[Pharo-users] polymorph question

hi, i make a text entry field with StandardWindow's method: newTextEntryFor: aModel getText: getSel setText: setSel getEnabled: enabledSel help: helpText how do i let that textentryfield verify the text for its validity (in its model) when one tries to leave that field and prevent the exiting from that field? or do i have to use another method (UITheme?) for this thing? werner