) { return; }

      for (int i = 0; i < DisplayCount; i++)
      {
        TryBindItem(this[i], i);
      }
    }

    private bool HasItem(int index)
    {
      return 0 <= index && index < ItemSource.Count;
    }

    private void OnRotated(object sender, RecyclerViewModel.RotateEvent e)
    {
      var first = 0;
      var last = DisplayCount - 1;

      var (from, to) = e.Direction switch
      {
        RecyclerViewModel.Direction.Clockwise => (first, last),
        RecyclerViewModel.Direction.CounterClockwise => (last, first),
        RecyclerViewModel.Direction.None => (-1, -1)
      };

      if (from == -1 && to == -1) { return; }

      var element = this[from];

      RemoveAt(from);
      Insert(to, element);

      element.dataSource = viewModel.GetItem(viewModel.GetOffset(to));

      FocusFirst();
      NotifySelection();
    }

    private void NotifySelection()
    {
      using SelectEvent evt = SelectEvent.GetPooled();
      evt.target = this[0];
      evt.SelectedIndex = viewModel.SelectedIndex;
      evt.SelectedItem = viewModel.GetItem(evt.SelectedIndex);
      SendEvent(evt);
    }

    private void FocusFirst()
    {
      if (childCount > 0)
      {
        this[0].Focus();
      }
    }

    private void TryBindItem(VisualElement element, int index)
    {
      if (HasItem(index))
      {
        var idx = viewModel.WrapIndex(index);
        if (0 <= idx && idx < ItemSource.Count)
        {
          element.dataSource = ItemSource[idx];
        }
      }
      else
      {
        element.dataSource = null;
      }
    }

    private void OnDrag(Vector2 vector)
    {
      throw new NotImplementedException();
    }

    private void OnNavigationMove(NavigationMoveEvent evt)
    {
      var direction = evt.direction switch
      {
        NavigationMoveEvent.Direction.Left => RecyclerViewModel.Direction.CounterClockwise,
        NavigationMoveEvent.Direction.Right => RecyclerViewModel.Direction.Clockwise,
        NavigationMoveEvent.Direction.Next => RecyclerViewModel.Direction.Clockwise,
        NavigationMoveEvent.Direction.Previous => RecyclerViewModel.Direction.CounterClockwise,
        _ => RecyclerViewModel.Direction.None
      };

      viewModel.RotateCommand.TryExecute(direction);
    }
  }
}