code: public ViewPort then(Functions.Func1<ViewPortChange> then, ViewPortFallback fallback) {
          assert then != null : "'then' callback cannot be null";
          this.then = then;
          this.fallback = fallback;
          return load();
      }
docs: Load the view port execution.
  
  @param then callback when the view port is detected.
  @param fallback fallback when no view port detected or failure to detect the given
  {@link Boundary} (using {@link #propagateFallback(boolean)})