[Lazarus] TListView OnSelectItem event: Bug or feature?

[Giuliano Colla]

Il 22/08/2014 16:44, Michael Thompson ha scritto:
> > The implementation of  the OnSelectItem event of TListView doesn't 
> appear to me to be correct.
> > With MultiSelect false, selecting a new Item on the list generates, 
> as expected, two events: the unselect of the previous Item, and the 
> select of the new one.
> > But they're generated in the reverse order. i.e. first the selection 
> of the new one, then the unselect of the old one.
>
> Out of curiosity, which widgetset are you using?  My memory (haven't 
> checked this for at least 3 years) says the events were fired in 
> different sequences between different widgetsets.  Mind you, I can't 
> think of how a change in event order will affect any of the examples 
> you've given.
>

This behavior is common at least to GTK2 and Qt. That's why I think it 
originates in LCL.
I consider it a bug, because the last event generated doesn't represent 
the actual final status of TListView (i.e. an Item selected or no Item 
selected), but may represent a previous intermediate condition. Extra 
code in application is required to sort out things.

There are many differences between different widgetset TListview 
implementation, and a lot of care must be exercised if you want your 
application behave the same way in all condition.
e.g.: under Qt a programmatical select of an Item doesn't unselect the 
previous one, even if MultiSelect is not active, and a programmatical 
unselect of an Item doesn't trigger the OnSelectItem event if ViewStyle 
is vsList, but does trigger it if the ViewStyle is vsReport!

Giuliano