UIWindow+PazLabs.h (header file)

#import <UIKit/UIKit.h>

@interface UIWindow (PazLabs)

- (UIViewController *) visibleViewController;

@end

UIWindow+PazLabs.m (implementation file)

#import "UIWindow+PazLabs.h"

@implementation UIWindow (PazLabs)

- (UIViewController *)visibleViewController {
    UIViewController *rootViewController = self.rootViewController;
    return [UIWindow getVisibleViewControllerFrom:rootViewController];
}

+ (UIViewController *) getVisibleViewControllerFrom:(UIViewController *) vc {
    if ([vc isKindOfClass:[UINavigationController class]]) {
        return [UIWindow getVisibleViewControllerFrom:[((UINavigationController *) vc) visibleViewController]];
    } else if ([vc isKindOfClass:[UITabBarController class]]) {
        return [UIWindow getVisibleViewControllerFrom:[((UITabBarController *) vc) selectedViewController]];
    } else {
        if (vc.presentedViewController) {
            return [UIWindow getVisibleViewControllerFrom:vc.presentedViewController];
        } else {
            return vc;
        }
    }
}

@end

參考链接：http://stackoverflow.com/questions/11637709/get-the-current-displaying-uiviewcontroller-on-the-screen-in-appdelegate-m