petshop缓存依赖的工厂模式

接口：ICacheDependency

实现类：TableCacheDependency下的类

工厂类：CacheDependencyFactory.DataAccess

这个工厂类跟之前的工厂类差不多，实现起来没什么太大的区别，但还是记录下来吧，因为毕竟这种模式知道归知道罢了。

先看下这个接口ICacheDependency吧：

publicinterface IPetShopCacheDependency {

        ///<summary>
        /// Method to create the appropriate implementation of Cache Dependency
        ///</summary>
        ///<returns>CacheDependency object(s) embedded in AggregateCacheDependency</returns>
        AggregateCacheDependency GetDependency();
 }

然后就是实现类了，先来看下这个缓存数据表吧：

由此可见要实现缓存的类事Category、Item和Product这些类。那么还是先来看看这些类的实现吧，先不管AspNet_SqlCacheTablesForChangeNotification表的存储过程和触发器了。

就看下Category吧：

publicclass Category : TableDependency {

        ///<summary>
        /// Call its base constructor by passing its specific configuration key
        ///</summary>
public Category() : base("CategoryTableDependency") { }
    }

这个Category是继承TableDependency类的，而这TableDependency是继承ICacheDependency这个类，也算是间接继承的。看下这个TableDependency这个类事怎么实现缓存的：

publicabstractclass TableDependency : PetShop.ICacheDependency.IPetShopCacheDependency {

        // This is the separator that's used in web.config
protectedchar[] configurationSeparator =newchar[] { ',' };

        protected AggregateCacheDependency dependency =new AggregateCacheDependency();

        ///<summary>
        /// The constructor retrieves all related configuration and add CacheDependency object accordingly
        ///</summary>
        ///<param name="configKey">Configuration key for specific derived class implementation</param>
protected TableDependency(string configKey) {

            string dbName = ConfigurationManager.AppSettings["CacheDatabaseName"];//MSPetShop4
string tableConfig = ConfigurationManager.AppSettings[configKey];//Product,Category,Item
string[] tables = tableConfig.Split(configurationSeparator);//','

            foreach (string tableName in tables)
                dependency.Add(new SqlCacheDependency(dbName, tableName));//吧数据库表的查询结果放在SqlCacheDependency中
        }
        //实现父类的方法
public AggregateCacheDependency GetDependency() {
            return dependency;
        }
    }

这里用到了SqlCacheDependency类和AggregateCacheDependency类。那么这两个类到底做了写什么呢？待续。

现在来看下工厂类吧（这个工厂类还是利用放射依赖注入的方法来实例化Category等类的）：

publicstaticclass DependencyAccess {
        ///<summary>
        /// Method to create an instance of Category dependency implementation
        ///</summary>
        ///<returns>Category Dependency Implementation</returns>
publicstatic IPetShopCacheDependency CreateCategoryDependency() {
            return LoadInstance("Category");
        }

        ///<summary>
        /// Method to create an instance of Product dependency implementation
        ///</summary>
        ///<returns>Product Dependency Implementation</returns>
publicstatic IPetShopCacheDependency CreateProductDependency() {
            return LoadInstance("Product");
        }

        ///<summary>
        /// Method to create an instance of Item dependency implementation
        ///</summary>
        ///<returns>Item Dependency Implementation</returns>
publicstatic IPetShopCacheDependency CreateItemDependency() {
            return LoadInstance("Item");
        }

        ///<summary>
        /// Common method to load dependency class from information provided from configuration file 
        ///</summary>
        ///<param name="className">Type of dependency</param>
        ///<returns>Concrete Dependency Implementation instance</returns>
privatestatic IPetShopCacheDependency LoadInstance(string className) {

            string path = ConfigurationManager.AppSettings["CacheDependencyAssembly"];//PetShop.TableCacheDependency
string fullyQualifiedClass = path +"."+ className;

            // Using the evidence given in the config file load the appropriate assembly and class
return (IPetShopCacheDependency)Assembly.Load(path).CreateInstance(fullyQualifiedClass);
        }
    }
}

接下来就看实现类DependencyFacade了：

publicstaticclass DependencyFacade {
        privatestaticreadonlystring path = ConfigurationManager.AppSettings["CacheDependencyAssembly"];//PetShop.TableCacheDependency

        publicstatic AggregateCacheDependency GetCategoryDependency() {
            if (!string.IsNullOrEmpty(path))//PetShop.TableCacheDependency这个怎么判断是否为空？
return DependencyAccess.CreateCategoryDependency().GetDependency();//直接调用
else
                returnnull;
        }

        publicstatic AggregateCacheDependency GetProductDependency() {
            if (!string.IsNullOrEmpty(path))
                return DependencyAccess.CreateProductDependency().GetDependency();
            else
                returnnull;
        }

        publicstatic AggregateCacheDependency GetItemDependency() {
            if (!string.IsNullOrEmpty(path))
                return DependencyAccess.CreateItemDependency().GetDependency();
            else
                returnnull;
        }
    }

这个工厂类的实现有别于DALFactory吧:

因为在数据访问层的BLL.Category，因为Category的实现是直接继承接口的，而这个是间接继承的，所以我们要经过两步，如DependencyAccess.CreateItemDependency().GetDependency();