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Como: Declarar eventos personalizados para evitar bloqueio (Visual Basic)

There are several circumstances when it is important that one event handler not block subsequent event handlers. Custom events allow the event to call its event handlers asynchronously.

By default, the backing-store field for an event declaration is a multicast delegate that serially combines all the event handlers. This means that if one handler takes a long time to complete, it blocks the other handlers until it completes. (Well-behaved event handlers should never perform lengthy or potentially blocking operations.)

Instead of using the default implementation of events that Visual Basic provides, you can use a custom event to execute the event handlers asynchronously.

Exemplo

In this example, the AddHandler accessor adds the delegate for each handler of the Click event to an ArrayList stored in the EventHandlerList field.

When code raises the Click event, the RaiseEvent accessor invokes all the event handler delegates asynchronously using the BeginInvoke method. That method invokes each handler on a worker thread and returns immediately, so handlers cannot block one another.

Public NotInheritable Class ReliabilityOptimizedControl
    'Defines a list for storing the delegates
    Private EventHandlerList As New ArrayList

    'Defines the Click event using the custom event syntax.
    'The RaiseEvent always invokes the delegates asynchronously
    Public Custom Event Click As EventHandler
        AddHandler(ByVal value As EventHandler)
            EventHandlerList.Add(value)
        End AddHandler
        RemoveHandler(ByVal value As EventHandler)
            EventHandlerList.Remove(value)
        End RemoveHandler
        RaiseEvent(ByVal sender As Object, ByVal e As EventArgs)
            For Each handler As EventHandler In EventHandlerList
                If handler IsNot Nothing Then
                    handler.BeginInvoke(sender, e, Nothing, Nothing)
                End If
            Next
        End RaiseEvent
    End Event
End Class

Consulte também

Tarefas

Como: Declarar eventos personalizados para conservar memória (Visual Basic)

Referência

ArrayList

BeginInvoke

Outros recursos

Eventos (Visual Basic)