Animating changes in a SliverList

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心在旅途
心在旅途 2021-02-12 18:24

I currently have a SliverList whose items are loaded dynamically. The issue is that once these items are loaded, the SliverList updates without animati

3条回答
  •  野的像风
    2021-02-12 19:08

    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 {
      int itemCount = 2;
    
      // The key to be used when accessing SliverAnimatedListState
      final GlobalKey _listKey =
          GlobalKey();
    
      @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: [
              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,
                )));
      }
    }
    

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