WebClient.DownloadProgressChanged 事件

定義

命名空間:

System.Net

組件:

System.Net.WebClient.dll

來源:

WebClient.cs

在非同步下載作業成功傳輸部分或全部資料時發生。

public:
 event System::Net::DownloadProgressChangedEventHandler ^ DownloadProgressChanged;

public event System.Net.DownloadProgressChangedEventHandler? DownloadProgressChanged;

member this.DownloadProgressChanged : System.Net.DownloadProgressChangedEventHandler 

Public Custom Event DownloadProgressChanged As DownloadProgressChangedEventHandler 

事件類型

DownloadProgressChangedEventHandler

範例

下列程式碼範例示範如何設定 DownloadProgressChanged 事件的事件處理常式。

// Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
void DownLoadFileInBackground4( String^ address )
{
   WebClient^ client = gcnew WebClient;
   Uri ^uri = gcnew Uri(address);

   // Specify a DownloadFileCompleted handler here...

   // Specify a progress notification handler.
   client->DownloadProgressChanged += gcnew DownloadProgressChangedEventHandler( DownloadProgressCallback4 );
   
   client->DownloadFileAsync( uri, "serverdata.txt" );
}

static void DownloadProgressCallback4(Object^ sender, DownloadProgressChangedEventArgs^ e)
{
   // Displays the operation identifier, and the transfer progress.
   Console::WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
      (String ^)e->UserState,
      e->BytesReceived,
      e->TotalBytesToReceive,
      e->ProgressPercentage);
}

// Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
public static void DownLoadFileInBackground4(string address)
{
    WebClient client = new WebClient();
    Uri uri = new Uri(address);

    // Specify a DownloadFileCompleted handler here...

    // Specify a progress notification handler.
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(DownloadProgressCallback4);

    client.DownloadFileAsync(uri, "serverdata.txt");
}

private static void DownloadProgressCallback4(object sender, DownloadProgressChangedEventArgs e)
{
    // Displays the operation identifier, and the transfer progress.
    Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...",
        (string)e.UserState,
        e.BytesReceived,
        e.TotalBytesToReceive,
        e.ProgressPercentage);
}

' Sample call : DownLoadFileInBackground4 ("http://www.contoso.com/logs/January.txt");
Public Shared Sub DownLoadFileInBackground4(ByVal address As String)

    Dim client As WebClient = New WebClient()

    ' Specify a DownloadFileCompleted handler here...

    '  Specify a progress notification handler.
    AddHandler client.DownloadProgressChanged, AddressOf DownloadProgressCallback4

    Dim uri as Uri = New Uri(address)
    client.DownloadFileAsync(uri, "serverdata.txt")

End Sub

Private Shared Sub DownloadProgressCallback4(ByVal sender As Object, ByVal e As DownloadProgressChangedEventArgs)
    ' Displays the operation identifier, and the transfer progress.
    Console.WriteLine("{0}    downloaded {1} of {2} bytes. {3} % complete...", _
    CStr(e.UserState), e.BytesReceived, e.TotalBytesToReceive, e.ProgressPercentage)
End Sub

備註

每次非同步下載進行時，都會引發此事件。 使用下列任一方法啟動下載時，就會引發此事件。

方法	描述
DownloadDataAsync	從資源下載資料並傳回 Byte 陣列，而不會封鎖呼叫執行緒。
DownloadFileAsync	將資料從資源下載到本機檔案，而不會封鎖呼叫執行緒。
OpenReadAsync	從資源傳回資料，而不封鎖呼叫執行緒。

DownloadProgressChangedEventHandler是這個事件的委派。 類別 DownloadProgressChangedEventArgs 會提供事件處理常式與事件資料。

如需如何處理事件的詳細資訊，請參閱 處理和引發事件。

注意

被動 FTP 檔案傳輸一律會顯示進度百分比為零，因為伺服器未傳送檔案大小。 若要顯示進度，您可以覆 GetWebRequest(Uri) 寫虛擬方法，將 FTP 連線變更為使用中：

internal class MyWebClient : WebClientProtocol
{
    protected override WebRequest GetWebRequest(Uri address)
    {
        FtpWebRequest req = (FtpWebRequest)base.GetWebRequest(address);
        req.UsePassive = false;
        return req;
    }
}