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Textbox.Text not seen in loop

 
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Textbox.Text not seen in loop
k6elc
21 Jan, 2008 - 07:53 PM
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When I run the following code, the TextBox1 is passed over in the loop until the final iteration when it displays the final value of i. The beep occurs three times, but the TextBox1 is not updated for i=1 and i=2. I would like an explanation of why this is.

Thanks in advance!
CODE

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim i As Integer
        Dim j As Integer

        For i = 1 To 3
            TextBox1.Text = i
            Beep()
            For j = 1 To 1000000000
            Next j
        Next i

    End Sub

End Class
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Nayana
RE: Textbox.Text Not Seen In Loop
22 Jan, 2008 - 06:38 PM
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OK, what's happening is that code above is running in the same thread as the user interface. What that means is that the update events for the user interface will not be executed until your above code has finished.

This means that only the last state will be shown.

In VB you can allow the UI to be updated by using the DoEvents command.

However, having a big loop like
CODE
For j = 1 To 1000000000
Next j

Is pretty useless. If you want to wait for a certain amount of time, you should use a timer.

This post has been edited by Nayana: 22 Jan, 2008 - 06:51 PM
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k6elc
RE: Textbox.Text Not Seen In Loop
23 Jan, 2008 - 03:33 PM
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Thanks Nayana.... That works fine and another possibility, as I now realize, is multithreading. I was fluent in basic 30 years ago, but this GUI is taxing my feeble old mind. The long loop was just for the example and yes the timer is better solution.


QUOTE(Nayana @ 22 Jan, 2008 - 07:38 PM) *

OK, what's happening is that code above is running in the same thread as the user interface. What that means is that the update events for the user interface will not be executed until your above code has finished.

This means that only the last state will be shown.

In VB you can allow the UI to be updated by using the DoEvents command.

However, having a big loop like
CODE
For j = 1 To 1000000000
Next j

Is pretty useless. If you want to wait for a certain amount of time, you should use a timer.
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Nayana
RE: Textbox.Text Not Seen In Loop
23 Jan, 2008 - 05:21 PM
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No worries smile.gif

I wasn't putting down your example, for your query, it was a perfect example. I just meant that in production code, it is pretty useless.

Multi-threading can be quite complex at first (actually, always), so if you give it a go, and have any queries, don't hesitate to ask here again.

Good luck.
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LookNAO
RE: Textbox.Text Not Seen In Loop
24 Jan, 2008 - 06:59 AM
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CODE


TextBox1.Text = i
TextBox1.refresh
doevents
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PsychoCoder
RE: Textbox.Text Not Seen In Loop
24 Jan, 2008 - 07:08 AM
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I wrote a tutorial on Cross Process Communication, in this tutorial you will learn that you cannot access that textbox from a separate thread, UI controls can only be accessed directly from the thread that created them.

You can, however, as the tutorial shows, use a Delegate to marshal the call onto the thread controlling the UI controls.

PS: This is VB.Net so I am moving this to the proper forum, but VB,Net also has a DoEvents Method as well smile.gif . The tutorial above is written in C#, but for a seasoned programmer you should be able t get the gist of how the logic works smile.gif
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