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Cannot decode object of class

Posted by: admin November 30, 2017 Leave a comment

Questions:

I am trying to send a “Class” to my Watchkit extension but I get this error.

* Terminating app due to uncaught exception ‘NSInvalidUnarchiveOperationException’, reason: ‘*
-[NSKeyedUnarchiver decodeObjectForKey:]: cannot decode object of class (MyApp.Person)

Archiving and unarchiving works fine on the iOS App but not while communicating with the watchkit extension. What’s wrong?

InterfaceController.swift

    let userInfo = ["method":"getData"]

    WKInterfaceController.openParentApplication(userInfo,
        reply: { (userInfo:[NSObject : AnyObject]!, error: NSError!) -> Void in

            println(userInfo["data"]) // prints <62706c69 7374303...

            if let data = userInfo["data"] as? NSData {
                if let person = NSKeyedUnarchiver.unarchiveObjectWithData(data) as? Person {
                    println(person.name)
                }
            }

    })

AppDelegate.swift

func application(application: UIApplication!, handleWatchKitExtensionRequest userInfo: [NSObject : AnyObject]!,
    reply: (([NSObject : AnyObject]!) -> Void)!) {

        var bob = Person()
        bob.name = "Bob"
        bob.age = 25

        reply(["data" : NSKeyedArchiver.archivedDataWithRootObject(bob)])
        return
}

Person.swift

class Person : NSObject, NSCoding {
    var name: String!
    var age: Int!

    // MARK: NSCoding

    required convenience init(coder decoder: NSCoder) {
        self.init()
        self.name = decoder.decodeObjectForKey("name") as! String?
        self.age = decoder.decodeIntegerForKey("age")
    }

    func encodeWithCoder(coder: NSCoder) {
        coder.encodeObject(self.name, forKey: "name")
        coder.encodeInt(Int32(self.age), forKey: "age")
    }
}
Answers:

NOTE: While the information in this answer is correct, the way better answer is the one below by @agy.

This is caused by the compiler creating MyApp.Person & MyAppWatchKitExtension.Person from the same class. It’s usually caused by sharing the same class across two targets instead of creating a framework to share it.

Two fixes:

The proper fix is to extract Person into a framework. Both the main app & watchkit extension should use the framework and will be using the same *.Person class.

The workaround is to serialize your class into a Foundation object (like NSDictionary) before you save & pass it. The NSDictionary will be code & decodable across both the app and extension. A good way to do this is to implement the RawRepresentable protocol on Person instead.

Questions:
Answers:

According to Interacting with Objective-C APIs:

When you use the @objc(name) attribute on a Swift class, the class is made available in Objective-C without any namespacing. As a result, this attribute can also be useful when you migrate an archivable Objective-C class to Swift. Because archived objects store the name of their class in the archive, you should use the @objc(name) attribute to specify the same name as your Objective-C class so that older archives can be unarchived by your new Swift class.

By adding the annotation @objc(name), namespacing is ignored even if we are just working with Swift. Let’s demonstrate. Imagine target A defines three classes:

@objc(Adam)
class Adam:NSObject {
}

@objc class Bob:NSObject {
}

class Carol:NSObject {
}

If target B calls these classes:

print("\(Adam().classForCoder)")
print("\(Bob().classForCoder)")
print("\(Carol().classForCoder)")

The output will be:

Adam
B.Bob
B.Carol

However if target A calls these classes the result will be:

Adam
A.Bob
A.Carol

To resolve your issue, just add the @objc(name) directive:

@objc(Person)
class Person : NSObject, NSCoding {
    var name: String!
    var age: Int!

    // MARK: NSCoding

    required convenience init(coder decoder: NSCoder) {
        self.init()
        self.name = decoder.decodeObjectForKey("name") as! String?
        self.age = decoder.decodeIntegerForKey("age")
    }

    func encodeWithCoder(coder: NSCoder) {
        coder.encodeObject(self.name, forKey: "name")
        coder.encodeInt(Int32(self.age), forKey: "age")
    }
}

Questions:
Answers:

I had to add the following lines after setting up the framework to make the NSKeyedUnarchiver work properly.

Before unarchiving:

NSKeyedUnarchiver.setClass(YourClassName.self, forClassName: "YourClassName")

Before archiving:

NSKeyedArchiver.setClassName("YourClassName", forClass: YourClassName.self)

Questions:
Answers:

I had a similar situation where my app used my Core framework in which I kept all model classes. E.g. I stored and retrieved UserProfile object using NSKeyedArchiver and NSKeyedUnarchiver, when I decided to move all my classes to MyApp NSKeyedUnarchiver started throwing errors because the stored objects were like Core.UserProfile and not MyApp.UserProfile as expected by the unarchiver. How I solved it was to create a subclass of NSKeyedUnarchiver and override classforClassName function:

class SKKeyedUnarchiver: NSKeyedUnarchiver {
    override open func `class`(forClassName codedName: String) -> Swift.AnyClass? {
        let lagacyModuleString = "Core."
        if let range = codedName.range(of: lagacyModuleString), range.lowerBound.encodedOffset == 0  {
            return NSClassFromString(codedName.replacingOccurrences(of: lagacyModuleString, with: ""))
        }
        return NSClassFromString(codedName)
    }
}

Then added @objc(name) to classes which needed to be archived, as suggested in one of the answers here.

And call it like this:

if let unarchivedObject = SKKeyedUnarchiver.unarchiveObject(withFile: UserProfileServiceImplementation.archiveURL.path) as? UserProfile {
    currentUserProfile = unarchivedObject
}

It worked very well.

The reason why the solution NSKeyedUnarchiver.setClass(YourClassName.self, forClassName: "YourClassName") was not for me because it doesn’t work for nested objects such as when UserProfile has a var address: Address. Unarchiver will succeed with the UserProfile but will fail when it goes a level deeper to Address.

And the reason why the @objc(name) solution alone didn’t do it for me was because I didn’t move from OBJ-C to Swift, so the issue was not UserProfile -> MyApp.UserProfile but instead Core.UserProfile -> MyApp.UserProfile.