Sorry for filing one more issue :-(

Since you allow infinite scrolling on either side, when I scroll to the top of the list and pull it down, it gives a perception that the list view is hung since it's probably still processing the negativeinfinity scroll extent. I tried setting minScrollExtent = 0.0 and it seems to work fine.

Can you pl look into this as well?

Hello, This code works awesomely well except I hit another issue today. I am using scrollToIndex when I go from a list screen to edit a record's details and then return back. Now if I keep editing the same record, the second time I return to the list screen, it scrolls to the first row (your code senses that there's no change in the index and if so, scrolls to 0.0) this is annoying. What happens if this check is removed in your code? Thanks

Would it be possible to implement the shrinkwrap property, like ListView has, so that the IndexedListView doesn't expand to take up all of the available space given by the parent element when shrinkwrap=true?

Here is a workaround that memics the finite list, while at the same time preserves the jumpToIndex functionality. It's not perfect, and it requires some tweaks, but it works at least.

// main.dart
import 'package:flutter/material.dart';
import 'package:indexed_list_view/indexed_list_view.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int itemCount = 20; // <<< This is the only parameter that my solution requires

  int lastIndexRendered = 0;

  IndexedScrollController listController = IndexedScrollController();

  @override
  void initState() {
    void scrollListener() {
      if (lastIndexRendered < 0) {
        listController.jumpToIndex(0);
        setState(() {
          lastIndexRendered = 0;
        });
      } else if (lastIndexRendered > itemCount) {
        listController.jumpToIndexAndOffset(
          index: itemCount,
          offset: -listController.position.viewportDimension,
        );
        setState(() {
          lastIndexRendered = itemCount;
        });
      }
    }

    listController.addListener(scrollListener);

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: IndexedListView.builder(
        key: UniqueKey(),
        cacheExtent: 0.0,
        controller: listController,
        itemBuilder: (context, index) {
          lastIndexRendered = index;
          if (index < 0 || index > itemCount - 1) {
            return null;
          }
          return Padding(
            padding: const EdgeInsets.all(8.0),
            child: Card(
              elevation: 10.0,
              child: Padding(
                padding: const EdgeInsets.all(16.0),
                child: Text(
                    'Item ${index + 1}\nLorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.'),
              ),
            ),
          );
        },
      ),
    );
  }
}

Copy the code above and run the app to see the solution in action. Then let's consider the drawbacks and how can this solution be integrated in your package. I should have created a pull request, but I'm not very experienced with dart itself.

The first drawback is that we have to use a UniqueKey() in the IndexedListView.builder constructor. I don't know for sure if this could cause a performance issue. According to my tests I was able to create huge lists that scroll very well.

The second is that we have to set cacheExtent to 0.0. I don't know too if there is some use-cases where it should be set to a specific value. The default value is 250.0 by the way. But for my solution to work it has to be 0.0.

The third one, which is solvable but needs some work: suppose that we want to scroll to the last item. We have two cases here:

If the height of the last item is bigger than the viewport height: we can safely jumpToIndex(lastItemIndex);, like this:

But if the height of the last item is less than the viewport height, we will end up with something like this image:

When it should look like this (because it's supposed that we cannot scroll past the bottom of the last item):

The solution for this is to use jumpToIndexAndOffset, and calculate the offset as follows: offset = the height of the last item - the height of the viewport. This works in general. If we know the height we can just use it in the equation. If we don't, we can use a globalKey (for the last item alone) and calculate the offset like this:

ourController.jumpToIndexAndOffset(
  index: itemCount - 1,
  offset: (globalKey.currentContext.size.height - ourController.position.viewportDimension),
);

However, I found that if the scroll velocity is big: the globalKey.currentContext would be null sometimes.

The other solution is more "hacky" and I did not try it myself yet, but I think it should work. If we define an effectively invisible widget just before the last item. For instance: a SizeBox with height of 1.0 and child of null. And whenever we want to scroll to the last item, we scroll to that invisible item like this:

ourController.jumpToIndexAndOffset(
  index: itemCount - 1,
  offset: (1.0 - ourController.position.viewportDimension),
);

This would solve the problem, and we only have to deal with my solution as if itemCount is itemCount + 1, which is not that hard.

Let me know what you think.

@marcglasberg

Hi,

Can we use finite list ? becuase my existing code use ListView.builder with exact item....

or this plugin only support infinite list ?

thanks

I copied the example code as is and tried to run it. When I click on any of the 3 buttons to scroll, it throws this error and does nothing.

