Flutter - 開發高階應用

在本章中，我們將學習如何編寫一個完整的移動應用程式，expense_calculator。expense_calculator 的目的是儲存我們的支出資訊。應用程式的完整功能如下：

支出列表。
輸入新支出的表單。
編輯/刪除現有支出的選項。
任何時候的總支出。

我們將使用 Flutter 框架下面提到的高階功能來編寫 expense_calculator 應用程式。

高階 ListView 用法來顯示支出列表。
表單程式設計。
SQLite 資料庫程式設計來儲存我們的支出。
scoped_model 狀態管理來簡化我們的程式設計。

讓我們開始編寫expense_calculator應用程式。

在 Android Studio 中建立一個新的 Flutter 應用程式，expense_calculator。
開啟 pubspec.yaml 並新增包依賴項。

dependencies: 
   flutter: 
      sdk: flutter 
   sqflite: ^1.1.0 
   path_provider: ^0.5.0+1 
   scoped_model: ^1.0.1 
   intl: any

請注意以下幾點：

sqflite 用於 SQLite 資料庫程式設計。
path_provider 用於獲取特定於系統的應用程式路徑。
scoped_model 用於狀態管理。
intl 用於日期格式化。

Android Studio 將顯示以下警報，提示 pubspec.yaml 已更新。

點選“獲取依賴項”選項。Android Studio 將從網際網路獲取包併為應用程式正確配置它。
刪除 main.dart 中的現有程式碼。
新增新檔案 Expense.dart 以建立 Expense 類。Expense 類將具有以下屬性和方法。

屬性：id - 在 SQLite 資料庫中表示支出條目的唯一 ID。
屬性：amount - 支出的金額。
屬性：date - 支出日期。
屬性：category - 類別表示支出領域，例如食物、旅行等。
formattedDate - 用於格式化 date 屬性
fromMap - 用於將資料庫表中的欄位對映到支出物件中的屬性，並建立新的支出物件。

factory Expense.fromMap(Map<String, dynamic> data) { 
   return Expense( 
      data['id'], 
      data['amount'], 
      DateTime.parse(data['date']),    
      data['category'] 
   ); 
}

toMap - 用於將支出物件轉換為 Dart Map，這可以進一步用於資料庫程式設計

Map<String, dynamic> toMap() => { 
   "id" : id, 
   "amount" : amount, 
   "date" : date.toString(), 
   "category" : category, 
};

columns - 靜態變數，用於表示資料庫欄位。

將以下程式碼輸入 Expense.dart 檔案並儲存。

import 'package:intl/intl.dart'; class Expense {
   final int id; 
   final double amount; 
   final DateTime date; 
   final String category; 
   String get formattedDate { 
      var formatter = new DateFormat('yyyy-MM-dd'); 
      return formatter.format(this.date); 
   } 
   static final columns = ['id', 'amount', 'date', 'category'];
   Expense(this.id, this.amount, this.date, this.category); 
   factory Expense.fromMap(Map<String, dynamic> data) { 
      return Expense( 
         data['id'], 
         data['amount'], 
         DateTime.parse(data['date']), data['category'] 
      ); 
   }
   Map<String, dynamic> toMap() => {
      "id" : id, 
      "amount" : amount, 
      "date" : date.toString(), 
      "category" : category, 
   }; 
}

以上程式碼簡單易懂。
新增新檔案 Database.dart 以建立 SQLiteDbProvider 類。SQLiteDbProvider 類的目的是：

使用 getAllExpenses 方法獲取資料庫中所有可用的支出。它將用於列出所有使用者的支出資訊。

Future<List<Expense>> getAllExpenses() async { 
   final db = await database; 
   
   List<Map> results = await db.query(
      "Expense", columns: Expense.columns, orderBy: "date DESC"
   );
   List<Expense> expenses = new List(); 
   results.forEach((result) {
      Expense expense = Expense.fromMap(result); 
      expenses.add(expense); 
   }); 
   return expenses; 
}

