苹果说:UIModalPresentationFormSheetThewidthandheightofthepresentedviewaresmallerthanthoseofthescreenandtheviewiscenteredonthescreen.Ifthedeviceisinalandscapeorientationandthekeyboardisvisible,thepositionoftheviewisadjustedupwardsothattheviewremainsvisible.Alluncoveredareasaredimmedtopreventtheuserf