在 UIAlertView 和 UIActionSheet 的代理方法中給 [UIApplication sharedApplication].keyWindow 設置 rootViewController 時，假如使用下面代碼：

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    if (buttonIndex == 1) {
        TheViewController *vc = [[TheViewController alloc] init];
        [UIApplication sharedApplication].keyWindow.rootViewController = vc;
    }
}

And

- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex
{
    if (buttonIndex == 0) {
        TheViewController *vc = [[TheViewController alloc] init];
        [UIApplication sharedApplication].keyWindow.rootViewController = vc;
    }
}

Then，在程序執(zhí)行到這里的時候，你會發(fā)現(xiàn)，界面雖然會跳轉到 TheViewController，但是在一眨眼間又跳轉回原來的界面。

解決辦法：#####

1、要先導入 AppDelegate

#import "AppDelegate.h"

2、使用下面代碼來實現(xiàn)

TheViewController *vc = [[TheViewController alloc] init];
((AppDelegate *)[[UIApplication sharedApplication] delegate]).window.rootViewController = vc;