多线程故障

Question

I have a program that uses a label called Valve and a textbox called Variable

The gist is that if the Variable = 0 then the label colour is gray, If the Variable = 1 then the label uses a thread which flashes between gray and red.

This works almost perfectly apart from if changing between the two values very quickly (entering 0 then deleting it then entering 1, and so on) Then the thread speed increases (as if it is multi-threading).

The strange thing is that if swapping between the values 0 & 1 slowly (every 2 seconds +), then it doesn't increase the blinking speed (this is what the program needs to do)

This is expanded code from the following question: vb.net multi threading

Note: This is just a VB.NET conversion from my project on VisiWin.NET. In this example the TextBox Variable will be an actual variable read from a PLC and the label Valve will be triangles representing a process solenoid from a process flowsheet mimic. Every solenoid will be controlled by different variables.

Imports Microsoft.VisualBasic
Imports System
Imports System.Collections
Imports System.Collections.Generic
Imports System.Drawing
Imports System.Windows.Forms
Imports System.Threading
Imports System.Diagnostics

Public Class Form1

Private _flash As Boolean = False

Private Sub Variable1_TextChanged(sender As System.Object, e As System.EventArgs) Handles Variable.TextChanged


    If Variable.Text = "1" And Not _flash Then
        _flash = True
        Dim FlashThread As New Thread(New ThreadStart(AddressOf FlashLabel))
        FlashThread.Start()
    End If

    If Variable.Text = "0" Then
        _flash = False
        Valve.ForeColor = Color.Gray
    End If

End Sub


Private Sub FlashLabel()

    Dim _Color As Color = Color.Gray
    While _flash

        If Valve.ForeColor = _Color Then
            Valve.ForeColor = Color.Red
        Else
            Valve.ForeColor = Color.Gray
        End If
        System.Threading.Thread.Sleep(2000)

    End While

End Sub

End Class

Answer 1

What's happening here is that your first flashing thread is still running, it's just in the two second sleep phase. Your value changes to 0, it doesn't break out of the loop because it's asleep and then the variable changes back to 1 again, the thread wakes up and carries on, by which time you have spawned another thread doing the exact same thing, so it appears as if the thread is going faster.

I would suggest changing this to a timer instead, as you can stop the timer when the variable is 0, and then restart it when it is 1:

Imports Microsoft.VisualBasic
Imports System
Imports System.Collections
Imports System.Collections.Generic
Imports System.Drawing
Imports System.Windows.Forms
Imports System.Threading
Imports System.Diagnostics

Public Class Form1

Private _timer As New System.Windows.Forms.Timer()

Private Sub Variable1_TextChanged(sender As System.Object, e As System.EventArgs) Handles Variable.TextChanged


    If Variable.Text = "1" And Not _flash Then
        _flash = True
        _timer.Interval = 2000
        _timer.Enabled = True
        _timer.Start()
    End If

    If Variable.Text = "0" Then
        _flash = False
        _timer.Stop()
        _timer.Enabled = False
        Valve.ForeColor = Color.Gray
    End If

End Sub


Private Sub FlashLabel() Handles _timer.Tick

    Dim _Color As Color = Color.Gray

    If Valve.ForeColor = _Color Then
        Valve.ForeColor = Color.Red
    Else
        Valve.ForeColor = Color.Gray
    End If

End Sub

End Class

Docs for Timer: http://msdn.microsoft.com/en-gb/library/system.windows.forms.timer.aspx

Alternatively, you could store the thread in a field and terminate it when your variable is set to 0:

Imports Microsoft.VisualBasic
Imports System
Imports System.Collections
Imports System.Collections.Generic
Imports System.Drawing
Imports System.Windows.Forms
Imports System.Threading
Imports System.Diagnostics

Public Class Form1

Private _flash As Boolean = False
Private _flashThread as Thread

Private Sub Variable1_TextChanged(sender As System.Object, e As System.EventArgs) Handles Variable.TextChanged


    If Variable.Text = "1" And Not _flash Then
        _flash = True
        _flashThread As New Thread(New ThreadStart(AddressOf FlashLabel))
        _flashThread.Start()
    End If

    If Variable.Text = "0" Then
        _flash = False
        _flashThread.Abort()
        Valve.ForeColor = Color.Gray
    End If

End Sub


Private Sub FlashLabel()

    Dim _Color As Color = Color.Gray
    While _flash

        If Valve.ForeColor = _Color Then
            Valve.ForeColor = Color.Red
        Else
            Valve.ForeColor = Color.Gray
        End If
        System.Threading.Thread.Sleep(2000)

    End While

End Sub

End Class

See http://msdn.microsoft.com/en-GB/library/ty8d3wta.aspx for notes on aborting threads, although I don't think any of this really applies to you, if it doesn't abort the thread during it's sleeping time, it should abort before the next iteration of the loop.

Answer 2

The problem is the following:

1. You enter 1, _flash is False : The thread is started, changes the color and sleeps 2 seconds
2. You enter 0 quickly after entering 1, _flash is True : _flash will be set to False .
3. You enter 1 quickly after entering 0, _flash is False again: A new thread is started.

Now, if steps 2 and 3 happen while the first thread sleeps, you have two running threads. After the first thread is finished sleeping, it will see that _flash is True and will continue running.