C# 設計模式：觀察者模式完整指南（三）— Delegate/Event 機制簡化實作詳解

編輯：關於C語言

C# 內建 Delegate 與 Event 機制

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完整結構性代碼


using System;

// 1. 定義委派：規範通知方法簽章
delegate void UpdateDelegate();

// 2. 主題角色
class Subject
{
    public event UpdateDelegate UpdateHandler;

    public void Attach(UpdateDelegate ud) => UpdateHandler += ud;
    public void Detach(UpdateDelegate ud) => UpdateHandler -= ud;

    public void Notify()
    {
        if (UpdateHandler != null) UpdateHandler();
    }
}

// 3. 具體主題
class ConcreteSubject : Subject
{
    private string subjectState;
    public string SubjectState
    {
        get => subjectState;
        set => subjectState = value;
    }
}

// 4. 具體觀察者 A
class ConcreteObserver
{
    private string name;
    private string observerState;
    private ConcreteSubject subject;

    public ConcreteObserver(ConcreteSubject subject, string name)
    {
        this.subject = subject;
        this.name = name;
    }

    public void Update()
    {
        observerState = subject.SubjectState;
        Console.WriteLine($"Observer {name}'s new state is {observerState}");
    }
}

// 5. 另一種觀察者：方法名不同也可訂閱
class AnotherObserver
{
    public void Show() => Console.WriteLine("AnotherObserver got an Notification!");
}

// 6. 客戶端測試
public class Client
{
    public static void Main()
    {
        ConcreteSubject s = new ConcreteSubject();
        ConcreteObserver o1 = new ConcreteObserver(s, "1");
        ConcreteObserver o2 = new ConcreteObserver(s, "2");
        AnotherObserver o3 = new AnotherObserver();

        s.Attach(new UpdateDelegate(o1.Update));
        s.Attach(new UpdateDelegate(o2.Update));
        s.Attach(new UpdateDelegate(o3.Show));  // 關鍵：Show() 也能訂閱！

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

        Console.WriteLine("--------------------------");

        s.Detach(new UpdateDelegate(o1.Update)); // 移除 o1
        s.SubjectState = "DEF";
        s.Notify();
    }
}

執行結果

Observer 1's new state is ABC
Observer 2's new state is ABC
AnotherObserver got an Notification!
--------------------------
Observer 2's new state is DEF
AnotherObserver got an Notification!