使用 getExpenseById 方法根據資料庫中可用的支出標識獲取特定支出資訊。它將用於向用戶顯示特定的支出資訊。

Future<Expense> getExpenseById(int id) async {
   final db = await database;
   var result = await db.query("Expense", where: "id = ", whereArgs: [id]);
   
   return result.isNotEmpty ? 
   Expense.fromMap(result.first) : Null; 
}

使用 getTotalExpense 方法獲取使用者的總支出。它將用於向用戶顯示當前的總支出。

Future<double> getTotalExpense() async {
   final db = await database; 
   List<Map> list = await db.rawQuery(
      "Select SUM(amount) as amount from expense"
   );
   return list.isNotEmpty ? list[0]["amount"] : Null; 
}

使用 insert 方法將新的支出資訊新增到資料庫中。它將用於使用者在應用程式中新增新的支出條目。

Future<Expense> insert(Expense expense) async { 
   final db = await database; 
   var maxIdResult = await db.rawQuery(
      "SELECT MAX(id)+1 as last_inserted_id FROM Expense"
   );
   var id = maxIdResult.first["last_inserted_id"]; 
   var result = await db.rawInsert(
      "INSERT Into Expense (id, amount, date, category)" 
      " VALUES (?, ?, ?, ?)", [
         id, expense.amount, expense.date.toString(), expense.category
      ]
   ); 
   return Expense(id, expense.amount, expense.date, expense.category); 
}

使用 update 方法更新現有支出資訊。它將用於使用者編輯和更新系統中可用的現有支出條目。

update(Expense product) async {
   final db = await database; 
   
   var result = await db.update("Expense", product.toMap(), 
   where: "id = ?", whereArgs: [product.id]); 
   return result; 
}

使用 delete 方法刪除現有支出資訊。它將用於使用者刪除系統中可用的現有支出條目。

delete(int id) async {
   final db = await database;
   db.delete("Expense", where: "id = ?", whereArgs: [id]); 
}

SQLiteDbProvider 類的完整程式碼如下：

import 'dart:async'; 
import 'dart:io'; 
import 'package:path/path.dart'; 
import 'package:path_provider/path_provider.dart'; 
import 'package:sqflite/sqflite.dart'; 
import 'Expense.dart'; 

class SQLiteDbProvider {
   SQLiteDbProvider._(); 
   static final SQLiteDbProvider db = SQLiteDbProvider._(); 
   
