Tap tab bar to scroll to top of UITableViewController

SWIFT 3

Here goes..

First implement the UITabBarControllerDelegate in the class and make sure the delegate is set in viewDidLoad

class DesignStoryStreamVC: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UITabBarControllerDelegate {

 @IBOutlet weak var collectionView: UICollectionView!

 override func viewDidLoad() {
        super.viewDidLoad()

        self.tabBarController?.delegate = self

        collectionView.delegate = self
        collectionView.dataSource = self
    }
}

Next, put this delegate function somewhere in your class.

func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {

    let tabBarIndex = tabBarController.selectedIndex

    print(tabBarIndex)

    if tabBarIndex == 0 {
        self.collectionView.setContentOffset(CGPoint.zero, animated: true)
    }
}

Make sure to select the correct index in the "if" statement. I included the print function so you can double check.

Swift 5: no need for stored properties in the UITabBarController.

In MyTabBarController.swift, implement tabBarController(_:shouldSelect) to detect when the user re-selects the tab bar item:

protocol TabBarReselectHandling {
    func handleReselect()
}

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

    func tabBarController(
        _ tabBarController: UITabBarController,
        shouldSelect viewController: UIViewController
    ) -> Bool {
        if tabBarController.selectedViewController === viewController,
            let handler = viewController as? TabBarReselectHandling {
            // NOTE: viewController in line above might be a UINavigationController,
            // in which case you need to access its contents
            handler.handleReselect()
        }

        return true
    }
}

In MyTableViewController.swift, handle the re-selection by scrolling the table view to the top:

class MyTableViewController: UITableViewController, TabBarReselectHandling {
    func handleReselect() {
        tableView?.setContentOffset(.zero, animated: true)
    }
}

Now you can easily extend this to other tabs by just implementing TabBarReselectHandling.

Implement the UITabBarControllerDelegate method tabBarController:didSelectViewController: to be notified when the user selects a tab. This method is also called when the same tab button is tapped again, even if that tab is already selected.

A good place to implement this delegate would probably be your AppDelegate. Or the object that logically "owns" the tab bar controller.

I would declare and implement a method that can be called on your view controllers to scroll the UICollectionView.

- (void)tabBarController:(UITabBarController *)tabBarController 
 didSelectViewController:(UIViewController *)viewController
{
    static UIViewController *previousController = nil;
    if (previousController == viewController) {
        // the same tab was tapped a second time
        if ([viewController respondsToSelector:@selector(scrollToTop)]) {
            [viewController scrollToTop];
        }
    }
    previousController = viewController;
}