1、思路
通过最底层的ViewController依次向上寻找，直到找到最顶层的ViewController，也就是从UIApplication的keyWindow的rootViewController开始寻找（如果有多个UIWindow则要考虑UIWindow的选择问题。
在寻找的过程中，要分别考虑当前ViewController是UITabBarController和UINavigationController的情况，同时还要考虑到当前ViewController是否通过presentViewController:animated:completion:模态展示了其他ViewController
2、实现代码
- (UIViewController *)topViewController {
UIViewController *resultVC;
resultVC = [self _topViewController:[[UIApplication sharedApplication].keyWindow rootViewController]];
while (resultVC.presentedViewController) {
resultVC = [self _topViewController:resultVC.presentedViewController];
}
return resultVC;
}
- (UIViewController *)_topViewController:(UIViewController *)vc {
if ([vc isKindOfClass:[UINavigationController class]]) {
return [self _topViewController:[(UINavigationController *)vc topViewController]];
} else if ([vc isKindOfClass:[UITabBarController class]]) {
return [self _topViewController:[(UITabBarController *)vc selectedViewController]];
} else {
return vc;
}
return nil;
}
