Update UI async? - wpf

Question

Consider this example:
Private Sub Button_Click(
sender As Button, e As RoutedEventArgs) Handles btn.Click
sender.IsEnabled = False
Thread.Sleep(5000)
sender.IsEnabled = True
End Sub
In my scenario the Button_Click is a command delegate in the VM, and the Thread.Sleep is some long-running process (about 2-10 seconds).
I want, that when the user calls the command, it should immediately update the UI disabling the button so the user cannot execute it while it's running, then execute that operation, then, when operation completed, unblock the button.
I tried wrapping the middle line like the following:
Dispatcher.BeginInvoke(Sub() Thread.Sleep(5000))
But it didn't do the job.
What's the best way to do it?

Answer 1

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.

Answer 2

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

Answer 3

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.