快取裝飾器

此示例演示如何使用 Decorator 模式向 DbProductRepository 新增快取功能。這種方法遵循 SOLID 原則， 因為它允許你在不違反單一責任原則或開放/封閉原則的情況下新增快取。

public interface IProductRepository
{
    Product GetProduct(int id);
}

public class DbProductRepository : IProductRepository
{
    public Product GetProduct(int id)
    {
        //return Product retrieved from DB
    }
}

public class ProductRepositoryCachingDecorator : IProductRepository
{
    private readonly IProductRepository _decoratedRepository;
    private readonly ICache _cache;
    private const int ExpirationInHours = 1;

    public ProductRepositoryCachingDecorator(IProductRepository decoratedRepository, ICache cache)
    {
        _decoratedRepository = decoratedRepository;
        _cache = cache;
    }

    public Product GetProduct(int id)
    {
        var cacheKey = GetKey(id);
        var product = _cache.Get<Product>(cacheKey);
        if (product == null)
        {
            product = _decoratedRepository.GetProduct(id);
            _cache.Set(cacheKey, product, DateTimeOffset.Now.AddHours(ExpirationInHours));
        }
        
        return product;
    }

    private string GetKey(int id) => "Product:" + id.ToString();
}

public interface ICache
{
    T Get<T>(string key);
    void Set(string key, object value, DateTimeOffset expirationTime)
}

用法：

var productRepository = new ProductRepositoryCachingDecorator(new DbProductRepository(), new Cache());
var product = productRepository.GetProduct(1);

呼叫 GetProduct 的結果將是：從快取中檢索產品（裝飾器責任），如果產品不在快取中，則繼續呼叫 DbProductRepository 並從 DB 檢索產品。此產品可以新增到快取後，以便後續呼叫不會命中 DB。