type casting when creating an instance from a parent class

Question

I would like to ask something about type-casting in Swift.

There are 2 classes.

RootViewController
MyViewController

and the class hierarchy is like below:

class RootViewController: UIViewController {

}

class MyViewController: RootViewController {

}

and, I want to simply call instance function to create an instance from xib file. so I implemented below function in RootViewController.

Objective-C

+ (instancetype)instance {
    return [[[self class] alloc] initWithNibName:NSStringFromClass([self class]) bundle:nil];
}

Swift

public class func instance<T:RootViewController>() -> T {
    let type = self as UIViewController.Type
    let name = NSStringFromClass(type).components(separatedBy: ".").last!
    let instance = type.init(nibName: name, bundle: nil)
    return instance as! T
}

and, usage is like below.

Objective-C

MyViewController *vc = [MyViewController instance];

Swift

let vc = MyViewController.instance() as! MyViewController

Question:

Do I have to always cast the type of instance using as! MyViewController in Swift? Or can anybody advise me a better approach in Swift?

Any help would be appreciated!

I don't see any problem or question here. I would personally avoid using `instance` or any other factory methods in swift, because you can't use `Self` as a return type. Use regular `init` methods instead. — kelin, Jun 19 '17 at 09:48
And, I must add, this question is better to be asked on https://codereview.stackexchange.com/ — kelin, Jun 19 '17 at 09:53
Compare [Return instancetype in Swift](https://stackoverflow.com/q/33200035/2976878) – you want a return type of `Self`, using a generic helper function in order to perform the cast. — Hamish, Jun 19 '17 at 09:54

score 3 · Answer 1 · answered Jun 19 '17 at 09:49

3

You can Also use like this way

let vc:MyViewController = MyViewController.instance()

answered Jun 19 '17 at 09:49

hardikdevios

1,779
14
33

Orkhan Alikhanov · Accepted Answer · 2017-06-19T11:24:57.840

2

class RootViewController: UIViewController {
    public class func instance() -> Self {
        func inner<T: RootViewController>(type: T.Type) -> T {
            let name = NSStringFromClass(type).components(separatedBy: ".").last!
            let type1 = type as UIViewController.Type
            let instance = type1.init(nibName: name, bundle: nil)
            return instance as! T
        }
        return inner(type: self)
    }
}

I would suggest creating an extension method:

extension UIViewController {
    public class func instance() -> Self {
        func inner<T: UIViewController>(type: T.Type) -> T {
            let name = NSStringFromClass(type).components(separatedBy: ".").last!
            return T(nibName: name, bundle: nil)
        }
        return inner(type: self)
    }
}

edited Jun 19 '17 at 11:24

answered Jun 19 '17 at 09:46

Orkhan Alikhanov

9,122
3
39
60

1

This wouldn't even compile. – kelin Jun 19 '17 at 09:50
@kelin As if OP's does. – Orkhan Alikhanov Jun 19 '17 at 09:52
@kelin check now. – Orkhan Alikhanov Jun 19 '17 at 10:08
@OrkhanAlikhanov, now I'm getting the following error: "Cannot convert value of type 'Self.Type' to expected argument type 'RootViewController.Type'" – kelin Jun 19 '17 at 10:39
@kelin weird I am not getting it. – Orkhan Alikhanov Jun 19 '17 at 10:41
@kelin Although I was not getting that error, I removed the usage of `self` from the `inner` method. Can you try now? – Orkhan Alikhanov Jun 19 '17 at 10:50
1

@kelin I run it and it failed to cast, fixed. Now working. – Orkhan Alikhanov Jun 19 '17 at 11:28

score 0 · Answer 3 · answered Jun 19 '17 at 09:55

Okay you can instantiate in these three ways:

Swift inference to Type:

let myVC = RootViewController.instance()   //Swift will automatically infer the type

Explicitly telling the Type:

let myVC: RootViewController = RootViewController.instance()

Casting to your Type:

let myVC = RootViewController.instance() as! RootViewController

All these three are valid.

type casting when creating an instance from a parent class

3 Answers3