Syntriax.Engine/Engine.Core/Transform.cs

using System.Collections;
using System.Collections.Generic;

using Syntriax.Engine.Core.Abstract;

namespace Syntriax.Engine.Core;

[System.Diagnostics.DebuggerDisplay("Name: {GameObject.Name, nq} Position: {Position.ToString(), nq}, Scale: {Scale.ToString(), nq}, Rotation: {Rotation}")]
public class Transform : ITransform
{
    public event IAssignableGameObject.OnGameObjectAssignedEventHandler? OnGameObjectAssigned = null;

    public event IAssignable.OnUnassignedEventHandler? OnUnassigned = null;

    public event ITransform.OnPositionChangedEventHandler? OnPositionChanged = null;
    public event ITransform.OnScaleChangedEventHandler? OnScaleChanged = null;
    public event ITransform.OnRotationChangedEventHandler? OnRotationChanged = null;

    public event ITransform.OnParentChangedEventHandler? OnParentChanged = null;
    public event ITransform.OnChildrenAddedEventHandler? OnChildrenAdded = null;
    public event ITransform.OnChildrenRemovedEventHandler? OnChildrenRemoved = null;


    private Vector2D _position = Vector2D.Zero;
    private Vector2D _scale = Vector2D.One;
    private float _rotation = 0f;

    private Vector2D _localPosition = Vector2D.Zero;
    private Vector2D _localScale = Vector2D.One;
    private float _localRotation = 0f;

    private readonly List<ITransform> _children = [];

    public IGameObject GameObject { get; private set; } = null!;
    public ITransform? Parent { get; private set; } = null;

    public IReadOnlyList<ITransform> Children => _children;

    public Vector2D Position
    {
        get => _position;
        set
        {
            if (value == _position)
                return;

            _position = value;
            UpdateLocalPosition();
            OnPositionChanged?.Invoke(this);
        }
    }

    public Vector2D Scale
    {
        get => _localScale.Scale(Parent?.Scale ?? Vector2D.One);
        set
        {
            if (value == _scale)
                return;

            _scale = value;
            UpdateLocalScale();
            OnScaleChanged?.Invoke(this);
        }
    }

    public float Rotation
    {
        get => _localRotation + (Parent?.Rotation ?? 0f);
        set
        {
            if (value == _rotation)
                return;

            _rotation = value;
            UpdateLocalPosition();
            OnRotationChanged?.Invoke(this);
        }
    }

    public Vector2D LocalPosition
    {
        get => _localPosition;
        set
        {
            if (value == _localPosition)
                return;

            _localPosition = value;
            UpdatePosition();
            OnPositionChanged?.Invoke(this);
        }
    }

    public Vector2D LocalScale
    {
        get => _localScale;
        set
        {
            if (value == _localScale)
                return;

            _localScale = value;
            UpdateScale();
            OnScaleChanged?.Invoke(this);
        }
    }

    public float LocalRotation
    {
        get => _localRotation;
        set
        {
            if (value == _localRotation)
                return;

            _localRotation = value;
            UpdateRotation();
            OnRotationChanged?.Invoke(this);
        }
    }

    public void SetParent(ITransform? transform)
    {
        if (transform == this || Parent == transform)
            return;

        ITransform? previousParent = Parent;
        if (previousParent is not null)
        {
            previousParent.RemoveChild(this);
            previousParent.OnPositionChanged -= RecalculatePosition;
            previousParent.OnScaleChanged -= RecalculateScale;
            previousParent.OnRotationChanged -= RecalculateRotation;
            previousParent.OnParentChanged -= NotifyChildrenOnParentChange;
        }

        Parent = transform;

        if (transform is not null)
        {
            transform.AddChild(this);
            transform.OnPositionChanged += RecalculatePosition;
            transform.OnScaleChanged += RecalculateScale;
            transform.OnRotationChanged += RecalculateRotation;
            transform.OnParentChanged += NotifyChildrenOnParentChange;
        }

        UpdateLocalPosition();
        UpdateLocalScale();
        UpdateLocalRotation();

        OnParentChanged?.Invoke(this, previousParent, transform);
    }

    public void AddChild(ITransform transform)
    {
        if (_children.Contains(transform))
            return;

        _children.Add(transform);
        transform.SetParent(this);
        OnChildrenAdded?.Invoke(this, transform);
    }

    public void RemoveChild(ITransform transform)
    {
        if (!_children.Remove(transform))
            return;

        transform.SetParent(null);
        OnChildrenRemoved?.Invoke(this, transform);
    }

    public IEnumerator<ITransform> GetEnumerator() => _children.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => _children.GetEnumerator();

    private void NotifyChildrenOnParentChange(ITransform transform, ITransform? previousParent, ITransform? newParent)
    {
        // TODO No idea how logical this is to propagate this to the children the way I'm doing right now.
        // I was originally gonna just call `child.OnParentChanged?.Invoke(child, child.Parent);` but seems an unnecessary call too?
        foreach (var child in Children) // TODO CHECK ERRORS
            child.SetParent(this);
    }

    private void RecalculatePosition(ITransform _)
    {
        if (Parent is null)
            return;

        UpdatePosition();
        OnPositionChanged?.Invoke(this);
    }

    private void RecalculateScale(ITransform _)
    {
        if (Parent is null)
            return;

        UpdateScale();
        OnScaleChanged?.Invoke(this);
    }

    private void RecalculateRotation(ITransform _)
    {
        if (Parent is null)
            return;

        UpdatePosition();
        UpdateRotation();
        OnPositionChanged?.Invoke(this);
        OnRotationChanged?.Invoke(this);
    }

    private void UpdateLocalPosition()
    {
        if (Parent is null)
            _localPosition = Position;
        else
            _localPosition = Parent.Position.FromTo(Position).Scale(Parent.Scale);
    }

    private void UpdateLocalScale()
    {
        if (Parent is null)
            _localScale = Scale;
        else
            _localScale = Scale.Scale(new(1f / Parent.Scale.X, 1f / Parent.Scale.Y));
    }

    private void UpdateLocalRotation()
    {
        if (Parent is null)
            _localRotation = Rotation;
        else
            _localRotation = Rotation - Parent.Rotation;
    }

    private void UpdatePosition()
    {
        if (Parent is null)
            _position = LocalPosition.Rotate(0f * Math.DegreeToRadian);
        else
            _position = Parent.Position + LocalPosition.Scale(new(Parent.Scale.X, Parent.Scale.Y)).Rotate(Parent.Rotation * Math.DegreeToRadian);
    }

    private void UpdateScale()
    {
        if (Parent is null)
            _scale = LocalScale;
        else
            _scale = Vector2D.Scale(Parent.Scale, LocalScale);
    }

    private void UpdateRotation()
    {
        if (Parent is null)
            _rotation = LocalRotation;
        else
            _rotation = Parent.Rotation + LocalRotation;
    }

    public bool Assign(IGameObject gameObject)
    {
        if (GameObject is not null && GameObject.IsInitialized)
            return false;

        GameObject = gameObject;
        OnGameObjectAssigned?.Invoke(this);
        return true;
    }

    public bool Unassign()
    {
        if (GameObject is not null && GameObject.IsInitialized)
            return false;

        GameObject = null!;
        OnUnassigned?.Invoke(this);
        return true;
    }
}