     private func finishTracking() {
         UIView.animateWithDuration(0.4, animations: { () -> Void in
             self.resetCaliperRects()
-            self.calipersAndTracks.map({ $0.alpha = 0 })
+            (self.calipers + self.tracks).map({ $0.alpha = 0 })
         })
     }
 }

    private var trackRect = CGRectZero
    private let calipers:[UIView]
    private let tracks:[UIView]
    private let calipersAndTracks:[UIView]
    
    //MARK: - Setup Etc.
    
    required init(coder aDecoder: NSCoder) {
        calipers = [leftCaliper, rightCaliper]
        tracks = [leftTrack, rightTrack]
        calipersAndTracks = calipers + tracks
        super.init(coder: aDecoder)
    }
    

        calipers.map({ self.styleCaliperView($0) })
        tracks.map({ self.styleTrackView($0) })
        
        (calipers + tracks).map({ $0.alpha = 0 })
        (calipers + tracks).map({ self.superview?.addSubview($0) })
        
        resetCaliperRects()
    }
