Update badge of non selected tabBarItem in Swift

Shorter:

let tabItem = self.tabBarController?.tabBar.items![3]
let tabItem.badgeValue = "34"

No need to select that index to update badge value. Take an array of tab bar items. The select item at the index which you want to update and then set it badge value. See below I have done for 4th tab bar item.

Swift 5.0

if let items = self.tabBarController?.tabBar.items as NSArray? {
    let tabItem = items.object(at: 3) as! UITabBarItem
    tabItem.badgeValue = "34"
}

extension UITabBarController {
    func increaseBadge(indexOfTab: Int, num: String) {
        let tabItem = tabBar.items![indexOfTab]
        tabItem.badgeValue = num
    } 
}

and you can call it like this:

self.tabBarController?.increaseBadge(indexOfTab: 3, num: "34")