5_flutter_DropdownButton(下拉按鈕),SliverAppBar(摺疊工具欄),啓動頁面

1_DropdownButton(下拉按鈕)

---

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return MyAppState();
  }
}

class MyAppState extends State<MyApp> {
  List _fruits = ["Apple", "Banana", "Pineapple", "Mango", "Grapes"];

  List<DropdownMenuItem<String>> _dropDownMenuItems;
  String _selectedFruit;

  @override
  void initState() {
    _dropDownMenuItems = buildAndGetDropDownMenuItems(_fruits);
    _selectedFruit = _dropDownMenuItems[0].value;
    super.initState();
  }

  List<DropdownMenuItem<String>> buildAndGetDropDownMenuItems(List fruits) {
    List<DropdownMenuItem<String>> items = List();
    for (String fruit in fruits) {
      items.add(DropdownMenuItem(value: fruit, child: Text(fruit)));
    }
    return items;
  }

  void changedDropDownItem(String selectedFruit) {
    setState(() {
      _selectedFruit = selectedFruit;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: Text("DropDown Button Example"),
        ),
        body: Container(
          child: Center(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.center,
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Text("Please choose a fruit: "),
                  DropdownButton(
                    value: _selectedFruit,
                    items: _dropDownMenuItems,
                    onChanged: changedDropDownItem,
                  )
                ],
              )),
        ),
      ),
    );
  }
}
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2_SliverAppBar(摺疊工具欄)

---

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"),
            )),
          )
        ],
      ),
    );
  }
}
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3_啓動頁面

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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>
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