Sync difference between NestedScrollViewPlus and CustomScrollView while scrolling.

[fikretsengul]

Hello again, I hope you’re doing well. Sadly, I just discovered another bug related the scroll sync which again doesn't exist in NestedScrollView. If you swipe up, sometimes (2 times of 3 swipes, especially on fast swipes) the inner scroll view scrolls a little first before the outer one starts to scroll. Let me demonstrate the issue:

NestedScrollViewPlus:

NestedScrollViewPlus.mov

NestedScrollViewPlus2.mov

NestedScrollView:

NestedScrollView.mov

NestedScrollView2.mov

This happens on all versions including: 2.0.0, 1.0.3, 1.0.2, I didn't tested on older ones. You can check yourself too:

https://github.com/fikretsengul/nsw_plus_bug

Sorry for disturbing you again, thank for reading.

[fikretsengul]

With the help of the GPT-4, here is a working fix that ensures that the outer scroll view is given priority for initial gestures.

The condition for prioritizing the outer scroll view (shouldPrioritizeOuter) can be adjusted based on more complex criteria, including gesture velocity or other factors. For simplicity, it's currently set to prioritize the outer scroll view for upward swipes when it hasn't reached its maximum extent.

src/nested_scroll_view_outer.dart

  @override
  void applyUserOffset(double delta) {
    updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse);

    // Check for the presence of inner positions
    if (_innerPositions.isEmpty) {
      _outerPosition!.applyFullDragUpdate(delta);
      return;
    }

    // Prioritize the outer view based on specific conditions (custom logic can be inserted here)
    final shouldPrioritizeOuter = _outerPosition!.pixels < _outerPosition!.maxScrollExtent && delta < 0.0;

    var remainingDelta = delta;
    if (shouldPrioritizeOuter) {
      // Try to scroll the outer view first
      remainingDelta = _outerPosition!.applyFullDragUpdate(delta);
      // If there's any remaining delta, apply it to the inner views
      if (remainingDelta != 0) {
        for (final position in _innerPositions) {
          remainingDelta = position.applyFullDragUpdate(remainingDelta);
          if (remainingDelta == 0) {
            break;
          }
        }
      }
    } else {
      // Handle delta for upward swipes
      if (delta < 0.0) {
        remainingDelta = _outerPosition!.applyClampedDragUpdate(remainingDelta);
        for (final position in _innerPositions) {
          if (remainingDelta < 0) {
            remainingDelta = position.applyFullDragUpdate(remainingDelta);
          }
        }
      } else {
        // Handle delta for downward swipes
        if (_floatHeaderSlivers) {
          remainingDelta = _outerPosition!.applyClampedDragUpdate(remainingDelta);
        }
        for (final position in _innerPositions) {
          if (remainingDelta > 0) {
            remainingDelta = position.applyClampedDragUpdate(remainingDelta);
          }
        }
        if (remainingDelta > 0) {
          _outerPosition!.applyFullDragUpdate(remainingDelta);
        }
      }
    }
  }

[idootop]

Thank you for your effort! Unfortunately, the fix is more of a temporary solution. The code provided by GPT-4 is incorrect, as it assumes delta is fully absorbed by the outer layer without causing overflow before reaching maximum extent, which isn't always the case. This doesn't fully address the issue. You'll notice if you quickly scroll up, the outer scroll view overflows at the bottom before the inner reaches the top.

This seems to be caused by the outer scrollview moving quickly in the opposite direction after an overflow, which requires a more detailed analysis. My free time is limited lately, and since this scenario is not common in everyday use, it's imperceptible to most users. I'll try to address this properly when I can. In the meantime, if you can resolve this, please feel free to submit a PR.

[fikretsengul]

@idootop Hello, I hope you're doing well. Any progress on this? I tried to fix it but failed. Thank you.

[idootop]

@fikretsengul Hello! Long time no see. Unfortunately, I've shifted my focus to other projects outside of Flutter at the moment, so I don't have the bandwidth to fix this issue right now. Apologies for any inconvenience caused. Thank you!