[C#] 표준 이벤트 패턴

// 이벤트 데이터 클래스
class PriceChangedEventArgs : EventArgs
{
    public decimal OldPrice { get; }
    public decimal NewPrice { get; }
    public decimal ChangePercent { get; }

    public PriceChangedEventArgs(decimal oldPrice, decimal newPrice)
    {
        OldPrice = oldPrice;
        NewPrice = newPrice;
        if (oldPrice != 0)
        {
            ChangePercent = (newPrice - oldPrice) / oldPrice * 100;
        }
    }
}

class Stock
{
    private string _symbol;
    private decimal _price;

    // 표준 이벤트 패턴: EventHandler<TEventArgs>
    public event EventHandler<PriceChangedEventArgs> PriceChanged;

    public Stock(string symbol, decimal initialPrice)
    {
        _symbol = symbol;
        _price = initialPrice;
    }

    public decimal Price
    {
        get => _price;
        set
        {
            if (_price == value)
            {
                return;
            }

            decimal oldPrice = _price;
            _price = value;

            // 이벤트 발생
            OnPriceChanged(new PriceChangedEventArgs(oldPrice, _price));
        }
    }

    // protected virtual로 선언하여 상속 시 재정의 가능
    protected virtual void OnPriceChanged(PriceChangedEventArgs e)
    {
        PriceChanged?.Invoke(this, e);
    }

    public override string ToString()
    {
        return $"{_symbol}: {_price:C}";
    }
}

class Program
{
    static void Main()
    {
        Stock msStock = new Stock("MSFT", 100.00m);

        // 표준 패턴: sender와 EventArgs를 받음
        msStock.PriceChanged += (sender, e) =>
        {
            Stock stock = (Stock)sender;
            Console.WriteLine($"[{stock}]");
            Console.WriteLine($"  이전 가격: {e.OldPrice:C}");
            Console.WriteLine($"  현재 가격: {e.NewPrice:C}");
            Console.WriteLine($"  변동률: {e.ChangePercent:F2}%");
            Console.WriteLine();
        };

        msStock.Price = 110.00m;
        msStock.Price = 105.50m;
        msStock.Price = 120.00m;
    }
}

public sealed class ValueChangedEventArgs : EventArgs
{
    public DataSnapshot Snapshot { get; private set; }

    public DatabaseError DatabaseError { get; private set; }

    internal ValueChangedEventArgs(DataSnapshot snapshot)
    {
        Snapshot = snapshot;
    }

    internal ValueChangedEventArgs(DatabaseError error)
    {
        DatabaseError = error;
    }
}

private void OnValueChanged(object sender, ValueChangedEventArgs args)
    {
        if (args.DatabaseError != null)
        {
            Debug.LogError($"[Leaderboard] 리스너 오류: {args.DatabaseError}");
            return;
        }

        List<LeaderboardEntry> leaderboard = ParseEntries(args.Snapshot);
        DispatchUpdateAsync(leaderboard).Forget();
    }