如何等待獲取請求，而不凍結客戶端

Question

我有一個簡單的按鈕，發送獲取請求來檢索我的網站上的TXT文件。問題是它凍結了應用程序，同時檢索信息。我怎樣才能使它在等待結果時不凍結？

Private Sub cmd_ClickMe_Click(sender As Object, e As EventArgs) Handles cmd_ClickMe.Click 
    Dim request As String = String.Format("http://www.*****/database/test.txt") 
    Dim webClient As New System.Net.WebClient 
    Dim result As String = webClient.DownloadString(request) 

    MessageBox.Show(result) 
End Sub 

我也試過以下，但它不工作（稱 「webClient.DownloadStringAsync（myUri）」 不會產生一個值：

Private Sub cmd_ClickMe_Click_1(sender As Object, e As EventArgs) Handles cmd_ClickMe.Click 
    Dim request As String = String.Format("http://www.****.com/database/test.txt") 
    Dim webClient As New System.Net.WebClient 
    Dim myUri As Uri = New Uri(request) 

    Dim result As String = webClient.DownloadStringAsync(myUri) 

    MessageBox.Show(result) 
End Sub 

Answer 1

使用DownloadStringAsync(Uri)而不是DownloadString(uri)。

的DownloadStringAsync方法不會阻止調用線程

下面是一個例子，如何使用它：

Dim wc As New WebClient 
    ' Specify that you get alerted 
    ' when the download completes. 
    AddHandler wc.DownloadStringCompleted, AddressOf AlertStringDownloaded 

    Dim uri As New Uri("http:\\changeMe.com") 'Pass the URL to here. This is just an example 
    wc.DownloadStringAsync(uri) 

End Sub 

Public Shared Sub AlertStringDownloaded(ByVal sender As Object, ByVal e As DownloadStringCompletedEventArgs) 

    ' If the string request went as planned and wasn't cancelled: 
    If e.Cancelled = False AndAlso e.Error Is Nothing Then 

     Dim myString As String = CStr(e.Result) 'Use e.Result to get the String 
     MessageBox.Show(myString) 
    End If 

End Sub