EntityFramework之领域驱动设计实践（九）（转）

http://www.cnblogs.com/daxnet/archive/2010/07/10/1774706.html

仓储的实现：深入篇

早在年前的时候就已经在CSAI博客发表了上一篇文章：《仓储的实现：基础篇》。苦于日夜奔波于工作与生活之间，一直没有能够抽空继续探讨仓储的实现细节，也让很多关注EntityFramework和领域驱动设计的朋友们备感失望。

闲话不多说，现在继续考虑，如何让仓储的操作在相同的事物处理上下文中进行。DDD引入仓储模式，其目的之一就是能够通过仓储隐藏对象持久化的技术细节，使得领域模型变得更为“纯净”。由此可见，仓储的实现是需要基础结构层的组件支持的，表现为对数据库的操作。在传统的关系型数据库操作中，事务处理是一个很重要的概念，虽然从目前某些大型项目看，事务处理会降低效率，但它保证了数据的完整性。关系型数据库仍然是目前数据持久化机制的主流，事务处理的实现还是很有必要的。

为了迎合仓储模式，就需要对经典的ObjectContext使用方式作一些调整。比如，原本我们可以非常简单地使用using (EntitiesContainer ec = new EntitiesContainer())语句来界定LINQ to Entities的操作范围，并使用ObjectContext的SaveChanges成员方法提交事务，而在引入了仓储的实现中，就不能继续采用这种经典的使用方式。这让EntityFramework看上去变得很奇怪，也很牵强，我相信很多网友会批评我的做法，因为我把问题复杂化了。

其实，这应该是关注点不同罢了。关注EntityFramework的开发人员，自然觉得经典的调用方式简单明了，而从DDD的角度看呢？只能把关注点放在仓储上，而把EntityFramework当成是仓储的一种技术选型（当然从DDD角度讲，我们完全可以不选择 EntityFramework，而去选择其它技术）。所以本文暂且抛开EntityFramework，继续在上文的基础上，讨论仓储的实现。

前面提到，仓储的实现需要考虑事务处理，而且根据DDD的经验，针对每一个聚合根，都需要有个仓储对其进行持久化以及对象重新组装等操作。为此，我的想法是，将仓储操作“界定”在某一个事务处理上下文（Context）中，仓储的实例是由Context获得的，这有点像 EntityFramework中ObjectContext与EntityObject的关系那样。由于仓储是来自于transaction context，所以它知道目前处于哪个事务上下文中。我定义的这个transaction context如下：

隐藏行号复制代码？ Transaction Context

public interface IRepositoryTransactionContext : IDisposable

```
{ 
```

    IRepository<TEntity> GetRepository<TEntity>()

        where TEntity : EntityObject, IAggregateRoot;

```
    void BeginTransaction(); 
```
```
    void Commit(); 
```
```
    void Rollback(); 
```
```
} 
```

上面第三行代码定义了一个接口方法，这个方法的主要作用就是返回一个针对指定聚合根实体的仓储实例。剩下那三行代码就很明显了，那是标准的transaction操作：启动事务、提交事务以及回滚事务。

在设计上，可以根据需要，选择合适的技术来实现IRepositoryTransactionContext。我们现在讨论的是 EntityFramework，所以我将给出EntityFramework的具体实现。当然，如果你不选用EntityFramework，而是用 NHibernate实现数据持久化，这样的设计同样能够使你达到目的。以下是基于EntityFramework的实现：EdmRepositoryTransactionContext的伪代码。

隐藏行号复制代码？ EdmRepositoryTransactionContext

internal class EdmRepositoryTransactionContext : IRepositoryTransactionContext

```
{ 
```
```
    private ObjectContext objContext; 
```

    private Dictionary<Type, object> repositoryCache = new Dictionary<Type, object>();

    public EdmRepositoryTransactionContext(ObjectContext objContext)

```
    { 
```
```
        this.objContext = objContext; 
```
```
    } 
```

    #region IRepositoryTransactionContext Members

    public IRepository<TEntity> GetRepository<TEntity>() where TEntity : EntityObject, IAggregateRoot

```
 { 
```

        if (repositoryCache.ContainsKey(typeof(TEntity)))

```
        { 
```

            return (IRepository<TEntity>)repositoryCache[typeof(TEntity)];

```
        } 
```

        IRepository<TEntity> repository = new EdmRepository<TEntity>(this.objContext);

        this.repositoryCache.Add(typeof(TEntity), repository);

```
        return repository; 
```
```
    } 
```
```
    public void BeginTransaction()  
```
```
    {  
```

