How to Transfer Data Between Parent and Child View Controllers

How to pass data from parent view controller to child container view controller

You can try it more early like

let childVC = storyboard!.instantiateViewController(withIdentifier: "ChildVC") as! ChildVC
childVC.boolVariable = true

Swift - Pass data from parent view controller to child view controller

When you add a Container View in storyboard it adds a child view controller and a segue from the parent view controller to child view controller.

Sample Image

This segue is performed immediately after the parent view controller is loaded. Pass the data from EventDetails to InvitedController in prepare for segue method

class EventsMainController: UIViewController {
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "EventDetails" {
            if let eventDetails = segue.destination as? EventDetails {
                eventDetails.eventId = "FromEventsMainController"
            }
        }
    }
}
class EventDetails: UIViewController {
    var eventId = String()
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {    
        if segue.identifier == "InvitedController" {
            if let invitedController = segue.destination as? InvitedController {
                invitedController.eventId = self.eventId
            }
        }
    }
}
class InvitedController: UIViewController {
    var eventId = String()
    override func viewDidLoad() {
        super.viewDidLoad()
        print(eventId)//FromEventsMainController
    }
}

How to pass data to parent view controller

One strategy would be to set the parent as a delegate of the child. You would create this relationship in a prepare(for:sender:) function.

If you are unfamiliar with delegation, this article is a good guide: https://medium.com/@jamesrochabrun/implementing-delegates-in-swift-step-by-step-d3211cbac3ef

What is the better approach to communicate between parent and child view controllers?

If you simply want to invoke a method on your child view controllers, you could use:

[[self childViewControllers] makeObjectsPerformSelector:@selector(nameOfSelector:) withObject:objectToSend];

or one of the other methods for passing objects along with your selector.

As @Gianluca Tranchedone suggested, you could use delegates or an observer pattern but it really depends on what you need to do and how your code is structured. Making your parent view controller conform to delegates of your child view controllers would allow you to structure your code in a much cleaner way.

Swift - Pass data from Child View to Parent View

So as per the code you have done with declaration but not initialized the delegate. So go to you ParentView and assign the delegate in viewDidLoad or in the function where you prepare to move on child like:-

toChildView.delegate = self

And try again. :-)

Pass data from Parent UIviewController to a Container (ChildViewController) every time the value is changed

After a deeper digging I was able to find a solution for my own question.
here I am going to post if anyone else needs it in the future

so, first of all, I need it to lunch the ChildContoller from the parent controller and not from the storyboard ( so I deleted the segue between the parent and the child.
create a variable for childController like that:

 lazy  var firstChildViewController: FirstChildViewController =  {
        let storynoard = UIStoryboard(name: "Main", bundle: nil)
        let viewController = storynoard.instantiateViewController(identifier: "firstChild") as! FirstChildViewController
    self.addChild(viewController)
    self.view.addSubview(viewController.view)
        return viewController
        
    }()

same thing for the other one if you have two children

and then in the viewDidLoad:

   override func viewDidLoad() {
        
        super.viewDidLoad()
        firstChildViewController.view.isHidden  = false
        secondChildViewController.view.isHidden = true
    }

and then in the FirstChildViewController:

    override func viewDidLoad() {
    
    super.viewDidLoad()
    if let parent = self.parent as? ParentViewController {
     parent.delegate = self
 }

}

And the problem is solved
Hope it helps someone

How can I pass data from a parent view controller to an embedded view controller in Swift?

A way to achiеve this is to get the child view controller instance in the parent's viewDidLoad. It appears that the parent's viewDidLoad: gets called after the child's viewDidLoad:, which means the label is already created in the child's view.

override func viewDidLoad() {
    super.viewDidLoad()

    if let childVC = self.childViewControllers.first as? ChildVC {
        childVC.someLabel.text = "I'm here. Aye-aye."
    }   
}