C#——如何在派生类中引发基类事件

今天药忘吃喽~ 2023-01-03 01:47 161阅读 0赞

## C\#——如何在派生类中引发基类事件 ##

下面的简单示例演示用于在基类中声明事件，以便也可以从派生类引发它们的标准方法。 此模式广泛用于 .NET 类库中的 Windows 窗体类。

创建可以用作其他类的基类的类时，应考虑到以下事实：事件是特殊类型的委托，只能从声明它们的类中进行调用。 派生类不能直接调用在基类中声明的事件。 虽然有时可能需要只能由基类引发的事件，不过在大多数情况下，应使派生类可以调用基类事件。 为此，可以在包装事件的基类中创建受保护的调用方法。 通过调用或重写此调用方法，派生类可以间接调用事件。

不要在基类中声明虚拟事件并在派生类中重写它们。 C\# 编译器不会处理这些事件，并且无法预知派生事件的订阅者是否实际上会订阅基类事件。

namespace BaseClassEvents
    {
        
        // Special EventArgs class to hold info about Shapes.
        public class ShapeEventArgs : EventArgs
        {
        
            public ShapeEventArgs(double area)
            {
        
                NewArea = area;
            }
    
            public double NewArea {
         get; }
        }
    
        // Base class event publisher
        public abstract class Shape
        {
        
            protected double _area;
    
            public double Area
            {
        
                get => _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)
            {
        
                // Safely raise the event for all subscribers
                ShapeChanged?.Invoke(this, e);
            }
        }
    
        public class Circle : Shape
        {
        
            private double _radius;
    
            public Circle(double radius)
            {
        
                _radius = radius;
                _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)
            {
        
                _length = length;
                _width = width;
                _area = _length * _width;
            }
    
            public void Update(double length, double width)
            {
        
                _length = length;
                _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
        {
        
            private readonly List<Shape> _list;
    
            public ShapeContainer()
            {
        
                _list = new List<Shape>();
            }
    
            public void AddShape(Shape shape)
            {
        
                _list.Add(shape);
    
                // Subscribe to the base class event.
                shape.ShapeChanged += HandleShapeChanged;
            }
    
            // ...Other methods to draw, resize, etc.
    
            private void HandleShapeChanged(object sender, ShapeEventArgs e)
            {
        
                if (sender is Shape shape)
                {
        
                    // Diagnostic message for demonstration purposes.
                    Console.WriteLine($"Received event. Shape area is now {e.NewArea}");
    
                    // Redraw the shape here.
                    shape.Draw();
                }
            }
        }
    
        class Test
        {
        
            static void Main()
            {
        
                //Create the event publishers and subscriber
                var circle = new Circle(54);
                var rectangle = new Rectangle(12, 9);
                var container = new ShapeContainer();
    
                // Add the shapes to the container.
                container.AddShape(circle);
                container.AddShape(rectangle);
    
                // Cause some events to be raised.
                circle.Update(57);
                rectangle.Update(7, 7);
    
                // Keep the console window open in debug mode.
                Console.WriteLine("Press any key to continue...");
                Console.ReadKey();
            }
        }
    }
    /* Output:
            Received event. Shape area is now 10201.86
            Drawing a circle
            Received event. Shape area is now 49
            Drawing a rectangle
     */