The Case of the Jumping Dialog

We recently noticed a new bug in the IDE where a dialog that hadn’t been updated for quite some time suddenly started misbehaving: it would appear at the wrong coordinates (0,0), and then jump to the proper ones.

At first glance this this looked like a classic case of creating the dialog in a visible state, and then moving it during the CREATE event, but checking the dialog properties in the Form Designer showed that the dialog’s Visible property was “Hidden”, so this wasn’t the source of the problem.

Stepping through the CREATE event in the debugger showed that the dialog was indeed created hidden, but then became visible during an RList() call that performed a SELECT statement, ergo RList() was allowing some queued event code to execute (probably from an internal call to the Yield() procedure) and that was changing the dialog’s VISIBLE property.

Checking the other events revealed that the SIZE event contained the following code:

Call Set_Property( @Window, "REDRAW", FALSE$ )

// Move some controls around

Call Set_Property( @Window, "REDRAW", TRUE$ )

The REDRAW property works by toggling an object’s WS_VISIBLE style bit – when set to FALSE$ the style bit is removed but the object is not redrawn. When set to TRUE$ the object is marked as visible and redrawn.

So, putting all this together: creating the dialog generated a queued SIZE event, which under normal circumstances would run after the CREATE event. However, the Yield() call in RList() executed the SIZE event before the dialog was ready to be shown, and the REDRAW operation made the dialog visible before the CREATE event had moved it to the correct position.

The fix for this was to ensure that the REDRAW property wasn’t set to TRUE$ if it’s original value wasn’t TRUE$, like so:

bRedraw = Set_Property( @Window, "REDRAW", FALSE$ )

// Move some controls around

If bRedraw Then
   Call Set_Property( @Window, "REDRAW", TRUE$ )
End

Conclusion

  • Always protect calls to the REDRAW property by checking its state before you set it, and then only turn it back on again if it was set to TRUE$ originally.
  • Calling Yield() can cause events to run out of the normal sequence, and you should be aware of this when writing your application.

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