ios – 如何在显示模态视图控制器时更改modalPresentationStyle？

我希望将我的UIModalPresentationFormSheet样式模式视图设置为UIModalPresentationFullScreen样式,但是当它已经显示为UIModalPresentationFormSheet样式时,它无法使用代码“[xx setModalPresentationStyle：UIModalPresentationFullScreen];”转到全屏模式.

在显示模态视图后,有没有办法改变现在的风格？我的代码是这样的：

UITableDownload* vc = [[UITableDownload alloc] initWithStyle:UITableViewStylePlain];
UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:vc];
[nav setModalPresentationStyle:UIModalPresentationFormSheet];
[MainAppDelegate mainBrowserviewController] presentModalViewController:nav animated:YES];

...//after shown,sometime I implement following code:
[nav setModalPresentationStyle:UIModalPresentationFullScreen];//(The "nav" object is the same as above codE)
//But the change style code takes no effect.

[self.modalViewController setModalPresentationStyle:UIModalPresentationFullScreen];

[nav setModalPresentationStyle:UIModalPresentationFullScreen];