how to call a function from a view controller from UITabBarControllerDelegate when a tab bar item is pressed?

Question

How do I call a certain function from a UIViewController Here is my code file, I want to call a function say, viewController.somefunction() when a item is pressed or say when the viewController changes

import UIKit
import WebKit

class tabbarController: UITabBarController, UITabBarControllerDelegate{

    override func tabBar(_ tabBar : UITabBar, didSelect item: UITabBarItem){
        print("selected item")
        
    }
    
    func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController){
        
        print("calling custom function here from viewController");
        
    
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.delegate = self
    }


    
    
    

}

Answer 1

You need to get a reference to the view controller so you can call the function in it.

Quick example, assuming you have setup in Storyboard a TabBarController, assign its Custom Class as MyCustomTabBarController, with two tabs - FirstTabVC and SecondTabVC:

class MyCustomTabBarController: UITabBarController, UITabBarControllerDelegate {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        delegate = self
    }

    // this is called *when* the tab item is selected (tapped)
    override func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {

        // safely unwrap optionals    
        guard let theItems = self.tabBar.items,
              let idx = theItems.firstIndex(of: item),
              let controllers = self.viewControllers
        else { return }
        
        if let vc = controllers[idx] as? FirstTabVC {
            vc.someFunctionInFirst("From didSelect item")
        }
        if let vc = controllers[idx] as? SecondTabVC {
            vc.someFunctionInSecond("From didSelect item")
        }

    }

    // this is called when the tab's ViewController is selected (*after* the tab item is selected (tapped))
    func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
        if let vc = viewController as? FirstTabVC {
            vc.someFunctionInFirst("From didSelect viewController")
        }
        if let vc = viewController as? SecondTabVC {
            vc.someFunctionInSecond("From didSelect viewController")
        }
    }
    
}

class FirstTabVC: UIViewController {
    public func someFunctionInFirst(_ str: String) {
        print("In First Tab VC: ", str)
    }
}
class SecondTabVC: UIViewController {
    public func someFunctionInSecond(_ str: String) {
        print("In Second Tab VC: ", str)
    }
}