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


import Foundation

struct ClockTimer {
    let interval: NSTimeInterval
    var timer: dispatch_source_t?


        self.interval = interval
    }

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