HOW TO：在衍生類別中引發基底類別事件 (C# 程式設計手冊)
下列的簡單範例會示範在基底類別 (Base Class) 內宣告事件的標準方式，讓事件也可以從衍生類別 (Derived Class) 中引發。 這個模式會在 .NET Framework 類別庫 (Class Library) 的 Windows Form 類別中廣泛使用。
當您建立可當做其他類別之基底類別的類別時，您應該考量一項事實：事件是一種委派 (Delegate) 的特殊型別，只能從宣告事件的類別中予以叫用 (Invoke)。 衍生類別無法直接叫用在基底類別中宣告的事件。 雖然有時您可能會想要使用只能由基底類別所引發的事件，但是在多數情形下，您應該啟用衍生類別來叫用基底類別事件。 若要這樣做，您可以在包裝事件的基底類別內建立保護的 (Protected) 的叫用方法； 藉由呼叫或覆寫這個叫用方法，衍生類別便能夠間接地叫用該事件。
.gif "注意事項") 注意事項 |
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請勿在基底類別中宣告虛擬事件，以及進而在衍生類別中覆寫它們。C#編譯器會控制代碼這些正確而且衍生的事件訂閱者 」 實際上訂閱基底類別事件是否發生無法預期。 |
範例
namespace BaseClassEvents
{
using System;
using System.Collections.Generic;
// 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; }
}
}
// Base class event publisher
public abstract class Shape
{
protected double area;
public double Area
{
get { return area; }
set { area = value; }
}
// The event. Note that by using the generic EventHandler<T> event type
// we do not need to declare a separate delegate type.
public event EventHandler<ShapeEventArgs> ShapeChanged;
public abstract void Draw();
//The event-invoking method that derived classes can override.
protected virtual void OnShapeChanged(ShapeEventArgs e)
{
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<ShapeEventArgs> handler = ShapeChanged;
if (handler != null)
{
handler(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)
{
// Do any circle-specific processing here.
// Call the base class event invocation method.
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)
{
// Do any rectangle-specific processing here.
// Call the base class event invocation method.
base.OnShapeChanged(e);
}
public override void Draw()
{
Console.WriteLine("Drawing a rectangle");
}
}
// Represents the surface on which the shapes are drawn
// Subscribes 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;
}
// ...Other methods to draw, resize, etc.
private void HandleShapeChanged(object sender, ShapeEventArgs e)
{
Shape s = (Shape)sender;
// Diagnostic message for demonstration purposes.
Console.WriteLine("Received event. Shape area is now {0}", e.NewArea);
// Redraw the shape here.
s.Draw();
}
}
class Test
{
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);
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
}
/* Output:
Received event. Shape area is now 10201.86
Drawing a circle
Received event. Shape area is now 49
Drawing a rectangle
*/