how do you use unwind segue when you have multiple ViewController that calls it

Question

I started learning iOS/Swift recently and wondering how to use unwind segue.

Assume the following view controller A, B, C, E, F, X, BB, BBB

C -> A -> E
C -> B -> BB -> E
C -> B -> BBB -> E
C -> D -> E
F -> X -> E

How do I use unwind segue to go back to C or B or D from E?

Note: In ViewController E, there is only one Back button.

I have tried the following solution:

// Add this method in C, B, and D view controller
@IBAction func unwindFromE(sender: UIStoryboardSegue) {}

This seems to work some how, but why did it unwind back to C for F -> X -> E even though in view controller F/X there's no unwindFromE method.

Answer 1

As you have discovered, you can implement the unwind target @IBAction in multiple viewControllers and iOS will go up the call chain until it finds one. This allows viewController E to return to whomever called it as long as they implement the target function unwindFromE.

In the case of your E unwind returning to C when the call stack was F->X->E, this happens when C and F are viewControllers controlled by the same UITabBarController. Since F doesn't implement unwindFromE, iOS searches the other viewControllers controlled by the UITabBarController. In your case, it found C and switched to that tab.

More information about unwind segues can be found in Technical Note TN2298: Using Unwind Segues.