Flutter PreferredSize在标题栏中的应用

PreferredSize组件不对其子组件施加任何约束，并且不以任何方式影响子组件的布局。常用在AppBar布局中

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: _title,
      home: MyStatelessWidget(),
    );
  }
}

class AppBarContent extends StatelessWidget {
  const AppBarContent({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.end,
      children: <Widget>[
        Padding(
          padding: const EdgeInsets.symmetric(horizontal: 10),
          child: Row(
            children: <Widget>[
              const Text(
                'PreferredSize Sample',
                style: TextStyle(color: Colors.white),
              ),
              const Spacer(),
              IconButton(
                icon: const Icon(
                  Icons.search,
                  size: 20,
                ),
                color: Colors.white,
                onPressed: () {},
              ),
              IconButton(
                icon: const Icon(
                  Icons.more_vert,
                  size: 20,
                ),
                color: Colors.white,
                onPressed: () {},
              ),
            ],
          ),
        ),
      ],
    );
  }
}

class MyStatelessWidget extends StatelessWidget {
  const MyStatelessWidget({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: PreferredSize(
        preferredSize: const Size.fromHeight(80.0),
        child: Container(
          decoration: const BoxDecoration(
            gradient: LinearGradient(
              colors: <Color>[Colors.blue, Colors.pink],
            ),
          ),
          child: const AppBarContent(),
        ),
      ),
      body: const Center(
        child: Text('Content'),
      ),
    );
  }
}