DHJJ [Hatsune's Journal Japan] blog

[VB]Visual Basicには非同期が似合う

Visual Basic Advent Calendar 2011(12/10)

SilverlightやWindows Phoneのプログラミングを行っていると避けることができないのは非同期通信です。

非同期通信を」行うときのコード例としては次のようになります。

Private Sub MainPage_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs) Handles Me.Loaded
    Dim cli As New WebClient
    AddHandler cli.DownloadStringCompleted, AddressOf cli_DownloadStringCompleted
    cli.DownloadStringAsync(New Uri("http://blogs.msdn.com/b/shintak/rss.aspx", UriKind.Absolute))

End Sub

Private Sub cli_DownloadStringCompleted(sender As Object, e As DownloadStringCompletedEventArgs)
    If e.Error IsNot Nothing Then
        MessageBox.Show("エラー:" & e.Error.Message)
    Else
        Dim sr As New StringReader(e.Result)
        Dim xr As XmlReader = XmlReader.Create(sr)
        Dim sf As SyndicationFeed = SyndicationFeed.Load(xr)
        Me.DataContext = sf
    End If
End Sub

このコード例のようにAddHandlerで動的にイベントプロシージャを割り当てる方式であればVisual Basicの言語依存が最小になり、.NETらしいコードといえるかも知れません。

しかしVisual BasicならばもっとIDEの機能を引き出せる書き方が存在します。それがWithEventsです。

Friend WithEvents Cli As New WebClient

Private Sub MainPage_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs) Handles Me.Loaded
    Cli.DownloadStringAsync(New Uri("http://blogs.msdn.com/b/shintak/rss.aspx", UriKind.Absolute))
End Sub

Private Sub Clit_DownloadStringCompleted(sender As Object,
                                         e As System.Net.DownloadStringCompletedEventArgs) _
                                     Handles Cli.DownloadStringCompleted
    If e.Error IsNot Nothing Then
        MessageBox.Show("エラー:" & e.Error.Message)
    Else
        Dim sr As New StringReader(e.Result)
        Dim xr As XmlReader = XmlReader.Create(sr)
        Dim sf As SyndicationFeed = SyndicationFeed.Load(xr)
        Me.DataContext = sf
    End If
End Sub

WithEventsを使って変数を宣言しておけばIDEが関連するイベントを把握しイベントプロシージャのスケルトンが簡単に生成できるのです。

Private Sub Cli_DownloadProgressChanged(sender As Object,
                                        e As System.Net.DownloadProgressChangedEventArgs) _
                                    Handles Cli.DownloadProgressChanged

End Sub

Private Sub Cli_DownloadStringCompleted(sender As Object,
                                        e As System.Net.DownloadStringCompletedEventArgs) _
                                    Handles Cli.DownloadStringCompleted

End Sub

Private Sub Cli_OpenReadCompleted(sender As Object,
                                  e As System.Net.OpenReadCompletedEventArgs) _
                              Handles Cli.OpenReadCompleted

End Sub

Private Sub Cli_OpenWriteCompleted(sender As Object,
                                   e As System.Net.OpenWriteCompletedEventArgs) _
                               Handles Cli.OpenWriteCompleted

End Sub

Private Sub Cli_UploadProgressChanged(sender As Object,
                                      e As System.Net.UploadProgressChangedEventArgs) _
                                        Handles Cli.UploadProgressChanged

End Sub

Private Sub Cli_UploadStringCompleted(sender As Object,
                                      e As System.Net.UploadStringCompletedEventArgs) _
                                  Handles Cli.UploadStringCompleted

End Sub

Private Sub Cli_WriteStreamClosed(sender As Object,
                                  e As System.Net.WriteStreamClosedEventArgs) _
                              Handles Cli.WriteStreamClosed

End Sub

普段Visual Basicを使っていない方もぜひ実際に確認してみてください。

投稿日時 : 2011年12月10日 21:15