如何知道CrystalReport Preview是否已关闭

Question

我有这个VB代码来创建Crystal Reports预览：

Public Sub ShowReport(ByRef R As CrystalDecisions.CrystalReports.Engine.ReportClass, ByVal Page As Byte, ByVal Grup As Boolean, Optional ByVal Rate As Integer = 0)
        Dim formula As String = R.RecordSelectionFormula
        R.DataSourceConnections(0).SetConnection(PrmServer(), PrmDatabase(), True)
        R.RecordSelectionFormula = formula
        Dim f As New RptPreview
        f.Crw.ReportSource = R

        f.Tag = Grup

        f.Show()

    End Sub

我想知道用户何时关闭报告预览，以便我会进行一些操作。有f.PreviewClosed这样的函数吗？感谢。