改变view的大小

/**
*  切换城市
*/
- (IBAction)changeCity {
MTCityViewController *city = [[MTCityViewController alloc] init];
MTNavigationController *nav = [[MTNavigationController alloc] initWithRootViewController:city];
nav.modalPresentationStyle = UIModalPresentationFormSheet;
[self presentViewController:nav animated:YES completion:nil];

// modal出来的是MTNavigationController
// dismiss掉的应该也是MTNavigationController
}