I/flutter (32287): ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════ I/flutter (32287): The following assertion was thrown while dispatching notifications for IndexedScrollController: I/flutter (32287): A IndexedScrollController was used after being disposed. I/flutter (32287): Once you have called dispose() on a IndexedScrollController, it can no longer be used. I/flutter (32287): I/flutter (32287): When the exception was thrown, this was the stack: I/flutter (32287): #0 ChangeNotifier._debugAssertNotDisposed. (package:flutter/src/foundation/change_notifier.dart:61:9) I/flutter (32287): #1 ChangeNotifier._debugAssertNotDisposed (package:flutter/src/foundation/change_notifier.dart:67:6) I/flutter (32287): #2 ChangeNotifier.dispose (package:flutter/src/foundation/change_notifier.dart:134:12) I/flutter (32287): #3 ScrollController.dispose (package:flutter/src/widgets/scroll_controller.dart:198:11) I/flutter (32287): #4 _IndexedListViewState._addPositiveControllerListener. (package:indexed_list_view/indexed_list_view.dart:154:30) I/flutter (32287): #5 ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:161:21) I/flutter (32287): #6 IndexedScrollController.jumpToIndex (package:indexed_list_view/indexed_list_view.dart:72:7) I/flutter (32287): #7 _TestPageState.build. (file:///C:/Home/AndroidDevelopment/MoneyGuruFlutter/moneyguru/lib/ui/test_page.dart:212:47) I/flutter (32287): #8 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:503:14) I/flutter (32287): #9 _InkResponseState.build. (package:flutter/src/material/ink_well.dart:558:30) I/flutter (32287): #10 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24) I/flutter (32287): #11 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:242:9) I/flutter (32287): #12 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:175:7) I/flutter (32287): #13 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:315:9) I/flutter (32287): #14 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:73:12) I/flutter (32287): #15 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:11) I/flutter (32287): #16 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:143:19) I/flutter (32287): #17 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:121:22) I/flutter (32287): #18 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:101:7) I/flutter (32287): #19 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:64:7) I/flutter (32287): #20 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:48:7) I/flutter (32287): #21 _invoke1 (dart:ui/hooks.dart:142:13) I/flutter (32287): #22 _dispatchPointerDataPacket (dart:ui/hooks.dart:99:5) I/flutter (32287): I/flutter (32287): The IndexedScrollController sending notification was: I/flutter (32287): IndexedScrollController#f752d(one client, offset 0.0)

Hi, I have tried to use you library but now I am stucked here.

controller.addListener(() { print("Now which index we are in "+controller.position.pixels.toString()); }); I have set listener I notice you cant get what is the current index which I have scroll too.

Another thing. return (BuildContext context, int index) { // return Card( child: Container( //height: heights[index % 527], color: (index == 0) ? Colors.red : Colors.green, child: Center(child: Text('ITEM $index')), ), ); };

How is this build I dont see any for loop or any list array here?

ParentWidget(
    child: IndexedListView.builder(
        // ...
    )
)

While scrolling up, I have a logic that as loads more data from my backend. When new data arrives ParentWidget is rebuild and this makes the IndexedListView scroll up by itself without stopping. If I disable my logic to fetch new data then ParentWidget no longer gets rebuilt and the bug does not occur, so I think the issue happens when some parent of IndexedListView is rebuilt.

Thanks for your great work, been using it for a while. Working like a charm. I think if someone needs a long list in flutter, they definitely need this.

However, I need a scrollbar for the users to navigate quickly in the list. I think it's a common feature for a list, I found some libs to achieve the same features, but none of them is correctly functioning. Is that difficult to implement for the indexed list view?

such as I want to create a ListView contains 10 card .

"IndexedListView.builder()" have not such keyword like "itemCount" . I don't know how to create it ?

First of all, great package. After some research yours is the only package that supports positive and negative ListView, which should really be a default behaviour in flutter but I digress.

I noticed the following issues:

1. The animateTo() method is not forwarding the duration and curve parameter to the animateToIndexAndOffset() method. I've had to override this controller to be able to use it effectively with duration and curve.
2. It took me a lot of hours to try and get a snapping behaviour to work, mainly because of the confusion around the animateToIndex() method. Finally, after hours of frustration, and testing, the animatetToIndex does not really do what the name suggest. It only animates back to an anchor point known as the _originIndex, and jumps each time a new index is called. So I abandoned its usage and tried other methods. May I suggest that it is renamed to animateToAnchor(). This would make it more clear. If developer wants to jump to a specific index, they could just use jumpTo() instead. Baking in a use case for jumping to an index in an "animate" method is confusing.
3. I ended up using the animateToWithSameOriginIndex() method to achieve what I thought animateToIndex would achieve. I feel like this should have been named animateTo() though because it actually does the animation since the origin does not change. The animateTo() should really be renamed to jumpOrAnimateToAnchor() because it really only animates if the anchor hasn't changed from a previous programatic jump.
4. Overall, I just think the explicit use of the word jump and animate to do what they are suppose to do will make it less confusing.
5. May I suggest the change in the name of _originIndex to _anchorIndex as anchor seems to be more meaningful to understand the purpose of the index.
6. Wrapping the IndexedListView with a NotificationListener to listen for scroll events seem to clash with the animation/jump method. To overcome this issue I had to delay the call to the animation code. See below:

    if (scrollInfo is ScrollEndNotification) {
      if (widget.controller.offset != itemStartLocation) {
        // Controller will only register the animation after a short delay
        Future.delayed(
          const Duration(milliseconds: 1),
          () => widget.controller.animateToWithSameOriginIndex(itemStartLocation, duration: widget.snappingSpeed, curve: widget.animationCurve),
        );
      }
    }

Sorry for the long post, but I do think that this is a really great package and I want it to gain more popularity and should really be adopted as part of the flutter SDK.

Somehow, changing the controller, list gets rebuild but is not functional, as underlying scroll controller ends up being attached to multiple places.

When the exception was thrown, this was the stack: 
#2      ScrollController.position (package:flutter/src/widgets/scroll_controller.dart:108:12)
#3      ScrollController.offset (package:flutter/src/widgets/scroll_controller.dart:115:24)
#4      my code reading controller.offset
#5      ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:308:24)
#6      ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:308:24)
#7      ScrollPosition.notifyListeners (package:flutter/src/widgets/scroll_position.dart:969:11)
#8      ScrollPosition.setPixels (package:flutter/src/widgets/scroll_position.dart:278:9)
#9      ScrollPositionWithSingleContext.setPixels (package:flutter/src/widgets/scroll_position_with_single_context.dart:82:18)

I will try to dig deeper into the code to find an issue and suggest a fix, but maybe it is a known issue already?