使用具有通用数据类型的协议在屏幕之间传递数据

Question

I am Android Developer that started learning iOS. I am trying to pass data between the master-detail style app. I got controller1 that has a list of ToDo items, and controller2 that allows to create a new ToDo item and add it to the list on controller1 .

I have created a protocol:

protocol ListDataHolder {
    
    associatedtype T
    
    func addItem(item: T)
    
    func reloadData()
}

Assigned self in prepare of controller1 :

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if let controller2 = segue.destination as? Controller2{
            controller2.toDoDataHolder = self
        }
    } 

Declared delegate in controller2

// how do I tell to use ToDo class for generic type here
var toDoDataHolder: ListDataHolder? = nil

And use it like this:

@IBAction func onAddClicked(_ sender: Any) {
        let toDo = ToDo()
        ...
        toDoDataHolder?.addItem(item: toDo)
        toDoDataHolder?.reloadData()
        navigationController?.popViewController(animated: true)
    }

I got a few errors when going this way:

For delegate declaration:

Protocol 'ListDataHolder' can only be used as a generic constraint because it has Self or associated type requirements

When using addItem() :

Cannot convert value of type 'ToDo' to expected argument type 'ListDataHolder.T'
Insert ' as! ListDataHolder.T'
Member 'addItem' cannot be used on value of protocol type 'ListDataHolder'; use a generic constraint instead

When I remove generic from protocol and just have addItem(item: ToDo) , everything works fine. But I want to be able to use ListDataHolder with any data type.

This is just experimentation for me, I am not looking for a correct way to pass data between controllers.

EDIT: you can find complete code in this GitHub repo: github.com/Sermilion/ios_learning

Answer 1

What you need to do is to make the second view controller generic using the protocol and limit the type of objects being used (or held) by the class conforming to ListDataHolder

This can be done in the declaration of the view controller

class SecondViewController<Holder: ListDataHolder>: UIViewController where Holder.T == ToDo

then your method onAddClicked will work as is.

Answer 2

Instead of using associated type in ListDataHolder protocol use ToDo if your are always passing ToDo instance to the addItem method.

If you want this protocol addItem method will work with any genericType use following code.

protocol ListDataHolder {
    func addItem<T:Decodable>(item: T)
    func reloadData()
}

struct ToDo: Decodable {

}

class A: ListDataHolder {
   
    func addItem<T:Decodable>(item: T) {
          print("Add Item called")
    }
    
    func reloadData(){

    }
}

Here you need to implement Delegation design pattern. Please check Delegation design pattern in detail.