dotNet: Is there a way to do a Join statement on the UI Thread?

 

Questions


I’m coding a simple thread application: when clicking a start button, the application disable this button, run 5 threads simply making For iterations and updating 5 ProgressBars. A last thread is waiting for the end of the threads, to re-enable my start button.

Problem: The user is seeing the button enabled before the progressbars are at 100%… And the progressbars are still updating!

What’s the problem? Is there a way to make a join statement on the UI Thread?

Here is the code:

Imports System.Threading

Public Structure ThreadParameters
    Public ThreadId As Integer
    Public pgBar As ProgressBar
    Public iterations As Integer
End Structure

Public Structure SetPgValueParameters
    Public pgBar As ProgressBar
    Public value As Integer
End Structure

Public Class Form1

Private threads(4) As Thread
Private Shared FormThread As Thread

Private Sub StartButton_Click(sender As Object, e As EventArgs) Handles StartButton.Click
    StartButton.Enabled = False

    Dim MainThread As Thread = New Thread(AddressOf WaitThreads)
    MainThread.Start()

    For i = 0 To 4
        threads(i) = New Thread(AddressOf ThreadRun)

        Dim params As ThreadParameters

        params.pgBar = Me.Controls.Find("ProgressBar" & (i + 1), False)(0)
        params.iterations = 100
        params.ThreadId = i

        threads(i).Start(params)
    Next

End Sub

Private Sub ThreadRun(params As ThreadParameters)
    Dim invokeParams As SetPgValueParameters
    Dim lastIntegerVal As Integer = 1
    invokeParams.pgBar = params.pgBar
    For i = 0 To params.iterations
            invokeParams.value = (100 * i / params.iterations)
        setPgValue(invokeParams)
        Console.WriteLine(params.ThreadId & ":" & i & "/" & params.iterations)
    Next
End Sub

Private Delegate Sub setPgValueDelegate(params As SetPgValueParameters)
Private Sub setPgValue(params As SetPgValueParameters)
    If params.pgBar.InvokeRequired Then
        Dim d As New setPgValueDelegate(AddressOf setPgValue)
        Me.Invoke(d, New Object() {params})
    Else
        params.pgBar.Value = params.value
        'Application.DoEvents()
    End If
End Sub


Private Sub WaitThreads()
    For i = 0 To 4
        threads(i).Join()
    Next
    FormThread.Join()
    setButtonEnabled()
End Sub

Private Delegate Sub setButtonEnabledDelegate()
Private Sub setButtonEnabled()
    If StartButton.InvokeRequired Then
        Dim d As New setButtonEnabledDelegate(AddressOf setButtonEnabled)
        Me.Invoke(d, Nothing)
    Else
        Application.DoEvents()
        StartButton.Enabled = True
        Console.WriteLine("Bouton réactivé")
    End If
End Sub
End Class

EDIT: Thanks for the link Bjørn-Roger Kringsjå. So there’s a work-around: increase and directly dicrease the value skip the animation. For the max value, set it the value to the max, decrease by 1, and increase by 1…
There still a little lag, but it’s really better than having the button enabled when ProgressBars are in the middle 🙂

 

 

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Answer

Okay, I did some testing and it turns out that the problem lies with the progress bar animation. At the time your button gets enabled (which is correct) the progress bar is still in “animation mode”. This can be verified by disabling XP visual styles.

The progress bar have a property name MarqueeAnimationSpeed, but unfortunately this have no effect when using Blocks style.

I did a google search and found this SO post:

Disabling .NET progressbar animation when changing value?

Sample application

Imports System.Threading
Imports System.Threading.Tasks

Public Class Form1

    Public Sub New()
        Me.InitializeComponent()
        Me.StartButton = New Button() With {.TabIndex = 0, .Dock = DockStyle.Top, .Text = "Start", .Height = 30}
        Me.ProgressBar1 = New ProgressBar With {.TabIndex = 1, .Dock = DockStyle.Top, .Height = 30, .Name = "ProgressBar1"}
        Me.ProgressBar2 = New ProgressBar With {.TabIndex = 2, .Dock = DockStyle.Top, .Height = 30, .Name = "ProgressBar2"}
        Me.ProgressBar3 = New ProgressBar With {.TabIndex = 3, .Dock = DockStyle.Top, .Height = 30, .Name = "ProgressBar3"}
        Me.ProgressBar4 = New ProgressBar With {.TabIndex = 4, .Dock = DockStyle.Top, .Height = 30, .Name = "ProgressBar4"}
        Me.ProgressBar5 = New ProgressBar With {.TabIndex = 5, .Dock = DockStyle.Top, .Height = 30, .Name = "ProgressBar5"}
        Me.Controls.AddRange({Me.ProgressBar5, Me.ProgressBar4, Me.ProgressBar3, Me.ProgressBar2, Me.ProgressBar1, Me.StartButton})
    End Sub

    Private Sub HandleStartButtonClicked(sender As Object, e As EventArgs) Handles StartButton.Click

        Me.StartButton.Enabled = False

        Dim tasks As Task() = New Task(4) {}

        For i As Integer = 0 To 4
            Dim index As Integer = i
            tasks(i) = New Task(
                Sub()
                    Dim n As Integer = rnd.Next(100, 1001)
                    For j As Integer = 0 To n
                        Me.Invoke(
                            Sub()
                                DirectCast(Me.Controls("ProgressBar" & (index + 1).ToString()), ProgressBar).Value = CInt((j / n) * 100.0#)
                            End Sub
                        )
                        Thread.Sleep(10)
                    Next
                End Sub
                )
        Next

        Task.Factory.ContinueWhenAll(tasks,
            Sub() Me.Invoke(
                Sub()
                    MessageBox.Show(String.Join(Environment.NewLine, From pb As ProgressBar In Me.Controls.OfType(Of ProgressBar)() Select (pb.Name & ": " & pb.Value.ToString())))
                    Me.StartButton.Enabled = True
                End Sub
            )
        )

        Array.ForEach(Of Task)(tasks, Sub(t) t.Start())

    End Sub

    Private WithEvents StartButton As Button
    Private WithEvents ProgressBar1 As ProgressBar
    Private WithEvents ProgressBar2 As ProgressBar
    Private WithEvents ProgressBar3 As ProgressBar
    Private WithEvents ProgressBar4 As ProgressBar
    Private WithEvents ProgressBar5 As ProgressBar
    Private Shared rnd As New Random()

End Class

multithreading,synchronisation,user-interface,vb.net

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