code: public void dimAt(float alpha) {
          if (!mDimEnabled) {
              return;
          }
          if (mDimView == null) {
              mDimView = makeDimView();
          }
          if (!mDimView.isShown()) {
              mDimView.setVisibility(VISIBLE);
              mDimView.setBackgroundColor(mDimColor);
          }
          mDimView.setAlpha(alpha);
      }
docs: Set the dim-behind layer a specific opacity.
  
  @param alpha