        // We do not need to begin a transaction here because the object context,

        // which would handle the transaction, was created and injected into the

        // constructor by Castle Windsor framework.

```
 } 
```
```
    public void Commit() 
```
```
    { 
```
```
        this.objContext.SaveChanges(); 
```
```
    } 
```
```
    public void Rollback() 
```
```
    { 
```

        // We also do not need to perform the rollback operation because

        // entity framework will handle this for us, just when the execution

        // point is stepping out of the using scope.

```
 } 
```
```
    #endregion 
```
```
 
```
```
    #region IDisposable Members 
```
```
    public void Dispose() 
```
```
    { 
```
```
        this.repositoryCache.Clear(); 
```
```
        this.objContext.Dispose(); 
```
```
    } 
```
```
    #endregion 
```
```
} 
```

EdmRepositoryTransactionContext被定义为internal，这个设计是合理的，因为Domain层是不需要知道事务上下文的具体实现，它将会被IoC/DI容器注入到Domain层中（本系列文章采用Castle Windsor框架）。在EdmRepositoryTransactionContext的构造函数中，它需要EntityFramework的 ObjectContext对象来初始化实例。同样，由于IoC/DI的使用，我们在代码中也是不需要去创建这个ObjectContext的，交给 Castle Windsor就OK了。第13行的GetRepository方法简单地采用了Dictionary对象来实现缓存仓储实例的效果，当然这种做法还有待改进。

EdmRepositoryTransactionContext是不需要BeginTransaction的，我们将方法置空，因为EntityFramework的事务会由ObjectContext来管理，同理，Rollback也被置空。

EdmRepository的实现就比较显而易见了，请参见上文。

此外，我们还可以针对NHibernate实现仓储模式，只需要实现IRepositoryTransactionContext和IRepository接口即可，比如：

隐藏行号复制代码？ NHibernateRepositoryTransactionContext实现

internal class NHibernateRepositoryTransactionContext : IRepositoryTransactionContext

```
{ 
```
```
    ITransaction transaction; 
```

    Dictionary<Type, object> repositoryCache = new Dictionary<Type, object>();

    public ISession Session { get { return DatabaseSessionFactory.Instance.Session; } }

    #region IRepositoryTransactionContext Members

    public IRepository<TEntity> GetRepository<TEntity>()

        where TEntity : EntityObject, IAggregateRoot

```
 { 
```

        if (repositoryCache.ContainsKey(typeof(TEntity)))

```
        { 
```

            return (IRepository<TEntity>)repositoryCache[typeof(TEntity)];

```
        } 
```

        IRepository<TEntity> repository = new NHibernateRepository<TEntity>(this.Session);

        this.repositoryCache.Add(typeof(TEntity), repository);

```
        return repository; 
```
```
    } 
```
```
    public void BeginTransaction() 
```
```
    { 
```

        transaction = DatabaseSessionFactory.Instance.Session.BeginTransaction();

```
    } 
```
```
    public void Commit() 
```
```
    { 
```
```
        transaction.Commit(); 
```
```
    } 
```
```
    public void Rollback() 
```
```
    { 
```
```
        transaction.Rollback(); 
```
```
    } 
```
```
    #endregion 
```
```
 
```
```
    #region IDisposable Members 
```
```
    public void Dispose() 
```
```
    { 
```
```
        transaction.Dispose(); 
```

        ISession dbSession = DatabaseSessionFactory.Instance.Session;

        if (dbSession != null && dbSession.IsOpen)

```
            dbSession.Close(); 
```
```
    } 
```
```
    #endregion 
```
```
} 
```

隐藏行号复制代码？ NHibernateRepository实现

internal class NHibernateRepository<TEntity> : IRepository<TEntity>

    where TEntity : EntityObject, IAggregateRoot

```
{ 
```
```
    ISession session; 
```

    public NHibernateRepository(ISession session)

```
    { 
```
```
        this.session = session; 
```
```
    } 
```

    #region IRepository<TEntity> Members

```
    public void Add(TEntity entity) 
```
```
    { 
```
```
        this.session.Save(entity); 
```
```
    } 
```
```
    public TEntity GetByKey(int id) 
```
```
    { 
```

        return (TEntity)this.session.Get(typeof(TEntity), id);

```
    } 
```

    public IEnumerable<TEntity> FindBySpecification(Func<TEntity, bool> spec)

```
    { 
```

        throw new NotImplementedException();

```
    } 
```
```
    public void Remove(TEntity entity) 
```
```
    { 
```
```
        this.session.Delete(entity); 
```
```
    } 
```
```
    public void Update(TEntity entity) 
```
```
    { 
```

        this.session.SaveOrUpdate(entity);

```
    } 
```
```
    #endregion 
```
```
} 
```

