在派生類中引發基類事件

在基類中聲明可以從派生類引發的事件的標準方法。此模式廣泛應用於.Net Framework類庫中的Windows窗體類。

我們來看一個Button類的定義

    public class Button : ButtonBase, IButtonControl
    {
      protected override void OnClick(EventArgs e); //override基類的事件觸發程式
    }

我們通過ButtonBase最後找到基類Control

public class Control : Component, IDropTarget, ISynchronizeInvoke, IWin32Window, IArrangedElement, IBindableComponent, IComponent, IDisposable
{
    public event EventHandler Click;   //基類中定義事件
    protected virtual void OnClick(EventArgs e);//protected封裝事件觸發程式，允許繼承類調用或重寫
}

在包含事件的基類中創建一個受保護的調用方法。通過調用或重寫方法，派生類便可以間接調用該事件。

我們來看一個例子：

namespace BaseClassEvents
{
    
    //Special EventArgs class to hold info about Shapes.
    public class ShapeEventArgs:EventArgs
    {
        private double newArea;
        public ShapeEventArgs (double d)
        {
            newArea = d;
        }
        public double NewArea
        {
            get
            {
                return newArea;
            }
        }
    }
    //Declare a delegate
    public delegate void CustomEventHandler(object sender, ShapeEventArgs e);

    //Base class event publisher
    public abstract class Shape
    {
        protected double area;
        public double Area
        {
            get
            {
                return area;
            }
            set
            {
                area = value;
            }
        }
        //raise an event
        public event CustomEventHandler ShapeChanged;

        public abstract void Draw();

        //event-invoking method
        protected virtual void OnShapeChanged(ShapeEventArgs e)
        {
            if(ShapeChanged !=null)
            {
                ShapeChanged(this, e);
            }
        }
    }
    public class Circle:Shape
    {
        private double radius;
        public Circle (double d)
        {
            radius = d;
            area = 3.14 * radius * radius;
        }
        public void Update(double d)
        {
            radius = d;
            area = 3.14 * radius * radius;
            OnShapeChanged(new ShapeEventArgs(area));
        }
        protected override void OnShapeChanged(ShapeEventArgs e)
        {
            base.OnShapeChanged(e);
        }
        public override void Draw()
        {
            Console.WriteLine("Drawing a circle");
        }
    }
    public class Rectangle:Shape
    {
        private double length;
        private double width;
        public Rectangle (double length,double width)
        {
            this.length = length;
            this.width = width;
            area = length * width;
        }
        public void Update(double length,double width)
        {
            this.length = length;
            this.width = width;
            area = length * width;
            OnShapeChanged(new ShapeEventArgs(area));
        }
        protected override void OnShapeChanged(ShapeEventArgs e)
        {
            base.OnShapeChanged(e);
        }
        public override void Draw()
        {
            Console.WriteLine("Drawing a rectangle");
        }
    }

    //Subscriber
    //Represents the surface on which the shapes are drawn
    //Subscibes to shape events so that it knows
    //when to redraw a shape
    public class ShapeContainer
    {
        List<Shape> _list;
        public ShapeContainer ()
        {
            _list = new List<Shape>();
        }
        public void AddShape(Shape s)
        {
            _list.Add(s);
            //Subscribe to the base class event.
            s.ShapeChanged += HandleShapeChanged;
        }
        private void HandleShapeChanged(object sender,ShapeEventArgs e)
        {
            Shape s = (Shape)sender;
            Console.WriteLine("Received event. Shape area is now {0}", e.NewArea);
            s.Draw();
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            //Create the event publishers and subscriber
            Circle c1 = new Circle(54);
            Rectangle r1 = new Rectangle(12, 9);
            ShapeContainer sc = new ShapeContainer();

            //Add the shapes to the container
            sc.AddShape(c1);
            sc.AddShape(r1);

            //Cause some events to be raised
            c1.Update(57);
            r1.Update(7, 7);

            Console.ReadKey();
        }
    }
}