WebClient.DownloadData 进度报告答案

【问题标题】：WebClient.DownloadData Progress ReportWebClient.DownloadData 进度报告
【发布时间】：2020-07-04 09:13:12
【问题描述】：

我有一个 BackgroundWorker 来下载图像文件并在 PictureBox 中查看它。我想报告该下载的进度（因为它可能是一个大图像文件）并更新 ProgressBar。看来我找不到合适的方法。

我在 Microsoft 文档中找到了 this 文章，但它仅包含 C# 用法。谁能指出我正确的方向？

我在 BackgroundWorker 中的实际代码：

Private Sub BackgroundWorker3_DoWork(sender As Object, e As DoWorkEventArgs) Handles BackgroundWorker3.DoWork
    Dim Test1 As String = "DOWNLOAD URL"
    Dim tClient As WebClient = New WebClient
    Dim tImage As Bitmap = Bitmap.FromStream(New MemoryStream(tClient.DownloadData(Test1)))
    PictureBox1.SizeMode = PictureBoxSizeMode.Zoom
    PictureBox1.Image = tImage
End Sub

我猜它类似于 BackgroundWorker 的 ReportProgress，但由于我不知道实际文件大小，因此无法理解如何将其应用到下载数据中：

ReportProgress(Convert.ToInt32((contagem / count) * 100))

【问题讨论】：

参见WebClient.DownloadDataAsync 方法。下载过程引发DownloadProgressChanged 事件，该事件还提供进度百分比。顺便说一句，BackGroundWorker 并不像那样工作。几乎所有这些对象都不能以这种方式在DoWork 事件中使用。
我找到了这篇文章...您可以从该页面的语言下拉列表中选择 VB.NET。

标签： .net vb.net progress-bar webclient backgroundworker

【解决方案1】：

正如 cmets 中的开发人员所说，以异步模式下载数据的一种有效方法可能是这样的：有两个事件，一个用于进度，另一个用于下载结束。我整理了一些代码作为您可以开始改进的基础。希望这是你的目标:)

Event OnDownloadCompleted(downloadedDataAsByteArray() As Byte)
Event OnDownloadProgress(percentage As Integer)

Private Sub DownLoadFileAsync(address As String)

    Dim client As Net.WebClient = New Net.WebClient()

    AddHandler client.DownloadDataCompleted, Sub(sender As Object, e As System.Net.DownloadDataCompletedEventArgs)
                                                 RaiseEvent OnDownloadCompleted(e.Result)
                                             End Sub

    AddHandler client.DownloadProgressChanged, Sub(sender As Object, e As System.Net.DownloadProgressChangedEventArgs)
                                                   RaiseEvent OnDownloadProgress(e.ProgressPercentage)
                                                   Application.DoEvents()
                                               End Sub

    Dim uri As Uri = New Uri(address)
    client.DownloadDataAsync(uri)

End Sub

用法

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

    AddHandler Me.OnDownloadProgress, Sub(percet As Integer)
                                          Console.WriteLine("Percent reiceved: " & percet.ToString)
                                      End Sub


    AddHandler Me.OnDownloadCompleted, Sub(data() As Byte)
                                           Console.WriteLine("Download completed, total: " & data.Count)
                                           Using mStream As New MemoryStream(data)
                                               'Do what you want with this image
                                               Dim image As Image = Image.FromStream(mStream)
                                           End Using
                                       End Sub


    DownLoadFileAsync("https://images.pexels.com/photos/3952233/pexels-photo-3952233.jpeg?auto=compress&cs=tinysrgb&dpr=3&h=750&w=1260")

End Sub

【讨论】：