   static Database _database; Future<Database> get database async { 
      if (_database != null) 
         return _database; 
      _database = await initDB(); 
      return _database; 
   } 
   initDB() async {
      Directory documentsDirectory = await getApplicationDocumentsDirectory(); 
      String path = join(documentsDirectory.path, "ExpenseDB2.db"); 
      return await openDatabase(
         path, version: 1, onOpen:(db){}, onCreate: (Database db, int version) async {
            await db.execute(
               "CREATE TABLE Expense (
                  ""id INTEGER PRIMARY KEY," "amount REAL," "date TEXT," "category TEXT""
               )
            "); 
            await db.execute(
               "INSERT INTO Expense ('id', 'amount', 'date', 'category') 
               values (?, ?, ?, ?)",[1, 1000, '2019-04-01 10:00:00', "Food"]
            );
            /*await db.execute(
               "INSERT INTO Product ('id', 'name', 'description', 'price', 'image') 
               values (?, ?, ?, ?, ?)", [
                  2, "Pixel", "Pixel is the most feature phone ever", 800, "pixel.png"
               ]
            ); 
            await db.execute(
               "INSERT INTO Product ('id', 'name', 'description', 'price', 'image') 
               values (?, ?, ?, ?, ?)", [
                  3, "Laptop", "Laptop is most productive development tool", 2000, "laptop.png"
               ]
            );
            await db.execute(
               "INSERT INTO Product ('id', 'name', 'description', 'price', 'image') 
               values (?, ?, ?, ?, ?)", [
                  4, "Tablet", "Laptop is most productive development tool", 1500, "tablet.png"
               ]
            );
            await db.execute(
               "INSERT INTO Product ('id', 'name', 'description', 'price', 'image') 
               values (?, ?, ?, ?, ?)", [
                  5, "Pendrive", "iPhone is the stylist phone ever", 100, "pendrive.png"
               ]
            ); 
            await db.execute(
               "INSERT INTO Product ('id', 'name', 'description', 'price', 'image') 
               values (?, ?, ?, ?, ?)", [
                  6, "Floppy Drive", "iPhone is the stylist phone ever", 20, "floppy.png"
               ]
            ); */ 
         }
      );
   }
   Future<List<Expense>> getAllExpenses() async {
      final db = await database; 
      List<Map> 
      results = await db.query(
         "Expense", columns: Expense.columns, orderBy: "date DESC"
      );
      List<Expense> expenses = new List(); 
      results.forEach((result) {
         Expense expense = Expense.fromMap(result);
         expenses.add(expense);
      }); 
      return expenses; 
   } 
   Future<Expense> getExpenseById(int id) async {
      final db = await database;
      var result = await db.query("Expense", where: "id = ", whereArgs: [id]); 
      return result.isNotEmpty ? Expense.fromMap(result.first) : Null; 
   }
   Future<double> getTotalExpense() async {
      final db = await database;
      List<Map> list = await db.rawQuery(
         "Select SUM(amount) as amount from expense"
      );
      return list.isNotEmpty ? list[0]["amount"] : Null; 
   }
   Future<Expense> insert(Expense expense) async {
      final db = await database; 
      var maxIdResult = await db.rawQuery(
         "SELECT MAX(id)+1 as last_inserted_id FROM Expense"
      );
      var id = maxIdResult.first["last_inserted_id"]; 
      var result = await db.rawInsert(
         "INSERT Into Expense (id, amount, date, category)" 
         " VALUES (?, ?, ?, ?)", [
            id, expense.amount, expense.date.toString(), expense.category
         ]
      );
      return Expense(id, expense.amount, expense.date, expense.category); 
   }
   update(Expense product) async {
      final db = await database; 
      var result = await db.update(
         "Expense", product.toMap(), where: "id = ?", whereArgs: [product.id]
      ); 
      return result; 
   }
   delete(int id) async {
      final db = await database;
      db.delete("Expense", where: "id = ?", whereArgs: [id]);
   }
}

這裡，

database 是獲取 SQLiteDbProvider 物件的屬性。
initDB 是一個用於選擇和開啟 SQLite 資料庫的方法。

建立一個新檔案 ExpenseListModel.dart 以建立 ExpenseListModel。該模型的目的是在記憶體中儲存使用者支出的完整資訊，並在使用者記憶體中的支出發生變化時更新應用程式的使用者介面。它基於 scoped_model 包中的 Model 類。它具有以下屬性和方法：

_items - 支出的私有列表。
items - 作為 UnmodifiableListView<Expense> 的 _items 的 getter，以防止對列表進行意外或意外更改。
totalExpense - 基於 items 變數的總支出的 getter。

double get totalExpense {
   double amount = 0.0; 
   for(var i = 0; i < _items.length; i++) { 
      amount = amount + _items[i].amount; 
   } 
   return amount; 
}

load - 用於從資料庫載入完整支出並將其載入到 _items 變數中。它還會呼叫 notifyListeners 以更新 UI。

void load() {
   Future<List<Expense>> 
   list = SQLiteDbProvider.db.getAllExpenses(); 
   list.then( (dbItems) {
      for(var i = 0; i < dbItems.length; i++) { 
         _items.add(dbItems[i]); 
      } notifyListeners(); 
   });
}

byId - 用於從 _items 變數獲取特定支出。

Expense byId(int id) { 
   for(var i = 0; i < _items.length; i++) { 
      if(_items[i].id == id) { 
         return _items[i]; 
      } 
   }
   return null; 
}

add - 用於將新的支出項新增到 _items 變數以及資料庫中。它還會呼叫 notifyListeners 以更新 UI。

void add(Expense item) {
   SQLiteDbProvider.db.insert(item).then((val) { 
      _items.add(val); notifyListeners(); 
   }); 
}

