self is immutable - macos

Question

I have a class, Task, with a 'count' property. The end goal is to increase the count by 1.
In a playground, this is working:
class Task: NSObject {
var name: String
var count: Int
init(name:String, count:Int) {
self.name = name
self.count = count
}
}
var test = Task(name: "test", count: 0)
test.count += 1
// => 1
But in my actual project, it's getting this error:
Left side of mutating operator isn't mutable: 'self' is immutable
In my ViewController, I have an #IBAction function that I'd like to trigger the count increase, so that's where the error is occurring. Here's a trimmed down version:
#IBAction func addTask(sender: AnyObject) {
currentTask = newTaskInput.stringValue
let x = Task(name: currentTask, count: 0)
currentTaskObj = x
// error occurs here
currentTaskObj!.count += 1
}
Any idea how to fix this? I must be missing something because it's working in a playground exactly how I'd like. Huge thanks in advance!
Edit: here's the actual class from my project (I changed sessions to count for the examples above):
class Task: NSObject, NSCoding {
var name: String
var sessions: Int
var time: String = ""
init(name:String, sessions:Int, time:String? = "") {
self.name = name
self.sessions = sessions
self.time = time!
}
required convenience init(coder aDecoder: NSCoder) {
var name = aDecoder.decodeObjectForKey("name") as! String
var sessions = aDecoder.decodeObjectForKey("sessions") as! Int
var time = aDecoder.decodeObjectForKey("time") as! String
self.init(name: name, sessions: sessions, time: time)
}
func encodeWithCoder(aCoder: NSCoder) {
aCoder.encodeObject(self.name, forKey: "name")
aCoder.encodeObject(self.sessions, forKey: "sessions")
aCoder.encodeObject(self.time, forKey: "time")
}
}