code: protected synchronized void setupAd(final NativeExpressAdView adView) {
          if(mFetchFailCount > MAX_FETCH_ATTEMPT)
              return;
  
          if(!mPrefetchedAds.contains(adView))
              mPrefetchedAds.add(adView);
          adView.setAdListener(new AdListener() {
              @Override
              public void onAdFailedToLoad(int errorCode) {
                  super.onAdFailedToLoad(errorCode);
                  // Handle the failure by logging, altering the UI, etc.
                  onFailedToLoad(adView, errorCode);
              }
              @Override
              public void onAdLoaded() {
                  super.onAdLoaded();
                  onFetched(adView);
              }
          });
      }
docs: Subscribing to the native ads events
  @param adView