How to set minimum height of SliverAppBar within NestedScrollView in Flutter

Question

I'd like to use SliverAppBar within NestedScrollView in an ordinary Scaffold app. I also would like to have some minimum height of the app bar when user scrolls down.

I can't figure out how to use either PreferredSize widget nor any other solution found online e.g. this.

Here is my current simplified solution:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue, fontFamily: 'Oswald'),
      home: SliverHome(),
    );
  }
}

class SliverHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: testHome(),
    );
  }

  Widget testHome() {
    return Scaffold(
      body: NestedScrollView(
        headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
          return <Widget>[
            SliverAppBar( // this is where I would like to set some minimum constraint
              expandedHeight: 300,
              floating: false,
              pinned: true,
              flexibleSpace: Container(
                padding: EdgeInsets.all(10),
                height: 340,
                width: double.infinity,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.end,
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    Container(
                      height: 40,
                    ),
                    Container(
                      height: 60,
                    ),
                    Expanded(child: Container()),
                    Text('TEST'),
                  ],
                ),
                decoration: BoxDecoration(
                    image: DecorationImage(
                        image: NetworkImage('https://picsum.photos/400/400'),
                        fit: BoxFit.cover)),
              ),
            )
          ];
        },
        body: Container(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () => {},
        tooltip: '+',
        child: Icon(Icons.accessibility_new),
      ),
    );
  }
}

I'd like to stop shrinking of the app bar somewhere near 60 dp

Answer 1

Simply use collapsedHeight and specify your minimum height.

 SliverAppBar(
  // Display a placeholder widget to visualize the shrinking size.
  flexibleSpace: Placeholder(),
  expandedHeight: 300,
  collapsedHeight: 60,
);

Answer 2

I don't know since exactly when, but as of now, you can also use the collapsedHeight property, documented as below:

https://api.flutter.dev/flutter/material/SliverAppBar/collapsedHeight.html

Currently I am on flutter 1.20.1 and I have this property available.

Answer 3

This is Actually an requested Feature - https://github.com/flutter/flutter/issues/21298

A work Around is to use Bottom

SliverAppBar(
              // this is where I would like to set some minimum constraint
              expandedHeight: 300,
              floating: false,
              pinned: true,
              bottom: PreferredSize(                       // Add this code
                preferredSize: Size.fromHeight(60.0),      // Add this code
                child: Text(''),                           // Add this code
              ),                                           // Add this code
              flexibleSpace: Container(
                padding: EdgeInsets.all(10),
                height: 340,
                width: double.infinity,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.end,
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    Container(
                      height: 40,
                    ),
                    Container(
                      height: 60,
                    ),
                    Expanded(child: Container()),
                    Text('TEST'),
                  ],
                ),
                decoration: BoxDecoration(
                    image: DecorationImage(
                        image: NetworkImage('https://picsum.photos/400/400'),
                        fit: BoxFit.cover)),
              ),
            )