I am displaying a picture that changed every 10 seconds, image is not displaying with threading?

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I am using a do until loop that changes an image every 10 seconds. This will be adjusted but set only for 10 for testing purposes. I have 4 images (1.jpg-4.jpg) Theoretically it should change photo to 1st photo, wait 10 seconds, change to 2nd photo, wait 10 seconds and so on but no photo is displayed. I removed the threading and the last image is displayed.

VB

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim picNum As Integer = 0

    'Start Displaying images
    Do Until picNum = 4
        picNum = picNum + 1
        mainpbx1.ImageLocation = "F:\Programs\TV\" & picNum & ".jpg"
        Threading.Thread.Sleep(10000)
    Loop
End Sub

What I have tried:

I tried doing a Timer( I always use threading for stuff like this) and couldn't get it to be right. I had Timer enabled with 8 second interval but it didn't seem to be waiting after it Timer1.start()

Posted 23-Oct-19 5:22am

Cody O'Meara

Updated 23-Oct-19 5:49am

v3

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2 solutions

Solution 1

Ooh. This can be tricky if you're not used to multi-threading winforms. There are some gotcha's that we've all hit in our time.

So here's the problem: You have 1 thread that draws the GUI. You are creating another thread to change some property for the first thread to draw. Only problem is, the first thread doesn't know about it. You changed it on another thread and don't tell the first.

Here's a nice way of implemented a threaded process that tells the first thread that something changes.

Updating Your Form from Another Thread without Creating Delegates for Every Type of Update[^]

if you get stick then just let us know.

Good luck ^_^

Posted 23-Oct-19 5:47am

Andy Lanng

Comments

Dave Kreskowiak 23-Oct-19 11:53am

He's not creating a background thread in his code. He's just putting the UI thread to sleep for 10 seconds and wondering why it doesn't update the picturebox.

Andy Lanng 23-Oct-19 12:23pm

OOK - ok, but he should background it ^_^
Nah, your right. Maybe someone will find the link useful looking at this post

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Richard Deeming · Accepted Answer · 2019-10-23T05:49:00

Using Thread.Sleep causes the UI thread to freeze. It cannot process any messages, which means it cannot repaint itself to show the selected image.

Use a Timer to update your images instead of a loop:
Timer Class (System.Windows.Forms) | Microsoft Docs[^]

VB.NET

Private picNum As Integer
Private picTimer As System.Windows.Forms.Timer

Private Sub ShowCurrentPicture()
    mainpbx1.ImageLocation = "F:\Programs\TV\" & picNum & ".jpg"
End Sub

Private Sub ShowNextPicture()
    picNum = picNum + 1
    ShowCurrentPicture()
    If picNum = 4 Then 
        picTimer.Enabled = False
    End If
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    If picTimer Is Nothing Then
        picTimer = New System.Windows.Forms.Timer()
        AddHandler picTimer.Tick, AddressOf ShowNextPicture
        picTimer.Interval = 10000
    End If
    
    If Not picTimer.Enabled Then
        picNum = 1
        ShowCurrentPicture()
        picTimer.Enabled = True
    End If
End Sub