ios - What UIKeyboardNotification will be posted when the FirstResponse changed? -

i want receive uikeyboard notification when firstresponder changed(both uitextfield) coding prevent uikeyboard covering uitextfield.but no uikeyboard notification posted when size of keyboard did not change(i've tried receive these notifications: uikeyboardwillshownotification, uikeyboardwillhidenotification, uikeyboardwillchangeframenotification).i've received several uikeyboard notification when size of keyboard changed.did miss something?

1. no uikeyboardnotification posted when firstresponse changes.
2. uikeyboardnotification posted uikeyboardframe when size of keyboard changes.

this simple solution: store uikeyboardframe in property, send scrollcontentview message (implement yourself) when firstresponse changes , uikeyboard shown.