Flutter: call function during scroll

Question

I have a scroll controller which checks if the scroll position passed the threshold and then call the function:

final _scrollController = ScrollController();
final _scrollThreshold = 200.0;
_scrollController.addListener(_onScroll);

void _onScroll() {
  final maxScroll = _scrollController.position.maxScrollExtent;
  final currentScroll = _scrollController.position.pixels;
  if (maxScroll - currentScroll <= _scrollThreshold) {
    dummyFunction();
  }
}

Currently, dummyFunction() is called multiple times, How to make sure it is going to be called once?

drogel · Accepted Answer · 2020-02-19 08:46:30Z

1

I am not sure what you are trying to achieve exactly, but I think you could use controller.position, which is the currently visible segment of the scrollable. This variable contains info about its position inside the scrollable, such as extentBefore and extentAfter. extentAfter represents the remaining scroll space available, so you could do something like this to trigger your function:

  void _onScroll() {
    if (controller.position.extentAfter < someThreshold) {
      _dummyFunction()
    }
  }

answered Feb 19, 2020 at 8:46

drogel

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Augustin R · Accepted Answer · 2020-02-19 08:45:13Z

0

Use a boolean storing wheather the function was called or not :

final _scrollController = ScrollController();
final _scrollThreshold = 200.0;
_scrollController.addListener(_onScroll);
final _hasReachedPoint = false;

void _onScroll() {
  final maxScroll = _scrollController.position.maxScrollExtent;
  final currentScroll = _scrollController.position.pixels;
  if (maxScroll - currentScroll <= _scrollThreshold && !_hasReachedPoint) {
    _hasReachedPoint = true;
    dummyFunction();
  }
}

answered Feb 19, 2020 at 8:45

Augustin R

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