title: class AppDelegate: UIResponder, UIApplicationDelegate { SIGABIRT ERROR in SWIFT
body: <p>I got this error and I do not know why?
  I have started learning swift today</p>
  
  <pre><code>2016-06-28 02:40:03.393 Calculator[7072:173608] -[Calculator.ViewController touchDigit]: unrecognized selector sent to instance 0x7fc4a9465000
  2016-06-28 02:40:03.400 Calculator[7072:173608] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Calculator.ViewController touchDigit]: unrecognized selector sent to instance 0x7fc4a9465000'
  *** First throw call stack:
  (
      0   CoreFoundation                      0x0000000100c40e65 __exceptionPreprocess + 165
      1   libobjc.A.dylib                     0x0000000102980deb objc_exception_throw + 48
      2   CoreFoundation                      0x0000000100c4948d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
      3   CoreFoundation                      0x0000000100b9690a ___forwarding___ + 970
      4   CoreFoundation                      0x0000000100b964b8 _CF_forwarding_prep_0 + 120
      5   UIKit                               0x000000010145f194 -[UIApplication sendAction:to:from:forEvent:] + 92
      6   UIKit                               0x00000001015ce6fc -[UIControl sendAction:to:forEvent:] + 67
      7   UIKit                               0x00000001015ce9c8 -[UIControl _sendActionsForEvents:withEvent:] + 311
      8   UIKit                               0x00000001015cdaf8 -[UIControl touchesEnded:withEvent:] + 601
      9   UIKit                               0x00000001014ce49b -[UIWindow _sendTouchesForEvent:] + 835
      10  UIKit                               0x00000001014cf1d0 -[UIWindow sendEvent:] + 865
      11  UIKit                               0x000000010147db66 -[UIApplication sendEvent:] + 263
      12  UIKit                               0x0000000101457d97 _UIApplicationHandleEventQueue + 6844
      13  CoreFoundation                      0x0000000100b6ca31 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
      14  CoreFoundation                      0x0000000100b6295c __CFRunLoopDoSources0 + 556
      15  CoreFoundation                      0x0000000100b61e13 __CFRunLoopRun + 867
      16  CoreFoundation                      0x0000000100b61828 CFRunLoopRunSpecific + 488
      17  GraphicsServices                    0x0000000105256ad2 GSEventRunModal + 161
      18  UIKit                               0x000000010145d610 UIApplicationMain + 171
      19  Calculator                          0x0000000100a6361d main + 109
      20  libdyld.dylib                       0x000000010348992d start + 1
  )
  libc++abi.dylib: terminating with uncaught exception of type NSException
  (lldb) 
  </code></pre>
  
  <p>My main class has only this simple method don't know what's happening</p>
  
  <pre><code>import UIKit
  
  internal class ViewController : UIViewController {
  
      @IBAction internal func touchDigit(sender: UIButton)
  }
  </code></pre>
  
  <p>Thank you </p>