Swift 'open' keyword & overridable method/properties in extension?

Question

With introduction of open keyword in Swift 3.0 (What is the 'open' keyword in Swift?).

Note: Limited to extensions on NSObject derived classes or @objc attributed method/properties.

Code which declared and used public (class) methods/properties in extension across modules/frameworks broke, as public is no longer means 'overridable' outside of defining module.

Example:

public extension UIManagedDocument {

    public class func primaryDocumentName() -> String {
        return "Document"
    }

    public class func primaryStoreURL() -> URL {
        let documentsURL = FileManager.default.userDocumentsURL
        return URL(fileURLWithPath: self.primaryDocumentName(), isDirectory: false, relativeTo: documentsURL)
    }

    public class func primaryModelName() -> String? {
        return "Model"
    }

}

• Original proposal (SE-0117) is focused on subclassing and doesn't mention extensions.
• Currently extensions do not support open keyword (you can't write open extension NSObject as well as open func Method())

Question: Is there workaround to be able override extension provided methods/properties across modules/frameworks?

Answer 1

Unless I am mistaken, you can declare the extension methods as open in your framework if you just omit the public keyword in the extension declaration:

extension UIManagedDocument {

    open class func primaryDocumentName() -> String {
        return "Document"
    }
    // ...
}

And then (for NSObject subclasses or @objc members) you can override the method in your custom subclass in the main application (or in any module):

class MyManagedDocument: UIManagedDocument {

    override class func primaryDocumentName() -> String {
        return "MyDocument"
    }
    // ...
}

Answer 2

• 'Protocol-oriented' - declare protocol with desired methods/properties then refactor your extension for protocol compliance.
• 'Traditional' - implement intermediate (abstract) subclass with desired methods/properties.

Protocol example:

protocol PrimaryDocument {
    static func primaryDocumentName() -> String

    static func primaryStoreURL() -> URL

    static func primaryModelName() -> String?
}

extension UIManagedDocument : PrimaryDocument {

    open class func primaryDocumentName() -> String {
        return "Document"
    }

    open class func primaryStoreURL() -> URL {
        let documentsURL = FileManager.default.userDocumentsURL
        return URL(fileURLWithPath: self.primaryDocumentName(), isDirectory: false, relativeTo: documentsURL)
    }

    open class func primaryModelName() -> String? {
        return "Model"
    }

}