[Test]
public void RenderMainSpriteWithAttachedSubSprite()
{
	var mainSprite = new Sprite(logoMaterial, Rectangle.HalfCentered);
	mainSprite.Add(new Sprite(logoMaterial, Rectangle.FromCenter(0.5f, 0.5f, 0.1f, 0.1f)));
	mainSprite.Start<UpdateClientSpriteOnPositionChange>();
	new Command(Command.Click, position => mainSprite.Center = position);
}

private class UpdateClientSpriteOnPositionChange : UpdateBehavior
{
	public override void Update(IEnumerable<Entity> entities)
	{
		foreach (Entity2D entity in entities)
			entity.Get<Sprite>().Center = entity.Center;
	}
}

[Test]
public void RenderHierachyEntityWithAttachedSprites()
{
	var mainEntity = new Sprite(logoMaterial, Rectangle.HalfCentered);
	mainEntity.AddChild(new Sprite(logoMaterial, Rectangle.FromCenter(0.3f, 0.3f, 0.1f, 0.1f)));
	new Command(Command.Click, position => mainEntity.Center = position);
}

public class HierarchyEntity2D : Entity2D, HierarchyObject2D
{
	protected HierarchyEntity2D()
		: this(Rectangle.Zero) {}

	public HierarchyEntity2D(Vector2D position)
		: this(Rectangle.FromCenter(position, Size.Zero)) {}

	public HierarchyEntity2D(Rectangle drawArea, float orientation = 0.0f)
		: base(drawArea)
	{
		Children = new List<HierarchyObject2D>();
		Rotation = orientation;
		localPosition = Vector2D.Zero;
		localRotation = 0;
	}

	public Vector2D LocalPosition
	{
		get { return localPosition; }
		set
		{
			localPosition = value;
			UpdateGlobalsFromParent(Parent);
		}
	}

	private Vector2D localPosition;

	public float LocalRotation
	{
		get { return localRotation; }
		set
		{
			localRotation = value;
			UpdateGlobalsFromParent(Parent);
		}
	}

	private float localRotation;

	public HierarchyObject2D Parent { get; private set; }

	public List<HierarchyObject2D> Children { get; private set; }

	public void Add(HierarchyObject2D child)
	{
		Children.Add(child);
		child.UpdateGlobalsFromParent(this);
	}

	public void Remove(HierarchyObject2D child)
	{
		Children.Remove(child);
		child.UpdateGlobalsFromParent(null);
	}

	public void UpdateGlobalsFromParent(HierarchyObject2D parent)
	{
		if (Parent != parent)
		{
			Parent = parent;
			if (parent != null)
			{
				localPosition = Center - parent.Center;
				localRotation = Rotation - parent.Rotation;
			}
		}
		if (parent == null)
		{
			Center = localPosition;
			Rotation = localRotation;
			return;
		}
		Center = parent.Center + localPosition;
		Rotation = parent.Rotation + localRotation;
		foreach (var child in Children)
			child.UpdateGlobalsFromParent(this);
	}

	public object GetFirstChildOfType<T>() where T : HierarchyObject2D
	{
		for (int i = 0; i < Children.Count; i++)
			if (Children[i].GetType() == typeof(T))
				return (T)Children[i];
		return null;
	}

	protected override void OnPositionChanged()
	{
		UpdateChildren();
		base.OnPositionChanged();
	}

	protected override void OnRotationChanged()
	{
		UpdateChildren();
		base.OnRotationChanged();
	}

	private void UpdateChildren()
	{
		foreach (var child in Children)
			child.UpdateGlobalsFromParent(this);
	}

	public override bool IsActive
	{
		get { return base.IsActive; }
		set
		{
			foreach (var child in Children)
				child.IsActive = value;
			base.IsActive = value;
		}
	}
}