我们说我有一个 StockMarket 类，其中包含公司的列表。
class StockMarket : IStock
{
private static List<IObserverPush> observersPush;
private static List<IObserverPull> observersPull;
public static List<Company> Companies { get; private set; }
public StockMarket()
{
observersPush = new List<IObserverPush>();
observersPull = new List<IObserverPull>();
Companies = new List<Company>() { new Company("Unilever", "UNA", 47.72, 0.77, 1.63, -3.45, "135B"),
new Company("ING Groep", "INGA", 13.40, -0.07, -0.50, -12.38, "60.4B"),
new Company("ArcelorMittal", "MT", 29.50, 0.14, 0.48, 36.05, "54.6B"),
new Company("ASML Holding", "ASML", 167.40, 2.00, 1.21, 36.49, "53.3B"),
new Company("Heineken", "HEIA", 87.66, -0.02, -0.02, 2.80, "49B"),
new Company("RELX", "REN", 18.15, 0.17, 0.95, -0.22, "38.9B"),
new Company("Philips", "PHIA", 35.49, 0.17, 0.47, 7.61, "33.3B"),
new Company("Unibail Rodamco", "UL", 196.40, -0.15, -0.08, -16.78, "20.3B"),
new Company("Akzo Nobel", "AKZA", 75.68, -0.16, -0.21, 0.33, "19.4B"),
new Company("Altice", "ATC", 7.58, 0.16, 2.16, -66.30, "17.6B")};
Thread thread = new Thread(SimulateMarket);
thread.Start();
}
public void Subscribe(IObserverPull o)
{
observersPull.Add(o);
o.UpdateMarket();
}
public void Unsubscribe(IObserverPull o)
{
observersPull.Remove(o);
}
public void Subscribe(IObserverPush o)
{
observersPush.Add(o);
o.UpdateMarket(Companies);
}
public void Unsubscribe(IObserverPush o)
{
observersPush.Remove(o);
}
public void NotifyObservers()
{
foreach(IObserverPush o in observersPush)
{
o.UpdateMarket(Companies);
}
foreach(IObserverPull o in observersPull)
{
o.UpdateMarket();
}
}
public void SimulateMarket()
{
while(observersPush.Count + observersPull.Count > 0)
{
//randomly change property values of companies
//and notify the observers about the changes
}
}
}
公司类有一些属性。
public class Company
{
public string Name { get; private set; }
public string Symbol { get; private set; }
public double Price { get; set; }
public double Change { get; set; }
public double ChangePercentageDay { get; set; }
public double ChangePercentageYear { get; set; }
public string Capital { get; private set; }
public Company(string name, string symbol, double price, double change, double changePercentageDay,
double changePercentageYear, string capital)
{
Name = name;
Symbol = symbol;
Price = price;
Change = change;
ChangePercentageDay = changePercentageDay;
ChangePercentageYear = changePercentageYear;
Capital = capital;
}
}
表单引用了 StockMarket ，他们使用它来检索有关公司的数据并进行显示。
表单1
public partial class ConcreteObserverPush : Form, IObserverPush
{
private StockMarket stockMarket;
public ConcreteObserverPush()
{
InitializeComponent();
stockMarket = new StockMarket();
stockMarket.Subscribe(this);
}
public void UpdateMarket(List<Company> companies)
{
stockMarketListView.Items.Clear();
foreach(Company c in companies)
{
ListViewItem item = new ListViewItem(c.Symbol);
item.SubItems.Add(c.Price.ToString());
item.SubItems.Add(c.Change.ToString());
item.SubItems.Add(c.ChangePercentageDay.ToString() + "%");
stockMarketListView.Items.Add(item);
}
}
private void ConcreteObserverPush_FormClosing(object sender, FormClosingEventArgs e)
{
stockMarket.Unsubscribe(this);
}
}
表格2
public partial class ConcreteObserverPull : Form, IObserverPull
{
private StockMarket stockMarket;
public ConcreteObserverPull()
{
InitializeComponent();
stockMarket = new StockMarket();
stockMarket.Subscribe(this);
}
public void UpdateMarket()
{
stockMarketListView.Items.Clear();
foreach (Company c in StockMarket.Companies)
{
ListViewItem item = new ListViewItem(c.Symbol);
item.SubItems.Add(c.Name);
item.SubItems.Add(c.Price.ToString());
