code: @Override
      public void onResume() {
          super.onResume();
          if (!mFirstResume) {
              onBack();
          }
          if (mFirstResume) {
              mFirstResume = false;
          }
          if (DEBUG) {
              showStatus("onResume");
          }
      }
docs: Only when Activity resume, not very precise.
  When activity recover from partly invisible, onBecomesPartiallyInvisible will be triggered.