Update - 用於將支出項更新到 _items 變數以及資料庫中。它還會呼叫 notifyListeners 以更新 UI。

void update(Expense item) {
   bool found = false;
   for(var i = 0; i < _items.length; i++) {
      if(_items[i].id == item.id) {
         _items[i] = item; 
         found = true; 
         SQLiteDbProvider.db.update(item); break; 
      } 
   }
   if(found) notifyListeners(); 
}

delete - 用於從 _items 變數以及資料庫中刪除現有的支出項。它還會呼叫 notifyListeners 以更新 UI。

void delete(Expense item) { 
   bool found = false; 
   for(var i = 0; i < _items.length; i++) {
      if(_items[i].id == item.id) {
         found = true; 
         SQLiteDbProvider.db.delete(item.id); 
         _items.removeAt(i); break; 
      }
   }
   if(found) notifyListeners(); 
}

ExpenseListModel 類的完整程式碼如下：

import 'dart:collection'; 
import 'package:scoped_model/scoped_model.dart'; 
import 'Expense.dart'; 
import 'Database.dart'; 

class ExpenseListModel extends Model { 
   ExpenseListModel() { 
      this.load(); 
   } 
   final List<Expense> _items = []; 
   UnmodifiableListView<Expense> get items => 
   UnmodifiableListView(_items); 
   
   /*Future<double> get totalExpense { 
      return SQLiteDbProvider.db.getTotalExpense(); 
   }*/ 
   
   double get totalExpense {
      double amount = 0.0;
      for(var i = 0; i < _items.length; i++) { 
         amount = amount + _items[i].amount; 
      } 
      return amount; 
   }
   void load() {
      Future<List<Expense>> list = SQLiteDbProvider.db.getAllExpenses(); 
      list.then( (dbItems) {
         for(var i = 0; i < dbItems.length; i++) {
            _items.add(dbItems[i]); 
         } 
         notifyListeners(); 
      }); 
   }
   Expense byId(int id) {
      for(var i = 0; i < _items.length; i++) { 
         if(_items[i].id == id) { 
            return _items[i]; 
         } 
      }
      return null; 
   }
   void add(Expense item) {
      SQLiteDbProvider.db.insert(item).then((val) {
         _items.add(val);
         notifyListeners();
      }); 
   }
   void update(Expense item) {
      bool found = false; 
      for(var i = 0; i < _items.length; i++) {
         if(_items[i].id == item.id) {
            _items[i] = item; 
            found = true; 
            SQLiteDbProvider.db.update(item); 
            break; 
         }
      }
      if(found) notifyListeners(); 
   }
   void delete(Expense item) {
      bool found = false; 
      for(var i = 0; i < _items.length; i++) {
         if(_items[i].id == item.id) {
            found = true; 
            SQLiteDbProvider.db.delete(item.id); 
            _items.removeAt(i); break; 
         }
      }
      if(found) notifyListeners(); 
   }
}

開啟 main.dart 檔案。匯入如下所示的類：

import 'package:flutter/material.dart'; 
import 'package:scoped_model/scoped_model.dart'; 
import 'ExpenseListModel.dart'; 
import 'Expense.dart';

新增 main 函式並透過傳遞 ScopedModel<ExpenseListModel> widget 來呼叫 runApp。

void main() { 
   final expenses = ExpenseListModel(); 
   runApp(
      ScopedModel<ExpenseListModel>(model: expenses, child: MyApp(),)
   );
}

這裡，

expenses 物件從資料庫載入所有使用者的支出資訊。此外，當應用程式第一次開啟時，它將建立具有正確表的所需資料庫。
ScopedModel 在應用程式的整個生命週期中提供支出資訊，並確保在任何時候都維護應用程式的狀態。它使我們能夠使用 StatelessWidget 而不是 StatefulWidget。

使用 MaterialApp widget 建立一個簡單的 MyApp。

class MyApp extends StatelessWidget {
   // This widget is the root of your application. 
   @override 
   Widget build(BuildContext context) {
      return MaterialApp(
         title: 'Expense',
         theme: ThemeData(
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: 'Expense calculator'), 
      );
   }
}

