1_DropdownButton(下拉按钮)

main.dart

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 MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title)),
        body: Center(
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: const <Widget>[
              Text("Please choose a fruit: "),
              MyStatefulWidget(),
            ],
          ),
        ),
      ),
    );
  }
}

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

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  String dropdownValue = 'One';

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      value: dropdownValue,
      icon: const Icon(Icons.arrow_drop_down),

      onChanged: (String? newValue) {
        setState(() {
          dropdownValue = newValue!;
        });
      },

      items: <String>['One', 'Two', 'Free', 'Four']
          .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}

2_SliverAppBar(折叠工具栏)

import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(
  debugShowCheckedModeBanner: false,
  home: HomePage(),
));

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            expandedHeight: 150.0,
            floating: false,
            pinned: true,
            flexibleSpace: FlexibleSpaceBar(
              title: Text("Sliver App Bar"),
            ),
          ),
          SliverFixedExtentList(
            itemExtent: 150.0,
            delegate:
            SliverChildBuilderDelegate((context, index) => ListTile(
              title: Text("List item $index"),
            )),
          )
        ],
      ),
    );
  }
}

3_启动页面

android\app\src\main\res\drawable\launch_background.xml

  <!-- You can insert your own image assets here -->
     <item>
        <bitmap
            android:gravity="center"
            android:src="@mipmap/ic_launcher" />
    </item>