canto/Assets/Scripts/UI/Elements/Layout/DynamicLayout.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UIElements;

namespace KitsuneCafe.UI
{
  public record LayoutItem(float Position, float Size)
  {
    public float Min => Position;
    public float Max => Position + Size;
  }

  public class DynamicLayout : ILayout, IEnumerable<LayoutItem>
  {

    public struct LayoutEnumerator : IEnumerator<LayoutItem>
    {
      public readonly int Start;
      public int Index;
      private readonly DynamicLayout layout;

      public LayoutEnumerator(int index, DynamicLayout layout)
      {
        Start = index;
        Index = index;
        this.layout = layout;
      }

      public LayoutEnumerator(DynamicLayout layout) : this(0, layout) { }

      public readonly LayoutItem Current => new(
        layout.positions[Index - 1],
        layout.sizes[Index - 1]
      );

      readonly object IEnumerator.Current => Current;

      public readonly void Dispose() { }

      public bool MoveNext()
      {
        Index += 1;
        return Index < layout.positions.Count;
      }

      public void Reset()
      {
        Index = Start;
      }
    }

    public float DefaultItemSize { get; set; }
    public float GutterSize { get; set; }
    public FlowDirection Direction { get; set; }
    public float ContentSize { get; private set; }

    private readonly List<float> positions = new();
    private readonly List<float> sizes = new();

    public DynamicLayout(FlowDirection direction, float defaultItemSize = 22, float gutterSize = 0)
    {
      Direction = direction;
      GutterSize = gutterSize;
      DefaultItemSize = defaultItemSize;
    }

    public float GetItemPosition(int index)
    {
      if (index > 0)
      {
        return GetItemPosition(index - 1) + GetItemSize(index - 1) + GutterSize;
      }

      return 0;
    }

    public float GetItemSize(int index)
    {
      if (index >= 0 && index < sizes.Count)
      {
        return sizes[index];
      }

      return DefaultItemSize;
    }

    private float LastPositionOrDefault()
    {
      return positions.Count > 0 ? positions[^1] : DefaultItemSize + GutterSize;
    }

    public void Update(int itemCount, VisualElement container)
    {
      while (positions.Count < itemCount)
      {
        var last = LastPositionOrDefault();
        positions.Add(last + DefaultItemSize + GutterSize);
        sizes.Add(DefaultItemSize);
      }

      if (positions.Count > itemCount)
      {
        positions.RemoveRange(itemCount, positions.Count - itemCount);
        sizes.RemoveRange(itemCount, sizes.Count - itemCount);
      }

      ContentSize = LastPositionOrDefault();
    }

    private float CalculatePositions(int startingIndex = 0, float initialPosition = 0)
    {
      var position = initialPosition;

      for (int i = startingIndex; i < positions.Count; i++)
      {
        positions[i] = position;
        position += GetItemSize(i) + GutterSize;
      }

      return ContentSize = position;
    }

    public void SetMeasuredItemSize(int index, float size)
    {
      if (index < sizes.Count && Mathf.Approximately(sizes[index], size))
      {
        return;
      }

      sizes[index] = size;

      var position = index > 0 ? positions[index - 1] + sizes[index - 1] + GutterSize : 0;
      CalculatePositions(index, position);
    }

    public int GetFirstVisibleIndex(float scrollOffset)
    {
      if (scrollOffset <= 0) { return 0; }

      for (int i = 0; i < positions.Count; i++)
      {
        if (positions[i] >= scrollOffset)
        {
          return i;
        }
      }

      return positions.Count - 1;
    }

    public IEnumerable<LayoutItem> Enumerate(int startIndex = 0)
    {
      var len = positions.Count;
      for (int i = startIndex; i < len; i++)
      {
        yield return new LayoutItem(positions[i], sizes[i]);
      }
    }

    public IEnumerator<LayoutItem> GetEnumerator()
    {
      return new LayoutEnumerator(this);
    }

    public IEnumerator<LayoutItem> GetEnumerator(int startIndex)
    {
      return new LayoutEnumerator(startIndex, this);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
      return GetEnumerator();
    }
  }
}