UIPopoverController 非模式视图 ，好用

The UIPopoverController class is used to manage the presentation of content in a popover. You use popovers to present information temporarily but in a way that does not take over the entire screen like a modal view does. The popover content is layered on top of your existing content in a special type of window. The popover remains visible until the user taps outside of the popover window or you explicitly dismiss it. Popover controllers are for use exclusively on iPad devices. Attempting to create one on other devices results in an exception.