flutter - Flutter 滚动控制应用程序块（不明白这是做什么的）

Question

_scrollController ..addListener(() { final triggerFetchMoreSize = 0.7 * _scrollController.position.maxScrollExtent;

    if (_scrollController.position.pixels > triggerFetchMoreSize) {
      if (hasMore &&
          currentEndPosition < currentCount &&
          !loading &&
          !inErrorState) {
        setState(() {
          loading = true;
          currentStartPosition = currentEndPosition;
          currentEndPosition =
              min(currentStartPosition + pageCount, currentCount);
        });
      }
    }
  });