DDD简明入门之道 - 开篇
犹豫了很久才写下此文，一怕自己对DDD的理解和实践方式有偏差，二怕误人子弟被贻笑大方，所以纰漏之处还望各位谅解。不啰嗦，马上进入正题，如果你觉得此文不错就点个赞吧。
概述
“Domain-Driven Design领域驱动设计”简称DDD，是一套综合软件系统分析和设计的面向对象建模方法。关于DDD的学习资料园子里面有很多，大家可以自行参考，这里不过多介绍。
核心
DDD的核心是领域对象的建模，说白了就是怎么样从业务需求中抽象出我们需要的数据结构，通过这些数据结构之间的相互作用来实现我们的业务功能。这里的所说的数据结构是广义的，Domain里面的每一个类其实就是一个数据结构。这里说的有点抽象了，接下来我们将通过一个具体业务需求的开发来展开。
案例
假设需要开发一个电商平台，我们把平台按功能拆分成多个子系统，子系统之间以微服务形式进行交互调用。拆分后的子系统大致如下：
产品系统(PMS)
订单系统(OMS)
交易系统(TMS)
发货系统(DMS)
其他系统...
而你将会负责订单系统的开发工作，订单系统需要支撑的业务包括用户下单、支付、平台发货、用户确认收货、用户取消订单等业务场景，下面我们就围绕这些场景来对订单业务进行建模。
订单建模
//订单信息
public class Order
{
public int Id{get;set;}
public string OrderNo{get;set;}
public OrderStatus Status{get;set;}
public Address Address{get;set;}
public List<OrderLine> Lines{get;set;}
public decimal ShippingFee{get;set;}
public decimal Discount{get;set;}
public decimal GoodsTotal{get;set;}
public decimal DueAmount{get;set;}
}
//订单状态
public enum OrderStatus
{
PendingPayment = 0,
PendingShipment = 10,
PendingReceive = 20,
Received = 30,
Cancel = 40
}
//地址
public class Address
{
public string FullName{get;set;}
public string FullAddress{get;set;}
public string Tel{get;set;}
}
OrderLine.cs
//订单明细
public class OrderLine
{
public int Id{get;set;}
public int SkuId{get;set;}
public string SkuName{get;set;}
public string Spec{get;set;}
public int Qty{get;set;}
public decimal Cost{get;set;}
public decimal Price{get;set;}
public decimal Total{get;set;}
}
Txn.cs
//交易信息
public class Txn
{
....
}
Shipment.cs
//发货信息
public class Shipment
{
....
}
模型改进
类似上面的模型我们在传统的三层中经常使用，模型中只包含简单的业务属性，这些业务属性的赋值将会在服务层中去进行。这些模型只是用来装数据的壳子，或者叫做容器，完全就是为了和数据库表建立对应关系而存在的。还记得DataTable时代吗？我们完全可以连上面这些模型都不要也是一样可以操作数据库表的。
Class 不等于 OO
给模型赋予行为
深度面向对象编程
/// <summary>
/// 订单信息
/// </summary>
public class Order
{
private List<OrderLine> _lines;
public Order()
{
_lines = new List<OrderLine>();
}
/// <summary>
/// 创建订单（简单工厂）
/// </summary>
/// <param></param>
/// <param></param>
/// <param></param>
/// <returns></returns>
public static Order Create(string orderNo, Address address, SaleSkuInfo[] skus)
{
Order order = new Order();
order.OrderNo = orderNo;
order.Address = address;
order.Status = OrderStatus.PendingPayment;
foreach(var sku in skus)
{
order.AddLine(sku.Id,sku.Qty);
}
order.CalculateFee();
return order;
}
/// <summary>
/// Id
/// </summary>
public int Id{get; private set;}
/// <summary>
/// 订单号
/// </summary>
public string OrderNo{get; private set;}
/// <summary>
/// 订单状态
/// </summary>
public OrderStatus Status{get; private set;}
/// <summary>
/// 收货地址
/// </summary>
public Address Address{get; private set;}
/// <summary>
/// 订单明细
/// </summary>
public List<OrderLine> Lines
{
get{return this._lines;}
private set { this._lines = value; }
}
/// <summary>
/// 运费
/// </summary>
public decimal ShippingFee { get; private set; }
/// <summary>
/// 折扣金额
/// </summary>
public decimal Discount{ get; private set; }
/// <summary>
/// 商品总价值
/// </summary>
public decimal GoodsTotal { get; private set; }
/// <summary>
/// 应付金额
/// </summary>
public decimal DueAmount { get; private set; }
/// <summary>
/// 实付金额
/// </summary>
public decimal ActAmount { get; private set; }
/// <summary>
/// 添加明细
/// </summary>
/// <param></param>
/// <param></param>
public void AddLine(int skuId, int qty)
{
var product = ServiceProxy.ProductService.GetProduct(new GetProductRequest{SkuId = skuId});
if(product == null)
{
throw new SkuNotFindException(skuId);
}
OrderLine line = new OrderLine(skuId, product.SkuName, product.Spec, qty, product.Cost, product.Price);
this._lines.Add(line);
}
/// <summary>
/// 订单费用计算
/// </summary>
public void CalculateFee()
{
this.CalculateGoodsTotal();
this.CalculateShippingFee();
this.CalculateDiscount();
this.CalculateDueAmount();
}
/// <summary>
/// 订单支付
/// </summary>
/// <param></param>
public void Pay(decimal money)
{
if (money <= 0)
{
throw new ArgumentException("支付金额必须大于0");
}
this.ActAmount += money;
if (this.ActAmount >= this.DueAmount)
{
if (this.Status == OrderStatus.PendingPayment)
{
this.Status = OrderStatus.PendingShipment;
}
}
}
/// <summary>
/// 计算运费
/// </summary>
private decimal CalculateShippingFee()
{
//够买商品总价值小于100则收取8元运费
this.ShippingFee = this.CalculateGoodsTotal() > 100 ? 0 : 8m;
return this.ShippingFee;
}
/// <summary>
/// 计算折扣
/// </summary>
private decimal CalculateDiscount()
{
this.Discount = decimal.Zero; //todo zhangsan 暂未实现
return this.Discount;
}
/// <summary>
/// 计算商品总价值
/// </summary>
private decimal CalculateGoodsTotal()
{
this.GoodsTotal = this.Lines.Sum(line => line.CalculateTotal());
return this.GoodsTotal;
}
/// <summary>
/// 计算应付金额
/// </summary>
/// <returns></returns>
private decimal CalculateDueAmount()
{
this.DueAmount = this.CalculateGoodsTotal() + CalculateShippingFee() - CalculateDiscount();
return this.DueAmount;
}
}
