import 'package:flutter/material.dart';

class MyApp extends StatefulWidget {
 @override
 _MyApp createState() => new _MyApp();
}
class _MyApp extends State<MyApp> {
 /**
 *GlobalKey：整個(gè)應(yīng)用程序中唯一的鍵
 ScaffoldState：Scaffold框架的狀態(tài)
 解釋：_scaffoldKey的值是Scaffold框架狀態(tài)的唯一鍵
 */
 final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();

 // VoidCallback：沒有參數(shù)并且不返回?cái)?shù)據(jù)的回調(diào)
 VoidCallback _showBottomSheetCallback;

 @override
 void initState() {
 super.initState();
 _showBottomSheetCallback = _showBottomSheet;
 }

 void _showBottomSheet() {
 setState(() {
  // 禁用按鈕
  _showBottomSheetCallback = null;
 });
 /**
  *currentState：獲取具有此全局鍵的樹中的控件狀態(tài)
  showBottomSheet：顯示持久性的質(zhì)感設(shè)計(jì)底部面板
  解釋：聯(lián)系上文，_scaffoldKey是Scaffold框架狀態(tài)的唯一鍵，因此代碼大意為，
   在Scaffold框架中顯示持久性的質(zhì)感設(shè)計(jì)底部面板
  */
 _scaffoldKey.currentState.showBottomSheet<Null>((BuildContext context){
  final ThemeData themeData = Theme.of(context);
  return new Container(
  decoration: new BoxDecoration(
   border: new Border(top: new BorderSide(color: themeData.disabledColor))
  ),
  child: new Padding(
   padding: const EdgeInsets.all(32.0),
   child: new Text(
   '這是一個(gè)持久性的底部面板，向下拖動(dòng)即可將其關(guān)閉',
   textAlign: TextAlign.center,
   style: new TextStyle(
    color: themeData.accentColor,
    fontSize: 24.0
   )
   )
  )
  );
 })
 /**
  *closed：當(dāng)此對(duì)象控制的元素不再可見時(shí)完成
  whenComplete：注冊(cè)將在此未來完成時(shí)調(diào)用的函數(shù)
  解釋：聯(lián)系上文，closed控制的元素是新構(gòu)建的質(zhì)感設(shè)計(jì)底部面板，因此代碼大意為，
   注冊(cè)底部面板不再可見時(shí)調(diào)用的函數(shù)
  */
 .closed.whenComplete((){
  // mounted：bool值，這個(gè)State對(duì)象當(dāng)前是否在樹中
  if (mounted) {
  setState(() {
   // 重新啟用按鈕
   _showBottomSheetCallback = _showBottomSheet;
  });
  }
 });
 }

 void _showMessage() {
 // showDialog<T>：顯示應(yīng)用程序當(dāng)前內(nèi)容上方的對(duì)話框
 showDialog<Null>(
  context: context,
  // AlertDialog：質(zhì)感設(shè)計(jì)中的告警對(duì)話框
  child: new AlertDialog(
  // content：對(duì)話框的可選內(nèi)容，以淺色字體顯示在對(duì)話框的中心
  content: new Text('你點(diǎn)擊了浮動(dòng)按鈕'),
  // actions：顯示在對(duì)話框底部的可選操作
  actions: <Widget>[
   // FlatButton：質(zhì)感設(shè)計(jì)中的平面按鈕
   new FlatButton(
   onPressed: () { Navigator.pop(context); },
   child: new Text('確定')
   )
  ]
  ),
 );
 }
 @override
 Widget build(BuildContext context) {
 return new Scaffold(
  key: _scaffoldKey,
  appBar: new AppBar(
  title: new Text('底部面板')
  ),
  floatingActionButton: new FloatingActionButton(
  onPressed: _showMessage,
  backgroundColor: Colors.redAccent[200],
  child: new Icon(Icons.add)
  ),
  body: new Center(
  child: new RaisedButton(
   onPressed: _showBottomSheetCallback,
   child: new Text('顯示底部面板')
  )
  )
 );
 }
}

void main() {
 runApp(new MaterialApp(
 title: 'Flutter Demo',
 home: new MyApp()
 ));
}