Question 224965 in Flutter Question posted in
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Flutter stateless widget const inside constructor

When creating a stateless widget, the constructor is const

class MyWidget extends StatelessWidget {
  const MyWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: ListView(
      ),
    );
  }
}

If I add a ScrollController(), then it is not a const anymore

class MyWidget extends StatelessWidget {
  final ScrollController _scrollController = ScrollController();
  MyWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      controller: _scrollController,
      child: ListView(
        controller: _scrollController,
      ),
    );
  }
}

Does it has an important impact on performances, as it is still a stateless widget. const in general really increase performances as it creates a blueprint that can be reused. Do I have a better solution performance-wise for my widget with ScrollController ?

Thank you

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Adding as a required argument does not change anything because it will required to ba called as not a const

    class MyWidget extends StatelessWidget {
  final ScrollController scrollController;
  const MyWidget({
    required this.scrollController,
    super.key,
  });

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      controller: scrollController,
      child: ListView(
        controller: scrollController,
      ),
    );
  }
}

    MyWidget(scrollController: ScrollController()) //no const

  2. 0

    If you are that concerned over the const keyword, you can "lift up state".

    Meaning, refactor the code to pass the controller one widget to another:

    class Parent extends StatefulWidget {
  const Parent({Key? key}) : super(key: key);

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

class _ParentState extends State<Parent> {
  late final ScrollController _scrollController;

  @override
  Widget build(BuildContext context) {
    return Child(
      scrollController: _scrollController,
    );
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
  }
}

    And pass it to the child:

    class Child extends StatelessWidget {
  final ScrollController scrollController;

  const Child({Key? key, required this.scrollController}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      controller: scrollController,
      child: ListView(
        controller: scrollController,
      ),
    );
  }
}
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