Como: chamada um manipulador de eventos em Visual Basic

Artigo
08/08/2011

An event is an action or occurrence — such as a mouse click or a credit limit exceeded — that is recognized by some program component, and for which you can write code to respond. An event handler is the code you write to respond to an event.

An event handler in Visual Basic is a Sub procedure. However, you do not normally call it the same way as other Sub procedures. Instead, you identify the procedure as a handler for the event. You can do this either with a Cláusula Handles (Visual Basic) clause and a WithEvents (Visual Basic) variable, or with an Instrução AddHandler. Using a Handles clause is the default way to declare an event handler in Visual Basic. This is the way the event handlers are written by the designers when you program in the integrated development environment (IDE). The AddHandler statement is suitable for raising events dynamically at run time.

When the event occurs, Visual Basic automatically calls the event handler procedure. Any code that has access to the event can cause it to occur by executing a Instrução RaiseEvent.

You can associate more than one event handler with the same event. In some cases you can dissociate a handler from an event. For more information, see Eventos (Visual Basic).

To call an event handler using Handles and WithEvents

Make sure the event is declared with an Declaração de evento.
Declare an object variable at module or class level, using the WithEvents (Visual Basic) keyword. The As clause for this variable must specify the class that raises the event.
In the declaration of the event-handling Sub procedure, add a Cláusula Handles (Visual Basic) clause that specifies the WithEvents variable and the event name.
When the event occurs, Visual Basic automatically calls the Sub procedure. Your code can use a RaiseEvent statement to make the event occur.

The following example defines an event and a WithEvents variable that refers to the class that raises the event. The event-handling Sub procedure uses a Handles clause to specify the class and event it handles.
```
Public Class raisesEvent
    Public Event somethingHappened()
    Dim WithEvents happenObj As New raisesEvent
    Public Sub processHappen() Handles happenObj.somethingHappened
        ' Insert code to handle somethingHappened event.
    End Sub
End Class
```

To call an event handler using AddHandler

Make sure the event is declared with an Event statement.
Execute an Instrução AddHandler to dynamically connect the event-handling Sub procedure with the event.
When the event occurs, Visual Basic automatically calls the Sub procedure. Your code can use a RaiseEvent statement to make the event occur.

The following example defines a Sub procedure to handle the Closing event of a form. It then uses the Instrução AddHandler to associate the catchClose procedure as an event handler for Closing.
```
' Place these procedures inside a Form class definition.
Private Sub catchClose(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs)
    ' Insert code to deal with impending closure of this form.
End Sub
Public Sub formOpened()
    AddHandler Me.Closing, AddressOf catchClose
End Sub
```
You can dissociate an event handler from an event by executing the Instrução RemoveHandler.

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Como: chamada um manipulador de eventos em Visual Basic

To call an event handler using Handles and WithEvents

To call an event handler using AddHandler

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