 
 import Foundation
 
-struct ClockTimer {
+class ClockTimer {
     let interval: NSTimeInterval
     var timer: dispatch_source_t?
 

        timer = t
    }

    mutating func stop() {
        if let t = timer {
            dispatch_source_cancel(t)
        }

        self.interval = interval
    }

    func start(callback: (NSDate) -> ()) {
        if let t = timer {
            dispatch_source_cancel(t)
        }
