第一步：

#import "AppDelegate.h"

步骤二：

在须要跳转的地方：

    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

    UITabBarController *tabViewController = (UITabBarController *) appDelegate.window.rootViewController;

    [tabViewController setSelectedIndex:2];

參考：

http://*.com/questions/10307350/any-code-example-of-how-access-viewcontroller-from-uitabbarcontroller