在NHibernateRepositoryTransactionContext中使用了一个DatabaseSessionFactory的类，该类主要用于管理NHibernate的Session对象，具体实现如下（该实现已被用于我的Apworks应用开发框架原型中）：

隐藏行号复制代码？ DatabaseSessionFactory实现

```
/// <summary> 
```

/// Represents the factory singleton for database session.

```
/// </summary> 
```

internal sealed class DatabaseSessionFactory

```
{ 
```
```
    #region Private Static Fields 
```
```
    /// <summary> 
```

    /// The singleton instance of the database session factory.

```
 /// </summary> 
```

 private static readonly DatabaseSessionFactory databaseSessionFactory = new DatabaseSessionFactory();

```
    #endregion 
```
```
 
```
```
    #region Private Fields 
```
```
    /// <summary> 
```
```
    /// The session factory instance. 
```
```
 /// </summary> 
```

 private ISessionFactory sessionFactory = null;

```
    /// <summary> 
```
```
    /// The session instance. 
```
```
 /// </summary> 
```
```
 private ISession session = null; 
```
```
    #endregion 
```
```
 
```
```
    #region Constructors 
```
```
    /// <summary> 
```

    /// Privately constructs the database session factory instance, configures the

 /// NHibernate framework by using the assemblies listed in the configured spaces(paths)

 /// that are decorated by <see cref="EntityVisibleAttribute"/>.

```
 /// </summary> 
```
```
 private DatabaseSessionFactory() 
```
```
    { 
```

        Configuration nhibernateConfig = new Configuration();

```
        nhibernateConfig.Configure(); 
```

        nhibernateConfig.AddAssembly(typeof(IAggregateRoot).Assembly);

        sessionFactory = nhibernateConfig.BuildSessionFactory();

```
    } 
```
```
    #endregion 
```
```
 
```
```
    #region Public Properties 
```
```
    /// <summary> 
```

    /// Gets the singleton instance of the database session factory.

```
 /// </summary> 
```

 public static DatabaseSessionFactory Instance

```
    { 
```
```
        get 
```
```
 { 
```

            return databaseSessionFactory;

```
        } 
```
```
    } 
```
```
    /// <summary> 
```

    /// Gets the singleton instance of the session. If the session has not been

 /// initialized or opened, it will return a newly opened session from the session factory.

```
 /// </summary> 
```
```
 public ISession Session 
```
```
    { 
```
```
        get 
```
```
 { 
```
```
            ISession result = session; 
```

            if (result != null && result.IsOpen)

```
                return result; 
```
```
            return OpenSession(); 
```
```
        } 
```
```
    } 
```
```
    #endregion 
```
```
 
```
```
    #region Public Methods 
```
```
    /// <summary> 
```

    /// Always opens a new session from the session factory.

```
 /// </summary> 
```

    /// <returns>The newly opened session.</returns>

```
 public ISession OpenSession() 
```
```
    { 
```

        this.session = sessionFactory.OpenSession();

```
        return this.session; 
```
```
    } 
```
```
    #endregion 
```
```
 
```
```
} 
```

简单小结一下。通过上面的例子可以看到，仓储的实现是不能依赖于任何技术细节的，因为领域模型并不关心技术问题。这并不是DDD一书中怎么说，我们就得怎么做。事实上的确如此，因为DDD的思想，使得我们应该把关注点放在业务分析与领域建模上，而仓储实现的分离正是这一思想的具体表现形式。不管怎么样，采用其它的手段也罢，我们还是应该遵循“将关注点放在领域”这一宗旨。

接下来看如何在领域层结合IoC框架使用仓储。仍然以Castle Windsor为例。配置文件如下（超长部分我用省略号去掉了）：

隐藏行号复制代码？ Castle Windsor配置

```
<castle> 
```
```
  <components> 
```

    <!-- Object Context for Entity Data Model -->

```
    <component id="ObjectContext" 
```

               service="System.Data.Objects.ObjectContext, System.Data.Entity, Version=4.0.0.0,......"

               type="EasyCommerce.Domain.Model.EntitiesContainer, EasyCommerce.Domain"/>

```
 
```
```
    <component id="GeneralRepository" 
```

               service="EasyCommerce.Domain.IRepository`1[[EasyCommerce.Domain.Model.Customer, ......"

               type="EasyCommerce.Infrastructure.Repositories.EdmRepositories.EdmRepository`1[[EasyCo......">

      <objContext>${ObjectContext}</objContext>

```
    </component> 
```
```
 
```
```
    <component id="TransactionContext" 
```

               service="EasyCommerce.Domain.IRepositoryTransactionContext, EasyCommerce.Domain......"

