EventFieldHandler.cs

using System;

namespace RPGCore.Events;

public class EventFieldHandler<TValue> : IEventFieldHandler, IHandlerUsedCallback
{
	private IReadOnlyEventField<TValue>? eventField;

	protected virtual void OnBeforeChanged(TValue oldValue)
	{
	}

	protected virtual void OnAfterChanged(TValue newValue)
	{
	}

	void IEventFieldHandler.OnBeforeChanged()
	{
		if (eventField != null)
		{
			OnBeforeChanged(eventField.Value);
		}
	}

	void IEventFieldHandler.OnAfterChanged()
	{
		if (eventField != null)
		{
			OnAfterChanged(eventField.Value);
		}
	}

	void IHandlerUsedCallback.OnUse(IReadOnlyEventField field)
	{
		if (field == null)
		{
			throw new ArgumentNullException(nameof(field), $"{nameof(field)} is null");
		}
		if (eventField != null)
		{
			throw new InvalidOperationException($"Event handler of type {GetType()} cannot be used on multiple EventFields.");
		}

		if (field is IReadOnlyEventField<TValue> castedField)
		{
			eventField = castedField;
		}
		else
		{
			throw new InvalidOperationException($"Event handler for type {typeof(TValue)} cannot be used on event field of type {field.GetType()}.");
		}
	}
}