Update UI async?

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故里飘歌
故里飘歌 2021-01-13 19:28

Consider this example:

Private Sub Button_Click(
    sender As Button, e As RoutedEventArgs) Handles btn.Click

  sender.IsEnabled = False

  Thread.Sleep(50         


        
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  • 2021-01-13 19:53

    Instead of creating a thread of your own you can also use the BackgroundWorker Control. By calling the Method "RunWorkerAsync" the DoWork Event get's called in another Thread.

    By Calling the Method "CancelAsync" form your UI thread you can set the Backgroundworker to "Cancellation Pending" (Property of the Control "CancellationPending" is then true). In your long running background thread you can check for that property (e.g. if you have a loop: exit the loop as soon as CancellationPending is true). This is a quite nice feature to safely abort the thread.

    In addition with the Backgroundworker you can also report the progress of the thread (e.g. for use in a ProgressBar)

    Example:

    Public Class Form1
    
       Private Sub Form1_Load(sender As Object, e As System.EventArgs) Handles Me.Load
    
          '** Set to true if you want the ReportProgress Event
          BackgroundWorker1.WorkerReportsProgress = True
          BackgroundWorker1.WorkerSupportsCancellation = True
       End Sub
    
       Private Sub BackgroundWorker1_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
    
          Dim i As Integer
          Dim n As Integer = 100
          Dim iLastPerc As Integer
    
    
          While Not BackgroundWorker1.CancellationPending AndAlso i < n
    
             '** Do your time consuming actions here
             Threading.Thread.Sleep(500)
    
             If Math.Floor((i / n) * 100) > iLastPerc Then
                '** If the Progress has changed. Report
                iLastPerc = CInt(Math.Floor((i / n) * 100))
                BackgroundWorker1.ReportProgress(iLastPerc)
             End If
    
             i += 1
          End While
    
       End Sub
    
       Private Sub btnStart_Click(sender As System.Object, e As System.EventArgs) Handles btnStart.Click
    
          '** Run the Backgroundworker
          BackgroundWorker1.RunWorkerAsync()
    
       End Sub
    
       Private Sub BackgroundWorker1_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
    
          '** Update the ProgressBar
          ProgressBar1.Value = e.ProgressPercentage
    
       End Sub
    
       Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
    
          '** Worker is done. Check for Exceptions or evaluate the Result Object if you like
    
       End Sub
    
       Private Sub btnCancel_Click(sender As System.Object, e As System.EventArgs) Handles btnCancel.Click
    
          '** Cancel the worker
          BackgroundWorker1.CancelAsync()
    
          MsgBox("Finished!")
    
       End Sub
    End Class
    

    In reference to your question the code should be:

    Private Sub btn_Click(sender As Button, e As RoutedEventArgs) Handles btn.Click
      sender.IsEnabled = False
      Using bw As New BackgroundWorker()
        AddHandler bw.DoWork, Sub(s, ea) Thread.Sleep(5000)
        AddHandler bw.RunWorkerCompleted, Sub(s, ea) sender.IsEnabled = True
        bw.RunWorkerAsync()
      End Using
    End Sub
    
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  • 2021-01-13 19:59

    Bind the button enabled property to a property in your VM (say ProcessComplete).

    Use the button onclick event to trigger a method in your VM that starts up your long winded process. Keep the ProcessComplete False whilst the process is running and then set it True when it completes.

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  • 2021-01-13 20:01

    The button click event is handled by the UI thread, hence when you invoke thread.sleep you make the UI thread sleep, and you see no changes until the method ends.

    Therefore you need to run the process on a new thread, and when the process ends, make the UI changes using the dispatcher.

    For example:

    Private event TaskEnded()
    Private Sub Button_Click(sender As Button, e As RoutedEventArgs) Handles btn.Click
      btn.IsEnabled = False
      dim l as new Thread(sub()
                           Thread.Sleep(5000)
                           RaiseEvent TaskEnded
                           End Sub)
      l.start()
    End Sub
    
    Private Sub bla() Handles Me.TaskEnded
      dispatcher.BeginInvoke(sub()
                               btn.IsEnabled = True
                             end sub)
    End Sub
    

    The MVVM way you'll bind your button IsEnabled property to a boolean property in your viewModel, and update the VM propety instead on the button directly.

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