public void addTestingCallback(OneSheeldTestingCallback testingCallback){
  if (testingCallback != null && !testingCallbacks.contains(testingCallback))   testingCallbacks.add(testingCallback);
}

public void addTestingCallback ( OneSheeldTestingCallback testingCallback ) { if ( testingCallback != null && ! testingCallbacks . contains ( testingCallback ) ) testingCallbacks . add ( testingCallback ) ; }
add a testing callback .
