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Aprove answer 
-(BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController {
tmpController = viewController;
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Title" message:@"Message" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:@"OK", nil];
[alert show];
[alert release];
return NO;
}

-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
if (buttonIndex) {
    self.tabBarController.selectedViewController = tmpController;
}
}

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