Tricky Swift: Subclass adoption of protocol by binding to superclass.

How to debug different things?

Well, I have several questions about protocol CustomDebugStringConvertible. Is it really enough to describe all variates and types of information? Is it really cool if all debug data is a string?

You could answer on this question as you wish, I just show an illustration of another protocol which helps you to present debug data differently.

Introduction

Firts of all, assume that you have a complex object which you want to debug. For example, you start debug Core Data Entity or an UIView. And when you stop process execution at breakpoint, you take a deep breath and look at debug inspector. It is not a best place to dig into. You would like to group all related data in one method and print to console it.
For example, I would like to know the count of subviews or the types or any information about its sizes. And it is only the information that I would like to know.
Or I would like to be able to inspect only a group of fields in Core Data Entity by input only one method.

Is it a good decision to use DebugDescription?

A Debug Protocol

Let me add another protocol

protocol DebugInformationProtocol {
    typealias InformationType = [AnyHashable: CustomDebugStringConvertible]
    func debugInformation() -> InformationType
}

This protocol could show you a debugInformation about an object. Moreover, It could show you a set of different information about an object. You could add what you want.

Without loss of generality we could adopt this item by NSObject as an ancestor of UIView and NSManagedObject.

extension NSObject: DebugInformationProtocol {
    func debugInformation() -> DebugInformationProtocol.InformationType {
        return ["debugDescription": debugDescription]
    }
}

That’s all! You has one method for all cases of debugging process? What if you want to extend this protocol by only one method for debug only for a specific subclass or subclass hierarchy. NSObject hierarchy is so huge!

Extension only for a subset of subclasses

Suppose, that you would like to add different methods for UIView hierarchy and for NSManagedObject hierarchy.

For example, you would like to output a count of UIButtons in its subviews layer. Or you would like to output attributesByName property for NSManagedObject subclasses.

Let’s do it via protocol extension!

extension DebugInformationProtocol where Self: UIView {
    func debugInformationAboutSubviews() -> Int {
        return self.subviews.filter { $0 is UIButton }.count
    }
}

extension DebugInformationProtocol where Self: NSManagedObject {
    func debugInformationAboutEntity() -> InformationType {
        return entity.attributesByName
    }
}

Don’t forget about some tests.

class CustomControl: UIView {
    func test() {
        self.debugInformationAboutSubviews()
        self.debugInformation()
    }
}

class DatabaseObject: NSManagedObject {
    func test() {
        self.debugInformationAboutEntity()
        self.debugInformation()
    }
}

The Conclusion

An interesting protocol feature was described in this post. You could expand protocols on any class by adding constraint which binds Self to custom class hierarchy.

This is a test case which shows a desired behavior.

class DebugInformationTests {
    class CustomControl: UIView {
        func test() {
            self.debugInformationAboutSubviews()
            self.debugInformation()
        }
    }
    
    class DatabaseObject: NSManagedObject {
        func test() {
            self.debugInformationAboutEntity()
            self.debugInformation()
        }
    }
}