item.SubItems.Add(c.Change.ToString());
item.SubItems.Add(c.ChangePercentageDay.ToString() + "%");
item.SubItems.Add(c.ChangePercentageYear.ToString() + "%");
item.SubItems.Add(c.Capital);
stockMarketListView.Items.Add(item);
}
}
private void ConcreteObserverPull_FormClosing(object sender, FormClosingEventArgs e)
{
stockMarket.Unsubscribe(this);
}
}
问题在于，如果表单通过 StockMarket 上的属性获取公司列表，则可以更改其状态。但是，我只希望StockMarket能够改变公司的状态。
那么在请求时使用表单分享公司状态的最佳方式是什么，并阻止表单进行修改。
我知道可能的解决方案是返回公司对象的克隆，但我相信应该有更好的解决方案。
感谢任何帮助！
答案 0 :(得分：1)
这样做的一般要点是使Company对象不可变。然后，您可以向StockMarket对象添加方法以操作公司列表，并在您想要更改值时将条目替换为新条目。
这是LINQPad中一个快速示例，它使Company类不可变，并向UpdatePrice类添加StockMarket方法。
是否希望能够从Companies之外操作StockMarket属性，可以通过将列表返回为ReadOnlyCollection来处理，以便无法通过操作来处理它的大小消费者。
void Main()
{
var sm = new StockMarket();
sm.Companies.Add(new Company("Test", "TST", 50, 0));
sm.UpdatePrice("Test", 45);
var testCompany = sm.Companies.First(x => x.Name == "Test");
Console.WriteLine($"{testCompany.Name},{testCompany.Symbol},{testCompany.Price},{testCompany.Change}");
//Output: Test,TST,45,-5
}
class StockMarket
{
public List<Company> Companies { get; private set; } = new List<Company>();
public void UpdatePrice(string name, double price) {
var index = Companies.FindIndex(x => x.Name == name);
if(index >= 0)
{
var previous = Companies[index];
Companies[index] = new Company(previous.Name, previous.Symbol, price, price - previous.Price);
}
}
}
class Company
{
public Company(string name, string symbol, double price, double change) {
Name = name;
Symbol = symbol;
Price = price;
Change = change;
}
public string Name { get; }
public string Symbol { get; }
public double Price { get; }
public double Change { get; }
///...
}
答案 1 :(得分：1)
这将是一个解决方案：
在StockMarket类中创建Company类作为私有内部类，这样它只能在其内部访问，然后提供仅包含所有属性的获取并使公司实现的接口它。您必须使StockMarket公司列表成为界面类型。
您必须通过将界面的List对象强制转换为原始类类型来进行任何修改。
示例：
class Program
{
public static StockMarket stockMarket = new StockMarket();
static void Main(string[] args)
{
}
}
public interface ICompany
{
string Name { get; }
}
public class StockMarket
{
public StockMarket()
{
Companies = SomeWildFunctionThatRetrievesAllCompanies();
}
public void OneWildFunctionThatModifiesACompany()
{
Company dunno = (Company)Companies[0];
dunno.Name = "Modification Made Possible";
}
private List<ICompany> SomeWildFunctionThatRetrievesAllCompanies()
{
return new List<ICompany>(new List<Company>());
}
public List<ICompany> Companies { get; private set; }
private class Company : ICompany
{
public string Name { get; set; }
}
}
答案 2 :(得分：0)
试试这个：
class Company
{
public Company(Type type,string name,string symbol,double price, double change)
{
if (type.Name == "StockMarket")
{
Name = name;
Symbol = symbol;
Price = price;
Change = change;
}
}
private string Name { get; set; }
private string Symbol { get; set; }
private double Price { get; set; }
private double Change { get; set; }
///...
}
这将允许您仅在类型为StockMarket时更改状态 像：
class StockMarket
{
public List<Company> Companies { get; set; }
public StockMarket()
{
Companies = new List<Company>();
}
public StockMarket someMethod()
{
//You can change the state here
StockMarket s = new StockMarket();
s.Companies.Add(new Company(this.GetType(), "aa", "_", 123, 1234));
return s;
}
//...
}
现在你无法改变这里的状态：
public partial class Observer: Form
{
private StockMarket stockMarket;
public ConcreteObserverPull()
{
InitializeComponent();
stockMarket = new StockMarket();
//Here you cannot change the state
stockMarket.Companies.Add(new Company(this.GetType(), "aa", "_", 123,12));
}
//...
}
答案 3 :(得分：-1)
抱歉，我不知道C＃，但是作为一个想法，你可以用装饰器或代理包装返回的实体，这会在尝试修改公司状态时抛出异常。
答案 4 :(得分：-3)
返回字段设置为只读的克隆是最安全的方法。