Timer Event Question

[HARRY]

Hi,

A program stops from time to time with the message Error in <name of sub>. It cannot be based on user errors as he gives no input. In order to further investigate this problem, I want to be sure about the following.

After the user clicking a control a quite complicated action is taken which involves reading a picture file, making a number of scientific calculations, doing quite some drawing, ending in a form refresh.

During this process a timer may raise a tick event (to get GPS information update). Can I be sure that that the process of handling that tick event is postponed until all processing as a result of the user clicking a button is completed?

In the process of handling the user clicking a button no DoEvents occur.

Harry

 

[agraham]

I would expect it to be OK but you could check it like this.

InSub = False ' Global

Sub Button_Click
  InSub = True
  ...
  InSub = False
End Sub

Sub Time1_Tick
  If InSub then
    MsgBox("Unexpected entry!")
  End If
  ...
End Sub

 

[HARRY]

Hi Andrew,

Thanks for your suggestion. Meanwhile I have implemented also your exception library for the two subs where these errors occur. Now I hope that the problem shows up. Sometimes it runs well for hours, but it can also happen two times in a quarter.

Harry

 

[mjcoon]

During this process a timer may raise a tick event (to get GPS information update). Can I be sure that that the process of handling that tick event is postponed until all processing as a result of the user clicking a button is completed?

If you are concerned that the timer-tick action may alter some data that is being alerady operated upon, so producing an inconsistent result, I suggest that you protect it explicitly. Presumably (!) it is not difficult to set a global boolean at the start of the button operation, and cancel it at the finish. Then if a tick event finds the boolean set either the event is ignored (if another tick will subsequently substitute) or some separate flag is set to indicate a postponement.

Typically it was useful, if going to the operating system to fetch data, to be able to accept interrupts such as timers in the interim. This makes it possible to arrange a timeout so that a slow fetch does not hang the program.

Mike.

 

[agraham]

Presumably (!) it is not difficult to set a global boolean at the start of the button operation, and cancel it at the finish.

My code fragment above shows this?

interrupts such as timers.

A Timer is not an interrupt. It is a subroutine call from the Forms' message loop. As such it cannot interrupt anything unless the message loop is re-entered. DoEvents re-enters the message loop and can cause re-entrancy problems under some circumstances. Displaying a message box also runs a message loop as the message box, although modal, takes over pumping messages while it is displayed. You can see this in the application attached. These are the only two situations that I know of where the message loop can be run during Basic4ppc code execution and so potentially cause re-entrancy problems. Using Form.Refresh redraws a Form to, say, update a Textbox while running a long loop but calls the Forms' Paint event directly and so does not actually run the message loop and can't cause problems.

 

[sitajony]

Edit: Sorry an error happaned...