在做项目时，有时候，会写一个工具类来对项目进行操作：
这个时候如果需要使用当前控制器去进行一些操作，比如想去present一个alertController，
这时候就需要获取到当前控制器了，下面就是获取的方法：根据每个App都是一个单例来获取当前窗口的根控制器，从而获取到所有的控制器，找到

+ (UIViewController *)getCurrentVC {
    
    UIWindow *window = [[UIApplication sharedApplication].windows firstObject];
    if (!window) {
        return nil;
    }
    UIView *tempView;
    for (UIView *subview in window.subviews) {
        if ([[subview.classForCoder description] isEqualToString:@"UILayoutContainerView"]) {
            tempView = subview;
            break;
        }
    }
    if (!tempView) {
        tempView = [window.subviews lastObject];
    }
    
    id nextResponder = [tempView nextResponder];
    while (![nextResponder isKindOfClass:[UIViewController class]] || [nextResponder isKindOfClass:[UINavigationController class]] || [nextResponder isKindOfClass:[UITabBarController class]]) {
        tempView =  [tempView.subviews firstObject];
        
        if (!tempView) {
            return nil;
        }
        nextResponder = [tempView nextResponder];
    }
    return  (UIViewController *)nextResponder;
}

swift写法

class dynamic func getCurrentController() -> UIViewController? {
        
        guard let window = UIApplication.shared.windows.first else {
            return nil
        }
        
        var tempView: UIView?
        
        for subview in window.subviews.reversed() {
            
            
            if subview.classForCoder.description() == "UILayoutContainerView" {
                
                tempView = subview
                
                break
            }
        }
        
        if tempView == nil {
            
            tempView = window.subviews.last
        }
        
        var nextResponder = tempView?.next
        
        var next: Bool {
            return !(nextResponder is UIViewController) || nextResponder is UINavigationController || nextResponder is UITabBarController
        }
        
        while next{
            
            tempView = tempView?.subviews.first
            
            if tempView == nil {
                
                return nil
            }
            
            nextResponder = tempView!.next
        }
        
        return nextResponder as? UIViewController
    }
}