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}