利用依赖倒置和依赖注入实现应用程序解耦

首先，我们虚拟一个系统环境(e-Shop)，即网上商店的应用程序，其中有一个应用: 获取指定分类下的所有产品信息。我们按照传统的思路来实现。图1展示了系统的设计图。

新建一个类库工程EShop.Service. 然后添加相应的类到工程中。

namespace EShop.Service
{
    public class Product
    {
    }

    public class ProductRepository
    {
        public IList<Product> GetProductsByCategory(int categoryId)
        {
            IList<Product> products = new List<Product>();
            //进行数据库操作
            return products;
        }
    }

    public class ProductService
    {
        private ProductRepository productRepository;

        public ProductService()
        {
            productRepository = new ProductRepository();
        }

        public IList<Product> GetProductsByCategory(int categoryId)
        {
            IList<Product> products;
            string storageKey = String.Format("products_in_category_id_{0}", categoryId);
            products = (List<Product>)HttpContext.Current.Cache.Get(storageKey);
            if (products == null)
            {
                products = productRepository.GetProductsByCategory(categoryId);
                HttpContext.Current.Cache.Insert(storageKey, products);
            }
            return products;
        }
    }
}

从以上的程序段是否能发现不合理之处呢？我归纳了一下，大致有以下几点：

1. ProductService依赖于ProductRepository，一旦后者的API发生变化，则前者必然跟着发生变化。

2. 难以测试ProductService的方法，因为ProductRepository并未真正连接到数据库，另外还依赖于HttpContext，两者之间紧耦合。

3. 目前采用HttpContext来进行缓存，如果要更换缓存机制(如：Velocity或Memcached)，ProductService将进行更改。

基于以上几点不合理之处，我们将一一进行重构和优化。

针对1：我们采用依赖倒置原则(Dependency Inversion Principle)——依赖于抽象而不是具体实现来解决。我们加入了接口IProductRepository。

public interface IProductRepository
{
    IList<Product> GetProductsByCategory(int categoryId);
}
public class ProductRepository : IProductRepository
{
    //...
}
public class ProductService
{
    private IProductRepository productRepository;

    public ProductService()
    {
        productRepository = new ProductRepository();
    }
    //...
}

针对2：我们采用依赖注入原则(Dependency Injection Principle)——通过将抽象注入到构造函数、方法或属性来解决。我们修改ProductService构造函数。

public class ProductService
{
    private IProductRepository productRepository;

    public ProductService(IProductRepository productRepository)
    {
        this.productRepository = productRepository;
    }
    //...
}

针对3：我们采用适配器模式(Adapter Pattern)——转换已有接口与客户期望的目标接口使之兼容来解决。我们添加了接口ICacheStorage和类HttpContextCacheAdapter。

public interface ICacheStorage
{
    void Remove(string key);
    void Store(string key, object data);
    T Retrieve<T>(string key);
}

public class HttpContextCacheAdapter : ICacheStorage
public class HttpContextCacheAdapter : ICacheStorage
{
    public void Remove(string key)
    {
        HttpContext.Current.Cache.Remove(key);
    }

    public void Store(string key, object data)
    {
        HttpContext.Current.Cache.Insert(key, data);
    }

    public T Retrieve<T>(string key)
    {
        T item = (T)HttpContext.Current.Cache.Get(key);
        if (item == null)
        {
            item = default(T);
        }
        return item;
    }
}

public class ProductService
{
    private IProductRepository productRepository;
    private ICacheStorage cacheStorage;

    public ProductService(IProductRepository productRepository, ICacheStorage cacheStorage)
    {
        this.productRepository = productRepository;
        this.cacheStorage = cacheStorage;
    }

    public IList<Product> GetProductsByCategory(int categoryId)
    {
        IList<Product> products;
        string storageKey = String.Format("products_in_category_id_{0}", categoryId);
        products = cacheStorage.Retrieve<List<Product>>(storageKey);
        if (products == null)
        {
            products = productRepository.GetProductsByCategory(categoryId);
            cacheStorage.Store(storageKey, products);
        }
        return products;
    }
}

综合以上的解决方案，这里贴出重构后完整的代码：

namespace EShop.Service
{
    public class Product
    {
    }

    public interface IProductRepository
    {
        IList<Product> GetProductsByCategory(int categoryId);
    }

    public class ProductRepository : IProductRepository
    {
        public IList<Product> GetProductsByCategory(int categoryId)
        {
            IList<Product> products = new List<Product>();
            //进行数据库操作
             return products;
        }
    }

    public interface ICacheStorage
    {
        void Remove(string key);
        void Store(string key, object data);
        T Retrieve<T>(string key);
    }

    public class HttpContextCacheAdapter : ICacheStorage
    {
        public void Remove(string key)
        {
            HttpContext.Current.Cache.Remove(key);
        }

        public void Store(string key, object data)
        {
            HttpContext.Current.Cache.Insert(key, data);
        }

        public T Retrieve<T>(string key)
        {
            T item = (T)HttpContext.Current.Cache.Get(key);
            if (item == null)
            {
                item = default(T);
            }
            return item;
        }
    }
    
    //有时为方便起见，我们定义空对象(空缓存适配器)
    public class NullCacheAdapter : ICacheStorage
    {
        public void Remove(string key)
        {
        }

        public void Store(string key, object data)
        {
        }

        public T Retrieve<T>(string key)
        {
            return default(T);
        }
    }
    
    public class ProductService
    {
        private IProductRepository productRepository;
        private ICacheStorage cacheStorage;

        public ProductService(IProductRepository productRepository, ICacheStorage cacheStorage)
        {
            this.productRepository = productRepository;
            this.cacheStorage = cacheStorage;
        }

        public IList<Product> GetProductsByCategory(int categoryId)
        {
            IList<Product> products;
            string storageKey = String.Format("products_in_category_id_{0}", categoryId);
            products = cacheStorage.Retrieve<List<Product>>(storageKey);
            if (products == null)
            {
                products = productRepository.GetProductsByCategory(categoryId);
                cacheStorage.Store(storageKey, products);
            }
            return products;
        }
    }
}

附上重构后的设计图：