nsobject_prefer_isequal.md

Pattern: Implementing == for NSObject

Issue: -

Description

NSObject already exposes an implementation of == that calls out to isEqual. Adding a new implementation of == to an NSObject subclass can lead to odd situations where a == b is true, but (a as NSObject) == b (or, equivalently a.isEqual(b)) is false.

Examples of correct code:

class AClass: NSObject {
}


@objc class AClass: SomeNSObjectSubclass {
}


class AClass: Equatable {
    static func ==(lhs: AClass, rhs: AClass) -> Bool {
        return true
    }


class AClass: NSObject {
    override func isEqual(_ object: Any?) -> Bool {
        return true
    }
}


@objc class AClass: SomeNSObjectSubclass {
    override func isEqual(_ object: Any?) -> Bool {
        return false
    }
}


class AClass: NSObject {
    func ==(lhs: AClass, rhs: AClass) -> Bool {
        return true
    }
}


class AClass: NSObject {
    static func ==(lhs: AClass, rhs: BClass) -> Bool {
        return true
    }
}


struct AStruct: Equatable {
    static func ==(lhs: AStruct, rhs: AStruct) -> Bool {
        return false
    }
}


enum AnEnum: Equatable {
    static func ==(lhs: AnEnum, rhs: AnEnum) -> Bool {
        return true
    }
}

Examples of incorrect code:

class AClass: NSObject {
    ↓static func ==(lhs: AClass, rhs: AClass) -> Bool {
        return false
    }
}


@objc class AClass: SomeOtherNSObjectSubclass {
    ↓static func ==(lhs: AClass, rhs: AClass) -> Bool {
        return true
    }
}


class AClass: NSObject, Equatable {
    ↓static func ==(lhs: AClass, rhs: AClass) -> Bool {
        return false
    }
}


class AClass: NSObject {
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? AClass else {
            return false
        }
        return true
    }

    ↓static func ==(lhs: AClass, rhs: AClass) -> Bool {
        return false
    }
}

Further Reading

• SwiftLint - NSObject Prefer isEqual