如何在VB.NET中编写事件处理程序？

Question

我想在VB.NET中测试非常简单的事件处理。

到目前为止，我有：

Public Delegate Sub TestEventDelegate() 
Public Event TestEvent As TestEventDelegate 

Sub MySub 
    Raise TestEvent 
End Sub 

你会如何写，只是显示一个简单的MessageBox上述事件的事件处理程序？

Answer 1

写处理程序的方法是简单的 - 只写一个Sub其中不带参数，显示一个消息框。

然后您需要处理程序订阅的事件，你可以不添加Handles子句的方法：

Sub ShowMessageBox() Handles foo.TestEvent 

，或者通过使用AddHandler声明：

AddHandler foo.TestEvent, AddressOf ShowMessageBox 

注为了遵循.NET约定，您的委托应该有两个参数 - Object类型之一来指定哪个对象引发事件，以及一个类型为EventArgs或一个子类，以提供任何额外的信息。这不是要求的语言，但它是一个广泛遵循的惯例。

Answer 2

在VB中，我们有两种方法来订阅Publisher类的事件。

'Delegate 
Public Delegate Sub TestEventDelegate() 

'Event publisher class that publishes and raises an event 

Public Class EventPublisher 
    Private _num As Integer 
    Public Event NumberChanged As TestEventDelegate 
    Public Property Number As Integer 
     Get 
      Return _num 
     End Get 
     Set(value As Integer) 
      _num = value 
      RaiseEvent NumberChanged() 
     End Set 
    End Property 
End Class 

'Event subscriber class 

Public Class EventSubscriber 
    'instance of EventPublisher class 
    Private WithEvents myObject As New EventPublisher 

    'Handler of myObject.NumberChanged event 
    Public Sub ShowMessage() Handles myObject.NumberChanged 
     Console.WriteLine("Value has been changed") 
    End Sub 

    Shared Sub Main() 
     Dim es As New EventSubscriber 
     es.myObject.Number = 10 
     es.myObject.Number = 20 

     'Handle the events dynamically using AddHandler 
     Dim ep1 As New EventPublisher 
     ep1.Number = 101 
     'Attach an event to the handler 
     AddHandler ep1.NumberChanged, AddressOf TestIt 
     ep1.Number = 102 
    End Sub 
    Shared Sub TestIt() 
     Console.WriteLine("Number is modified") 
    End Sub 
End Class