C# 設計模式：觀察者模式完整指南（四）— Delegate/Event 簡化實作與股票通知實戰

C# Delegate 與 Event 簡化觀察者模式

.NET 內建機制可大幅簡化傳統介面式觀察者實作。

關鍵代碼解析

// 1. 定義委派規範方法簽章
delegate void UpdateDelegate();

// 2. 主題類別定義事件
public event UpdateDelegate UpdateHandler;

// 3. 訂閱事件
public void Attach(UpdateDelegate ud) => UpdateHandler += ud;
public void Detach(UpdateDelegate ud) => UpdateHandler -= ud;

// 4. 觸發通知
public void Notify()
{
    if (UpdateHandler != null) UpdateHandler();
}

使用範例

ConcreteSubject s = new ConcreteSubject();
ConcreteObserver o1 = new ConcreteObserver(s, "1");
ConcreteObserver o2 = new ConcreteObserver(s, "2");
AnotherObserver o3 = new AnotherObserver();

// 訂閱：任何符合 UpdateDelegate 簽章的方法皆可
s.Attach(new UpdateDelegate(o1.Update));
s.Attach(new UpdateDelegate(o2.Update));
s.Attach(new UpdateDelegate(o3.Show));  // 方法名可不同！

s.SubjectState = "ABC";
s.Notify();

s.Detach(new UpdateDelegate(o1.Update)); // 取消訂閱
s.SubjectState = "DEF";
s.Notify();

優勢對比

實戰：股票價格變動通知投資人

// 傳統介面式實作（留給讀者練習轉換為 Event 版本）
abstract class Stock
{
    protected string symbol;
    protected double price;
    private ArrayList investors = new ArrayList();

    public Stock(string symbol, double price) { this.symbol = symbol; this.price = price; }

    public void Attach(Investor investor) => investors.Add(investor);
    public void Detach(Investor investor) => investors.Remove(investor);

    public void Notify()
    {
        foreach (Investor i in investors) i.Update(this);
    }

    public double Price
    {
        get => price;
        set { price = value; Notify(); } // 價格變動自動通知
    }
}

class IBM : Stock { public IBM(string s, double p) : base(s, p) { } }

interface IInvestor { void Update(Stock stock); }

class Investor : IInvestor
{
    private string name;
    public Investor(string name) => this.name = name;
    public void Update(Stock stock)
    {
        Console.WriteLine($"Notified investor {name} of {stock.Symbol}'s change to {stock.Price:C}");
    }
}

// 測試
public class ObserverApp
{
    public static void Main()
    {
        Investor s = new Investor("Sorros");
        Investor b = new Investor("Berkshire");
        IBM ibm = new IBM("IBM", 120.00);
        ibm.Attach(s); ibm.Attach(b);
        ibm.Price = 120.10; ibm.Price = 121.00;
        ibm.Price = 120.50; ibm.Price = 120.75;
    }
}

輸出結果

Notified investor Sorros of IBM's change to $120.10
Notified investor Berkshire of IBM's change to $120.10
Notified investor Sorros of IBM's change to $121.00
Notified investor Berkshire of IBM's change to $121.00
Notified investor Sorros of IBM's change to $120.50
Notified investor Berkshire of IBM's change to $120.50
Notified investor Sorros of IBM's change to $120.75
Notified investor Berkshire of IBM's change to $120.75