         self.interval = interval
     }
 
-    mutating func start(callback: (NSDate) -> ()) {
+    func start(callback: (NSDate) -> ()) {
         if let t = timer {
             dispatch_source_cancel(t)
         }

        timer = t
    }

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


import Foundation

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

