我正在尝试在 C# 上实现观察者模式，想知道我的做法是否正确？

Question

我对观察者模式很陌生，认为它在某些情况下很有帮助，所以我写了一个简单的项目，我不确定我是否以正确的方式做了它，任何建议将不胜感激。

class Program
{
public delegate void ProductAddedEventHandler(Object sender, ProductAddedEventArgs e);
public class Products
{
private double subtotal;
public event ProductAddedEventHandler ProductAdded;
public void Add(Product product)
{
ProductModels model = new ProductModels();
IEnumerable<Product> products = model.Products();
IEnumerable<Product> new_Collection = products.Concat(new List<Product>() { product });
Calculate calculate = new Calculate();
this.subtotal = calculate.create(new_Collection);
if (ProductAdded != null)
{
ProductAddedEventArgs e = new ProductAddedEventArgs(subtotal);
ProductAdded(this, e);
}
Console.WriteLine("Subtotal Amount: {0}", subtotal);
}
}
/// <summary>
/// Calculate the Subtotal
/// </summary>
public class Calculate
{
public double create(IEnumerable<Product> products)
{
double result = 0;
foreach (Product p in products)
{
result += p.Price;
}
return result;
}
}
public class ProductAddedEventArgs : EventArgs
{
public readonly double SubTotal;
public ProductAddedEventArgs(double subtotal) => SubTotal = subtotal;
}
public class Shipping
{
const double Shipping_Rate = 0.05;
public void create(Object sender, ProductAddedEventArgs e)
{
Console.WriteLine("Shipping Amount: {0}", e.SubTotal * Shipping_Rate);
}
}
public class Discount
{
const double Discount_Rate = 0.2;
public void create(Object sender, ProductAddedEventArgs e)
{
Console.WriteLine("Discount Amount: {0}", e.SubTotal * Discount_Rate);
}
}
public class Tax
{
const double Tax_Rate = 0.07;
public void create(Object sender, ProductAddedEventArgs e)
{
Console.WriteLine("Tax Amount: {0}", e.SubTotal * Tax_Rate);
}
}
static void Main(string[] args)
{
Products products = new Products();
Shipping shipping = new Shipping();
Discount discount = new Discount();
Tax tax = new Tax();
products.ProductAdded += shipping.create;
products.ProductAdded += discount.create;
products.ProductAdded += tax.create;
products.Add(new Product()
{
Id = 50,
Name = "Watermelon",
Price = 9.21
});
}
}

顺便说一句，如何获得总金额？

Answer 1

虽然这个问题属于 https://codereview.stackexchange.com/但我会尝试写一个简短的答案。

来自DoFactory(GOF(的观察者模式的定义，

观察者模式定义了对象之间的一对多依赖关系，以便在一个对象更改状态时，会自动通知和更新其所有依赖项。

在您的示例中，您没有定义任何观察器，而是有更改对象本身状态的事件。在 Add(( 方法中要做太多的责任或工作。

观察者的例子可以是购物车(电子商务(，例如，当产品计数更改时，订单摘要对象应该知道它并重新计算摘要(税金、折扣、总计等(。请参阅以下示例，了解购物车和订单摘要被更新为购物车项目的观察者。它并不完美，但它会给你一个观察者模式的概念：

class Product
{
// Product properties
}
class ProductCollection
{
private List<Observer> _observers = new List<Observer>();
List<Product> _internalcol = new List<Product>();
public void Attach(Observer observer)
{
_observers.Add(observer);
}
public void Detach(Observer observer)
{
_observers.Remove(observer);
}
public void Add(Product p)
{
_internalcol.Add(p)
Notify();
}
public void Remove(Product p)
{
}
private void Notify()
{
foreach (Observer o in _observers)
{
o.Update();
}
}
}
class Cart
{
public ProductionCollection products = new ProductionCollection();
public OrderSummary summary = new OrderSummary();
public Cart()
{
products.Attach(new OrderSummary(products));
}
public void AddProduct(Product p, int count)
{
if(count > 1){
foreach(int i in count){
products.Add(p);
}
else if(count == 1){
products.Add(p);
}
else if(count == 0)
{
products.Remove(p);
}
}
}
abstract class Observer()
{
abstract void Update();
}
class OrderSummary: Observer
{
ProductionCollection _products;
// Constructor
public OrderSummary(ProductionCollection products)
{
this._products = products;
}
public override void Update()
{
//Recalculate taxes, discount etc. here.
// Update Summary object
}
public void GetSummary()
{
// return/print Summary object;
}
}
class program
{
static void Main(string[] args)
{
Cart c = new Cart();
c.products.AddProduct(
new Product()
{
Id = 50,
Name = "Watermelon",
Price = 9.21
});
}
}

它可能无法编译我在记事本中编写的。