Cannot decode object of class

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.

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")
    }
}

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)