为延迟加载注入数据访问依赖项的正确方法是什么？

Question

在进行延迟加载时,注入数据访问依赖项的正确方法是什么？

例如,我有以下类结构

class CustomerDao : ICustomerDao
  public Customer GetById(int id) {...}

class Transaction {
  int customer_id; //Transaction always knows this value
  Customer _customer = null;
  ICustomerDao _customer_dao;
  Customer GetCustomer() {
    if(_customer == null)
      _customer = _customer_dao.GetById(_customer_id);
    return _customer
  }

如何将_customer_dao引用到事务对象中？如果我希望事务至少看起来像POCO,那么对构造函数的要求似乎就没有意义.是否可以让Transaction对象直接引用Inversion of Control Container？这也似乎很尴尬.

像NHibernate这样的框架如何处理这个？

Answer 1

我建议不同的东西...使用延迟加载类:

public class Lazy<T>
{
   T value;
   Func<T> loader;

   public Lazy(T value) { this.value = value; }
   public Lazy(Func<T> loader { this.loader = loader; }

   T Value
   {
     get 
    {
       if (loader != null)
       {
         value = loader();
         loader = null;
       }

       return value;
    }

    public static implicit operator T(Lazy<T> lazy)
    {
        return lazy.Value;
    }

    public static implicit operator Lazy<T>(T value)
    {
        return new Lazy<T>(value);
    }
}

一旦你得到它,你就不需要在你的对象中注入dao了:

public class Transaction
{
    private static readonly Lazy<Customer> customer;

    public Transaction(Lazy<Customer> customer)
    {
      this.customer = customer;
    }

    public Customer Customer
    {
       get { return customer; } // implicit cast happen here
    }
}

创建未绑定到数据库的Transcation对象时:

new Transaction(new Customer(..)) // implicite cast 
                                  //from Customer to Lazy<Customer>..

从存储库中的数据库重新生成事务时:

public Transaction GetTransaction(Guid id)
{
   custmerId = ... // find the customer id 
   return new Transaction(() => dao.GetCustomer(customerId));
}

发生了两件有趣的事情: - 您的域对象可以在有或没有数据访问的情况下使用,它会变得无知的数据访问.唯一的小麻烦就是能够传递给对象而不是对象本身的函数. - Lazy类是内部可变的,但可以用作不可变值.readonly关键字保持其语义,因为其内容不能在外部更改.

如果希望字段可写,只需删除readonly关键字即可.在分配新值时,由于隐式转换,将使用新值创建新的Lazy.

编辑:我在这里写博客:

http://www.thinkbeforecoding.com/post/2009/02/07/Lazy-load-and-persistence-ignorance

Answer 2

我通常会像上面那样在构造函数中进行依赖项注入，但仅在调用“get”时才进行延迟加载，如下所示。不确定这是否是您正在寻找的纯粹方法，但它确实一步消除了“脏”构造函数 DI/延迟加载；）

public class Product
{
    private int mProductID;
    private Supplier mSupplier;
    private ISupplierService mSupplierService;

    public Product()
    {
      //if you want your object to remain POCO you can use dual constr
      //this constr will be for app use, the next will be for testing
    } 

    public Product(ISupplierService SupplierService)
    {
        mSupplierService = SupplierService;
    }

    public Supplier Supplier {
        get {
            if (mSupplier == null) {
                if (mSupplierService == null) {
                    mSupplierService = new SupplierService();
                }
                mSupplier = mSupplierService.GetSupplierByProductID(mProductID);
            }
            return mSupplier;
        }
        set { mSupplier = value; }
    }
}