如何在保留数据的同时将UIViewController下转换为子类的子类呢？

Question

Currently I'm working on a project and attempting to downcast from a viewcontroller instantiated from the storyboard using this method

let cl = self.storyboard?.instantiateViewController(withIdentifier: "contact_list_view")
let contact_list_controller = cl as? ContactListViewController

And yes currently the identifier and the view controller type for the view is set in the storyboard. However the problem I'm having is once it downcast the member variables don't persist.

Right now: ContactListViewController inherits from UITableViewController and that inherits from UIViewController. Is there some sort of issue downcasting in regards to multiple inheritance in an earlier project I was working on I didn't come across this issue using a single level inheritance (A to B instead of currently A to C).

@IBAction func switchToContractViewForAdd(_ sender: Any) 
    isAdd_or_edit = true;string = "hello"   
    let contact_view=self.storyboard?.instantiateViewController(withIdentifier:"contact_view")
    self.navigation?.pushViewController(contact_view!, animated: true)
}

override func viewDidLoad() {
    super.viewDidLoad()
    let cl = (self.storyboard?.instantiateViewController(withIdentifier: "contact_list_view"))
    contact_list_controller = cl as? ContactListViewController
    contact_list_controller?.string ?? "default")
}

Alright so here is a snippet of my code basically whats happening is the first function is acting as a segue transition to show the "contact_view". Once the contact view is loaded I want to access some of the values from the previous view "contact_list_view" (for simplicity purposes I added a string variable called string with a default value of "hi" and on the first function call I change it "hello" before performing the segue). After attempting to downcast the the "contact_list_view" when retrieving it from the storyboard the value of string is "hi" once again.

Answer 1

The reason (well, part of the reason) you are not getting the changed value of "hello" is because:

override func viewDidLoad() {
    super.viewDidLoad()
    let cl = (self.storyboard?.instantiateViewController(withIdentifier: "contact_list_view"))
    contact_list_controller = cl as? ContactListViewController
    contact_list_controller?.string ?? "default")
}

instantiateViewController creates a new instance of the specified view controller each time you call it. So, no matter what else you are doing to it after that line, you will never get the value you were trying to set in switchToContractViewForAdd()

Instead, you need to look into "passing data between view controllers" (you'll find uncountable examples and explanations out there by simple searching).