Swift weak var of protocol type (No @objc required)

WeakClassProtocol.swift

import UIKit



// A swift protocol can apply to a class, struct, or enum,
// but only a reference type can be stored in a `weak` var,
// so a weak var cannot be of a typical protocol type.

protocol MyProcotol {
}

class MyViewControllerWithError: UIViewController {
    // ERROR: 'weak' cannot be applied to non-class type 'MyProtocol'
    weak var delegate: MyProcotol?
}



// Swift protocols can be declared to only apply to class types,
// so that any value conforming to the protocol must be a class object.
// This allows the class protocol to be used as the type of a weak var.

protocol MyDelegate: class {
    // ...
}

class MyViewController: UIViewController {
    weak var delegate: MyDelegate? // This compiles! 😄🎉
}



// A class can be declared which conforms to the class protocol,
// but a struct cannot declare a class protocol without causing
// a compiler error:

class MyClass: MyDelegate {
}

// ERROR: Non-class type 'MyStruct' cannot conform to class protocol 'MyDelegate'
struct MyStruct: MyDelegate {
}



// The @objc keyword on a protocol implicitly makes it a class protocol,
// so an @objc protocol cannot be applied to a value type,
// but can be used as the type of a weak var.

@objc protocol MyObjCProtocol {
}

class MyClass2: MyObjCProtocol {
}

// ERROR: Non-class type 'MyStruct2' cannot conform to class protocol 'MyObjCProtocol'
struct MyStruct2: MyObjCProtocol {
}

Reference: The Swift Programming Language: Class-Only Protocols