建立 MyHomePage widget 以顯示所有使用者的支出資訊以及頂部的總支出。右下角的浮動按鈕將用於新增新的支出。

class MyHomePage extends StatelessWidget { 
   MyHomePage({Key key, this.title}) : super(key: key); 
   final String title; 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar( 
            title: Text(this.title), 
         ), 
         body: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return ListView.separated(
                  itemCount: expenses.items == null ? 1 
                  : expenses.items.length + 1, 
                  itemBuilder: (context, index) { 
                     if (index == 0) { 
                        return ListTile(
                           title: Text("Total expenses: " 
                           + expenses.totalExpense.toString(), 
                           style: TextStyle(fontSize: 24,
                           fontWeight: FontWeight.bold),) 
                        );
                     } else {
                        index = index - 1; 
                        return Dismissible( 
                           key: Key(expenses.items[index].id.toString()), 
                              onDismissed: (direction) { 
                              expenses.delete(expenses.items[index]); 
                              Scaffold.of(context).showSnackBar(
                                 SnackBar(
                                    content: Text(
                                       "Item with id, " 
                                       + expenses.items[index].id.toString() + 
                                       " is dismissed"
                                    )
                                 )
                              ); 
                           },
                           child: ListTile( onTap: () { 
                              Navigator.push(
                                 context, MaterialPageRoute(
                                    builder: (context) => FormPage(
                                       id: expenses.items[index].id,
                                       expenses: expenses, 
                                    )
                                 )
                              );
                           }, 
                           leading: Icon(Icons.monetization_on), 
                           trailing: Icon(Icons.keyboard_arrow_right), 
                           title: Text(expenses.items[index].category + ": " + 
                           expenses.items[index].amount.toString() + 
                           " \nspent on " + expenses.items[index].formattedDate, 
                           style: TextStyle(fontSize: 18, fontStyle: FontStyle.italic),))
                        ); 
                     }
                  },
                  separatorBuilder: (context, index) { 
                     return Divider(); 
                  }, 
               );
            },
         ),
         floatingActionButton: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return FloatingActionButton( onPressed: () {
                  Navigator.push( 
                     context, MaterialPageRoute(
                        builder: (context) => ScopedModelDescendant<ExpenseListModel>(
                           builder: (context, child, expenses) { 
                              return FormPage( id: 0, expenses: expenses, ); 
                           }
                        )
                     )
                  ); 
                  // expenses.add(new Expense( 
                     // 2, 1000, DateTime.parse('2019-04-01 11:00:00'), 'Food')
                  ); 
                  // print(expenses.items.length); 
               },
               tooltip: 'Increment', child: Icon(Icons.add), ); 
            }
         )
      );
   }
}

這裡，

ScopedModelDescendant 用於將支出模型傳遞到 ListView 和 FloatingActionButton widget 中。
ListView.separated 和 ListTile widget 用於列出支出資訊。
Dismissible widget 用於使用滑動手勢刪除支出條目。
Navigator 用於開啟支出條目的編輯介面。可以透過點選支出條目來啟用它。

建立一個 FormPage widget。FormPage widget 的目的是新增或更新支出條目。它也處理支出條目的驗證。

class FormPage extends StatefulWidget { 
   FormPage({Key key, this.id, this.expenses}) : super(key: key); 
   final int id; 
   final ExpenseListModel expenses; 
   
   @override _FormPageState createState() => _FormPageState(id: id, expenses: expenses); 
}
class _FormPageState extends State<FormPage> {
   _FormPageState({Key key, this.id, this.expenses}); 
   
   final int id; 
   final ExpenseListModel expenses; 
   final scaffoldKey = GlobalKey<ScaffoldState>(); 
   final formKey = GlobalKey<FormState>(); 
   
   double _amount; 
   DateTime _date; 
   String _category; 
   
