Animating changes in a SliverList

Question

I currently have a SliverList whose items are loaded dynamically. The issue is that once these items are loaded, the SliverList updates without animating the changes, making the transition between loading & loaded very jarring.

I see that AnimatedList exists but it isn't a sliver so I can't place it directly in a CustomScrollView.

Answer 1

You probably know about this now, but might as well mention it here to help people.

You can use SliverAnimatedList. It achieves the required result.

SliverAnimatedList Construction:

itemBuilder defines the way new items are built. The builder should typically return a Transition widget, or any widget that would use the animation parameter.

SliverAnimatedList(
     key: someKey,
     initialItemCount: itemCount,
     itemBuilder: (context, index, animation) => SizeTransition(
              sizeFactor: animation,
              child: SomeWidget()
     )
)

Adding/removing dynamically

You do that by using insertItem and removeItem methods of SliverAnimatedListState. You access the state by either:

1. providing a Key to the SliverAnimatedList and use key.currentState
2. using SliverAnimatedList.of(context) static method.

In cases where you need to make changes from outside of the list, you're always going to need to use the key.

Here's a full example to clarify things. Items are added by tapping the FloatingActionButton and are removed by tapping the item itself. I used both the key and of(context) ways to access the SliverAnimatedListState.

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class SliverAnimatedListTest extends StatefulWidget {
  @override
  _SliverAnimatedListTestState createState() => _SliverAnimatedListTestState();
}

class _SliverAnimatedListTestState extends State<SliverAnimatedListTest> {
  int itemCount = 2;

  // The key to be used when accessing SliverAnimatedListState
  final GlobalKey<SliverAnimatedListState> _listKey =
      GlobalKey<SliverAnimatedListState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Sliver Animated List Test")),

      // fab will handle inserting a new item at the last index of the list.
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          _listKey.currentState.insertItem(itemCount);
          itemCount++;
        },
      ),


      body: CustomScrollView(
        slivers: <Widget>[
          SliverAnimatedList(
              key: _listKey,
              initialItemCount: itemCount,

              // Return a widget that is wrapped with a transition
              itemBuilder: (context, index, animation) => 
                 SizeTransition(
                    sizeFactor: animation,
                    child: SomeWidget(
                        index: index,

                        // Handle removing an item using of(context) static method.
                        // Returned widget should also utilize the [animation] param
                        onPressed: () {
                          SliverAnimatedList.of(context).removeItem(
                              index,
                              (context, animation) => SizeTransition(
                                  sizeFactor: animation,
                                  child: SomeWidget(
                                    index: index,
                                  )));

                          itemCount--;
                        }),
                  ))
        ],
      ),
    );
  }
}

class SomeWidget extends StatelessWidget {
  final int index;

  final Function() onPressed;

  const SomeWidget({Key key, this.index, this.onPressed}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
        padding: const EdgeInsets.all(20.0),
        child: Center(
            child: FlatButton(
              child: Text("item $index"), 
              onPressed: onPressed,
            )));
  }
}

Answer 2

You can use Implicitly Animated Reorderable List

import 'package:implicitly_animated_reorderable_list/implicitly_animated_reorderable_list.dart';
import 'package:implicitly_animated_reorderable_list/transitions.dart';
...

SliverImplicitlyAnimatedList<Comment>(
  items: comments,
  areItemsTheSame: (a, b) => a.id == b.id,
  itemBuilder: (BuildContext context, Animation<double> animation, Comment item, int index) {
    return SizeFadeTransition(
      sizeFraction: 0.7,
      curve: Curves.easeInOut,
      animation: animation,
      child: CommentSliver(
        comment: item,
      ),
    );
  },
);