package flutter/src/widgets/framework.dart failed assertion: line 4479 pos 14

Issue

I’m starting on Flutter, I have this error that appeared from the moment I created a SliverPersistentHeader. I tried to find an explanation but nothing.

Does the error come from the Sliver ?

I don’t have the error regularly but I have to save my work 2 times in a row to have a normal display otherwise the error comes back

Also, is it simply an error when debugging or could it be displayed in the eyes of users ?

My page:

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

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

  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return homepageState();
  }
}

const BackgroundColor = const Color(0xFFF2F2F2);

class homepageState extends State<homepage> {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      backgroundColor: Colors.white,
      body: CustomScrollView(
        slivers: <Widget>[
          SliverPersistentHeader(
              delegate: _topBarContent(),
            pinned: true,
          ),
          SliverToBoxAdapter(
            child: Container(
              color: BackgroundColor,
              padding: EdgeInsets.all(20),
              child: Text(
                "Lorem ipsum"
              ),
            )
          )
        ],
      ),
    );
  }
}

const _maxHeaderExtent = 280.0;
const _minHeaderExtent = 100.0;

const _maxIconSize = 150.0;
const _minIconSize = 60.0;

class _topBarContent extends SliverPersistentHeaderDelegate{
  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
    // TODO: implement build
    final size = MediaQuery.of(context).size;
    return Container(
      color: Colors.white,
      padding: EdgeInsets.all(20),
      child: Stack(
        children: <Widget>[
          Positioned(
              top: 50.0,
              left: 0,
              height: _maxIconSize,
              child: Column(
                  children: <Widget>[
                    Text(
                      "My text",
                      textAlign: TextAlign.start,
                      style: TextStyle(
                        fontSize: 20,
                      ),
                    ),
                    Text(
                      "My second text",
                      textAlign: TextAlign.start,
                      style: TextStyle(
                        fontSize: 20
                      ),
                    )
                  ],
              ),
          ),
          Positioned(
              bottom: 20.0,
              left: 35.0,
              height: _maxIconSize,
              child: Icon(
                  Icons.cloud_outlined,
                size: 50,
              ),
          )
        ],
      ),
    );
  }

  @override
  // TODO: implement maxExtent
  double get maxExtent => _maxHeaderExtent;

  @override
  // TODO: implement minExtent
  double get minExtent => _minHeaderExtent;

  @override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) {
    // TODO: implement shouldRebuild
    throw UnimplementedError();
  }
  
}

here, the error that tells me an internal file to flutter so the error must come from else:

package flutter/src/widgets/framework.dart failed assertion: line 4479 pos 14

Solution

Probably because you haven’t implemented shouldRebuild:

@override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) {
    // TODO: implement shouldRebuild
    throw UnimplementedError();
  }

You should return true if anything has changed which requires the header to repaint itself, and false otherwise. In your case it looks like you can just return false:

@override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) {
    return false;
  }

Answered By – James Allen

Answer Checked By – David Marino (FlutterFixes Volunteer)