               type="EasyCommerce.Infrastructure.Repositories.EdmRepositories.EdmRepositoryTransactionContext, ...">

```
    </component> 
```
```
 
```
```
  </components> 
```
```
</castle> 
```
```
 
```

以下是调用代码：

隐藏行号复制代码？调用方代码

```
[TestMethod] 
```
```
public void TestCreateCustomer() 
```
```
{ 
```

    IWindsorContainer container = new WindsorContainer(new XmlInterpreter());

    using (IRepositoryTransactionContext tx = container.GetService<IRepositoryTransactionContext>())

```
    { 
```
```
        tx.BeginTransaction(); 
```
```
        try 
```
```
 { 
```

            Customer customer = Customer.CreateCustomer("daxnet", "12345",

                new Name { FirstName = "Sunny", LastName = "Chen" },

                new Address(), new Address(), DateTime.Now.AddYears(-29));

            IRepository<Customer> customerRepository = tx.GetRepository<Customer>();

            customerRepository.Add(customer);

```
            tx.Commit(); 
```
```
        } 
```
```
        catch 
```
```
 { 
```
```
            tx.Rollback(); 
```
```
            throw; 
```
```
        } 
```
```
    } 
```
```
} 
```

测试结果及数据库数据结果：

【注意】：在使用NHibernate的仓储实现时，由于NHibernate的延迟加载特性，需要将实体的属性设置为virtual，以便由NHibernate产生Proxy Class进而实现延迟加载；但是由EntityFramework自动生成的源代码并不会将实体属性设置为virtual，而采用partial class也无法解决这个问题。因此需要在代码生成技术上做文章。我的做法是，针对edmx产生一个基于T4的代码生成模板，然后修改这个模板，分别在 WritePrimitiveTypeProperty和WriteComplexTypeProperty方法中的适当位置加上virtual关键字：

隐藏行号复制代码？ WritePrimitiveTypeProperty

    private void WritePrimitiveTypeProperty(EdmProperty primitiveProperty, CodeGenerationTools code)

```
    { 
```

        MetadataTools ef = new MetadataTools(this);

```
#> 
```
```
    /// <summary> 
```

    /// <#=SummaryComment(primitiveProperty)#>

    /// </summary><#=LongDescriptionCommentElement(primitiveProperty, 1)#>

    [EdmScalarPropertyAttribute(EntityKeyProperty=<#=code.CreateLiteral(ef.IsKey(primitiveProperty))#>,

IsNullable=<#=code.CreateLiteral(ef.IsNullable(primitiveProperty))#>)]

```
    [DataMemberAttribute()] 
```

    <#=code.SpaceAfter(NewModifier(primitiveProperty))#><#=Accessibility.ForProperty(primitiveProperty)#> virtual

 <#=code.Escape(primitiveProperty.TypeUsage)#> <#=code.Escape(primitiveProperty)#>

```
    { 
```

        <#=code.SpaceAfter(Accessibility.ForGetter(primitiveProperty))#>get

```
        { 
```

<#+             if (ef.ClrType(primitiveProperty.TypeUsage) == typeof(byte[]))

```
                { 
```
```
#> 
```

            return StructuralObject.GetValidValue(<#=code.FieldName(primitiveProperty)#>);

隐藏行号复制代码？ WriteComplexTypeProperty

    private void WriteComplexTypeProperty(EdmProperty complexProperty, CodeGenerationTools code)

```
    { 
```
```
#> 
```
```
    /// <summary> 
```

    /// <#=SummaryComment(complexProperty)#>

    /// </summary><#=LongDescriptionCommentElement(complexProperty, 1)#>

```
    [EdmComplexPropertyAttribute()] 
```

    [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]

```
    [XmlElement(IsNullable=true)] 
```
```
    [SoapElement(IsNullable=true)] 
```
```
    [DataMemberAttribute()] 
```

    <#=code.SpaceAfter(NewModifier(complexProperty))#><#=Accessibility.ForProperty(complexProperty)#> virtual

<#=MultiSchemaEscape(complexProperty.TypeUsage, code)#><#=code.Escape(complexProperty)#>

```
    { 
```

        <#=code.SpaceAfter(Accessibility.ForGetter(complexProperty))#>get

```
        { 
```

            <#=code.FieldName(complexProperty)#> = GetValidValue(<#=code.FieldName(complexProperty)#>,

```
"<#=complexProperty.Name#>", 
```

false, <#=InitializedTrackingField(complexProperty, code)#>);

            <#=InitializedTrackingField(complexProperty, code)#> = true;

始终坚持一个原则：不要在生成的代码上直接修改，一是工作量巨大，另一方面就是，代码是自动生成的，今后模型修改了，代码将会重新生成。