It seems for me that best place for validation of user input is in focus-out-event handler of entry type widgets (but this is only my thinking). I try to return TRUE from focus-out-event with idea that this will avoid to transfer focus to next control and keep focus on current widget, but this is not OK (not work as needed).
Which is proper way to suppress of transfering focus to next control in case when validation don’t pass and where to put validation code on such widgets (gtkEntry, gtkSpinButton, etc) to be usable with signals invoked with both – mouse and keyboard actions?
That is hardly a good idea… Focus is (should be) always owned by the user. That is, the users should be able to move the focus wherever they see fit. If you try to direct the focus, or prevent moving it, based on any logic you can imagine, you will likely frustrate them.
It will surely frustrate me! Say for example that I wrote a wrong data in a box, and then I say, “hey, that’s wrong! I want to select that data over there, copy it and paste it here”. But I cannot do that because the program will not let me leave the box where I am now without writing an acceptable data. I have to delete the wrong data and enter something… just not good.
The best way to validate the user entry is when you actually do something with it. Either when you save it or when you make it effective. Then you can even make a pop-up saying that there is an error, and when it is close, move the focus to the first error.
If you feel that the user can benefit from an early warning that he is doing wrong, you can use colors (paint the box yellow/red if there is an error), or add a small error icon next to the box. That is waaaay less disruptive than moving the focus around.