Flutter: build list from sharedPreferences-list with immediate visible effects

Question

I am trying to learn flutter and building a small "shopping list app". For this purpose I save the state of my shopping list to the sharedPreferences for later use. This way I was able to restore the same list after closing and opening the app again, but only after "triggering a rebuild"(?) by starting to type something in a text field, using the following code:

class _ItemChecklistState extends State<ItemChecklist> {
  final List<ShoppingItem> _items = [];
  final _itemNameController = TextEditingController();
  final _amountController = TextEditingController()..text = '1';
  final Map<int, bool> checkedMap = new Map();
  bool _isComposing = false;

  ...

  @override
  Widget build(BuildContext context) {

    // calling the method to "preload" my state from the shared preferences
    _loadPrefs();

    return Scaffold(
        appBar: AppBar(
          title: Text('Shopping List'),
          actions: <Widget>[
            IconButton(
                onPressed: () => _removeCheckedItems(),
                icon: Icon(Icons.remove_done)),
            IconButton(
              icon: const Icon(Icons.remove_circle_outline),
              tooltip: 'Remove all items',
              onPressed: () => _removeAllItems(),
            ),
          ],
        ),
        body: Column(children: [
          Flexible(
            child: ListView.builder(
              itemBuilder: (_, int index) => _items[index],
              padding: EdgeInsets.all(8.0),
              itemCount: _items.length,
            ),
          ),
          Divider(height: 1.0),
          Container(child: _buildTextComposer())
        ]));
  }
  ...

  // the method I use to "restore" my state
  void _loadPrefs() async {
    String key = 'currentItemList';
    SharedPreferences prefs = await SharedPreferences.getInstance();
    if (!prefs.containsKey(key)) { return; }

    _items.clear();
    checkedMap.clear();

    Map stateAsJson = jsonDecode(prefs.getString(key));
    final itemsKey = 'items';
    final checkedMapKey = 'checkedMap';
    List items = stateAsJson[itemsKey];
    Map checkedMapClone = stateAsJson[checkedMapKey];

    for (Map item in items){
      ShoppingItem newItem = ShoppingItem(
        id: item['id'],
        name: item['name'],
        amount: item['amount'],
        removeFunction: _removeItemWithId,
        checkedMap: checkedMap,
        saveState: _saveListToSharedPrefs,
      );
      _items.add(newItem);
      checkedMap.putIfAbsent(newItem.id, () => checkedMapClone[newItem.id.toString()]);
    }
  }
  ...

}

Now at this point loading the state and setting the lists works fine, so _items list is updated correctly, as well as the checkedMap, but the ListView does not contain the corresponding data. How can I for example "trigger a rebuild" immediatlly or make sure that the "first" build of the ListView already contains the correct state? Thanks :)

woprandi · Accepted Answer · 2021-03-11 13:30:23Z

1

You have to use FutureBuilder when your UI depends on a async task

Future<List<ShoppingItem>> _getShoppingItems;

@override
void initState() {
  _getShoppingItems = _loadPrefs();
  super.initState();
}

@override
Widget build(BuildContext context) {
  FutureBuilder<List<ShoppingItem>>(
    future: _getShoppingItems,
    builder: (context, snapshot) {
      // Data not loaded yet
      if (snapshot.connectionState != ConnectionState.done) {
        return CircularProgressIndicator();
      }

      // Data loaded
      final data = snapshot.data;
      return ListView(...);
    }
  }
);

More info : https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

answered Mar 11, 2021 at 13:30

woprandi

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