.NET 委托演進史話：從緊耦合到異步事件的睡前故事

第一幕：緊耦合 — 硬編碼回調

class Worker {
    public void Advise(Boss boss) { _boss = boss; }
    public void DoWork() {
        Console.WriteLine("工作開始");
        if (_boss != null) _boss.WorkStarted();
        Console.WriteLine("工作中...");
        if (_boss != null) _boss.WorkProgressing();
        Console.WriteLine("工作完成");
        if (_boss != null) {
            int grade = _boss.WorkCompleted();
            Console.WriteLine($"得分：{grade}");
        }
    }
    private Boss _boss;
}

class Boss {
    public void WorkStarted() { }
    public void WorkProgressing() { }
    public int WorkCompleted() { return 2; }
}

缺點：Worker 強依賴 Boss 具體類，無法擴展新監聽者 (如 Universe)。

第二幕：接口解耦 — 統一契約

interface IWorkerEvents {
    void WorkStarted();
    void WorkProgressing();
    int WorkCompleted();
}

class Worker {
    public void Advise(IWorkerEvents events) { _events = events; }
    // ... 調用 _events.xxx()
    private IWorkerEvents _events;
}

class Boss : IWorkerEvents { ... }
class Universe : IWorkerEvents { ... }

缺點：接口粒度粗，Boss 被迫實現不關心的方法 (空實現)，違反接口隔離原則。

第三幕：委托細粒度 — 方法級訂閱

delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted();

class Worker {
    public WorkStarted started;
    public WorkProgressing progressing;
    public WorkCompleted completed;
    public void DoWork() {
        Console.WriteLine("工作開始"); if (started != null) started();
        Console.WriteLine("工作中..."); if (progressing != null) progressing();
        Console.WriteLine("工作完成"); if (completed != null) { int g = completed(); Console.WriteLine($"得分：{g}"); }
    }
}

class Boss { public int WorkCompleted() { return 4; } }

class Universe {
    static void WorkerStartedWork() { Console.WriteLine("Universe 注意到..."); }
    static int WorkerCompletedWork() { return 7; }
    static void Main() {
        var p = new Worker();
        p.completed = new WorkCompleted(new Boss().WorkCompleted);
        p.started = new WorkStarted(Universe.WorkerStartedWork);
        p.completed = new WorkCompleted(Universe.WorkerCompletedWork); // 覆蓋 Boss!
        p.DoWork();
    }
}

缺點：public 委托字段可被任意覆蓋/直接調用 (破壞封裝)，無法多播。

第四幕：事件封裝 — 安全的多播

class Worker {
    public event WorkStarted started;
    public event WorkProgressing progressing;
    public event WorkCompleted completed;
    // DoWork 內部調用不變
}

// 使用 += / -= 訂閱/退訂，不可直接賦值或調用
p.completed += new WorkCompleted(boss.WorkCompleted);
p.started += new WorkStarted(Universe.WorkerStartedWork);

event 關鍵字編譯器生成 `add_XXX`/`remove_XXX` 存取器，僅暴露 `+=`/`-=`。

第五幕：收獲所有結果 — GetInvocationList

if (completed != null) {
    foreach (WorkCompleted wc in completed.GetInvocationList()) {
        int grade = wc();
        Console.WriteLine($"得分：{grade}");
    }
}

第六幕：異步通知三部曲

1. Fire-and-Forget (BeginInvoke 無回調)

foreach (var wc in completed.GetInvocationList())
    wc.BeginInvoke(null, null); // 線程池執行，主線程立即返回

2. 輪詢等待 (BeginInvoke + IsCompleted)

foreach (var wc in completed.GetInvocationList()) {
    var ar = wc.BeginInvoke(null, null);
    while (!ar.IsCompleted) Thread.Sleep(1);
    int grade = wc.EndInvoke(ar);
    Console.WriteLine($"得分：{grade}");
}

3. 異步回調模式 (BeginInvoke + AsyncCallback)

foreach (var wc in completed.GetInvocationList())
    wc.BeginInvoke(new AsyncCallback(WorkGraded), wc);

private void WorkGraded(IAsyncResult ar) {
    var wc = (WorkCompleted)ar.AsyncState;
    int grade = wc.EndInvoke(ar);
    Console.WriteLine($"得分：{grade}");
}

主線程完全解放，完成時自動回調處理結果。

結局：各得其所

• Boss/Universe：只關心感興趣的事件，無空實現負擔
• Worker：異步通知不阻塞，結果通過回調收集
• 架構：鬆耦合、可擴展、線程安全 (需注意跨線程調用 UI)

「他們永遠幸福地生活下去……直到下一個重構需求到來。」