我的 Design Pattern 之旅 [1]：Strategy Pattern (5) - C# Delegate 實現

Strategy Pattern C# Delegate 實現：函數指標與觀察者模式的語言級融合

核心代碼

class Grapher {
    private DrawDelegate _drawDelegate = null;
    public delegate void DrawDelegate();
    public Grapher() {}
    public Grapher(DrawDelegate drawDelegate) { _drawDelegate = drawDelegate; }
    public void drawShape() { if (_drawDelegate != null) _drawDelegate(); }
    public void setShape(DrawDelegate drawDelegate) { _drawDelegate = drawDelegate; }
}

class Triangle { public static void draw() { Console.WriteLine("Draw Triangle"); } }
class Circle { public static void draw() { Console.WriteLine("Draw Circle"); } }
class Square { public static void draw() { Console.WriteLine("Draw Square"); } }

class Program {
    public static void Main() {
        Grapher grapher = new Grapher(Square.draw);
        grapher.drawShape();
        grapher.setShape(Circle.draw);
        grapher.drawShape();
    }
}

執行結果

Draw Square
Draw Circle

Delegate 是 C# 對函數指標與觀察者模式的語言級實現，天然適合 Strategy Pattern。GoF 也展示 Template Method 實現 Strategy。