Wednesday, January 12, 2011

Validation and the disappearing caret

A common requirement in accounting software is on-the-fly text validation: run some code in the lost focus event to elaborate the control's content. One example would be validating a VAT number, another is typing a few letters and showing a list of customers whose names start with the typed text.

All there situations have one thing in common: the code shows a new dialog from within the lost focus event handler of the control.

I just discovered a problem with wxWidgets: after closing the dialog the caret (the blinking cursor that shows where the next letter will be inserted) disappears so it is difficult to know which control has the focus. Tabbing to another control makes the caret reappear.
This is rather confusing for the user, so I made some searches: it looks like this is a well known problem and it looks like it is not going to be fixed soon.

A mail message also suggested a workaround: just set a flag in the event handler, then do what you need in the idle event handler. That handler is called just after the focus has moved from a control to another: in the idle handler check the value of the flag and execute the related code.

That is what I did and now the program is working well. The workaround does not even add too much complication to the code so it is satisfactory. It is only a pity that this problem is not mentioned in the documentation: it would save some headaches from time to time.

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