Override swift protocol property to optional

I am using this library, https://github.com/gcharita/XMLMapper.

It contains protocol,

public protocol XMLBaseMappable {
    var nodeName: String! { get set }
}

I would like to make this nodeName optional for my struct/classes implementing it, like

public protocol CustomXMLBaseMappable {
    var nodeName: String? { return "default" }

    //I don't want struct/classes using it, implements `nodeName`.
}

Any suggestions will be helpful.

The existence of this property in XMLBaseMappable protocol is crucial in order to function correctly the whole library.

Having said that, you can't omit the implementation of this property in your structs and classes but you can "hide" it in a super class. Using this:

class BasicXMLMappable: XMLMappable {
    var nodeName: String!

    required init(map: XMLMap) {

    }

    func mapping(map: XMLMap) {

    }
}

You can have XMLMappable objects that extends BasicXMLMappable and they don't have to implement nodeName property:

class TestBasicXMLMappable: BasicXMLMappable {

    // Your custom properties

    required init(map: XMLMap) {
        super.init(map: map)
    }

    override func mapping(map: XMLMap) {
        // Map your custom properties
    }
}

Edit: As of version 1.5.1 you can use XMLStaticMappable for implementing XMLMapper in an extension. For example:

class CustomClass {
    var property: String?
}

extension CustomClass: XMLStaticMappable {
    var nodeName: String! {
        get {
            return "default"
        }
        set(newValue) {

        }
    }

    static func objectForMapping(map: XMLMap) -> XMLBaseMappable? {
        // Initialize CustomClass somehow
        return CustomClass()
    }

    func mapping(map: XMLMap) {
        property <- map["property"]
    }
}

Hope this helps.