Swift Delegation: unexpectedly found nil while unwrapping an Optional value - swift3

Question

Edit: Please note the question below discusses using delegation between
2 viewcontrollers that are also implemented in a UITabBarController.
I've done a fair bit of searching here and on YouTube, but haven't seen my issue replicated elsewhere. I'll keep it to the point.
I have 2 view controllers that I coded myself -not generated by XCode-; TabOneController, and TabTwoController
below are the coding for both...
import UIKit
class TabOneController: UIViewController{
private let instanceOfTabOneView = TabOneView()
var vc1Delegate: fromOneToTwo!
override func loadView() {
super.loadView()
view.addSubview(instanceOfTabOneView.buildTheVu())
view.backgroundColor = UIColor.white
runThisOnce()
}
func runThisOnce(){
vc1Delegate.passTheValue(heroNameIs: "pass this to TabTwoController")
}
}
protocol fromOneToTwo{
func passTheValue(heroNameIs: String)
}
as for tab 2...
import UIKit
class TabTwoController: UIViewController, fromOneToTwo{
private let instanceOfTabTwoView = TabTwoView()
override func loadView() {
super.loadView()
view.addSubview(instanceOfTabTwoView.buildTheVu())
assignDelegateToSelf()
}
func assignDelegateToSelf(){
let instanceTabOne = TabOneController()
instanceTabOne.vc1Delegate = self
}
func passTheValue(heroNameIs:String){
instanceOfTabTwoView.txtFld.text = heroNameIs
}
}
I'm getting the following error at runtime -the app builds successfully-...
fatal error: unexpectedly found nil while unwrapping an Optional value
on the following line...
vc1Delegate.passTheValue(heroNameIs: "pass this to TabTwoController")
When I comment out the above line, the app builds and runs successfully, but of course the app doesn't execute the delegation.
I kinda understand what the compiler is trying to tell me, that the
vc1Delegate
hasn't been instantiated -I guess-. But I searched under every rock, and can't seem to find how to get around this.
I'd appreciate any help or guidance. Sorry if my code seems immature, I'm new to Swift and programming in general. Thank you.

Answer 1

In a UITabBarController, the first tab is instantiating by default. The view controller initialization executes the loadView and finds a nil because the second tab did not initialize yet; this is normal behavior. My suggestion is making the delegate weak optional with the ? suffix and run the delegate code elsewhere. Also, always capitalize the first letter in class and protocol names.
weak var vc1Delegate: FromOneToTwo?
If this structure is mandatory, try with a custom notification observer instead.

Answer 2

First thing first, your error happens in line var vc1Delegate: fromOneToTwo! while you declared this delegate variable as not null but then calling passTheValue on it. A correct practice will be
var vc1Delegate: fromOneToTwo?
func runThisOnce(){
if let delegate = vc1Delegate {
vc1Delegate.passTheValue(heroNameIs: "pass this to TabTwoController")
}
}
Secondly, you are not using delegate correctly. In the assignDelegateToSelf() function, you are creating a new instance of TabOneController
and then assign delegate. Instead, you need to find out the existing TabOneController instance and assign delegate.

Answer 3

I try this and worked add delegate = self in table cellforRowAt
like this
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
{
let cell = tableView.dequeueReusableCell(withIdentifier: "ChapterPracticeTableViewCell") as! ChapterPracticeTableViewCell
cell.deligate = self
return cell
}