How do I set the default tab for tab bar controller in swift

User Omkar responded above with the correct answer. I can successfully switch the first tab using the following viewDidAppear in ThirdViewController.swft

override func viewDidAppear(animated: Bool) {         
    self.tabBarController?.selectedIndex = 0     
}

I followed @Paolo Sangregorio way The code to do it as follows

in appdelegate find applicationDidBecomeActive function and add this lines

let tabBarController = self.window?.rootViewController as! UITabBarController
tabBarController.selectedIndex = 0