   void _submit() {
      final form = formKey.currentState; 
      if (form.validate()) {
         form.save(); 
         if (this.id == 0) expenses.add(Expense(0, _amount, _date, _category)); 
            else expenses.update(Expense(this.id, _amount, _date, _category)); 
         Navigator.pop(context); 
      }
   }
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         key: scaffoldKey, appBar: AppBar(
            title: Text('Enter expense details'),
         ), 
         body: Padding(
            padding: const EdgeInsets.all(16.0), 
            child: Form(
               key: formKey, child: Column(
                  children: [
                     TextFormField( 
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.monetization_on), 
                           labelText: 'Amount', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        validator: (val) {
                           Pattern pattern = r'^[1-9]\d*(\.\d+)?$'; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) 
                           return 'Enter a valid number'; else return null; 
                        }, 
                        initialValue: id == 0 
                        ? '' : expenses.byId(id).amount.toString(), 
                        onSaved: (val) => _amount = double.parse(val), 
                     ), 
                     TextFormField( 
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.calendar_today),
                           hintText: 'Enter date', 
                           labelText: 'Date', 
                           labelStyle: TextStyle(fontSize: 18), 
                        ), 
                        validator: (val) {
                           Pattern pattern = r'^((?:19|20)\d\d)[- /.]
                              (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$'; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) 
                              return 'Enter a valid date'; 
                           else return null; 
                        },
                        onSaved: (val) => _date = DateTime.parse(val), 
                        initialValue: id == 0 
                        ? '' : expenses.byId(id).formattedDate, 
                        keyboardType: TextInputType.datetime, 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.category),
                           labelText: 'Category', 
                           labelStyle: TextStyle(fontSize: 18)
                        ),
                        onSaved: (val) => _category = val, 
                        initialValue: id == 0 ? '' 
                        : expenses.byId(id).category.toString(),
                     ), 
                     RaisedButton( 
                        onPressed: _submit, 
                        child: new Text('Submit'), 
                     ), 
                  ],
               ),
            ),
         ),
      );
   }
}

這裡，

TextFormField 用於建立表單條目。
TextFormField 的 validator 屬性用於使用 RegEx 模式驗證表單元素。
_submit 函式與 expenses 物件一起使用，將支出新增到資料庫或更新資料庫中的支出。

main.dart 檔案的完整程式碼如下：

import 'package:flutter/material.dart'; 
import 'package:scoped_model/scoped_model.dart'; 
import 'ExpenseListModel.dart'; 
import 'Expense.dart'; 

void main() { 
   final expenses = ExpenseListModel(); 
   runApp(
      ScopedModel<ExpenseListModel>(
         model: expenses, child: MyApp(), 
      )
   ); 
}
class MyApp extends StatelessWidget {
   // This widget is the root of your application. 
   @override
   Widget build(BuildContext context) {
      return MaterialApp(
         title: 'Expense',
         theme: ThemeData(
            primarySwatch: Colors.blue, 
         ), 
         home: MyHomePage(title: 'Expense calculator'), 
      );
   }
}
class MyHomePage extends StatelessWidget {
   MyHomePage({Key key, this.title}) : super(key: key);
   final String title;

   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(
            title: Text(this.title),
         ),
         body: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) { 
               return ListView.separated(
                  itemCount: expenses.items == null ? 1 
                  : expenses.items.length + 1, itemBuilder: (context, index) { 
                     if (index == 0) { 
                        return ListTile( title: Text("Total expenses: " 
                        + expenses.totalExpense.toString(), 
                        style: TextStyle(fontSize: 24,fontWeight: 
                        FontWeight.bold),) ); 
                     } else {
                        index = index - 1; return Dismissible(
                           key: Key(expenses.items[index].id.toString()), 
                           onDismissed: (direction) {
                              expenses.delete(expenses.items[index]); 
                              Scaffold.of(context).showSnackBar(
                                 SnackBar(
                                    content: Text(
                                       "Item with id, " + 
                                       expenses.items[index].id.toString() 
                                       + " is dismissed"
                                    )
                                 )
                              );
                           }, 
                           child: ListTile( onTap: () {
                              Navigator.push( context, MaterialPageRoute(
                                 builder: (context) => FormPage(
                                    id: expenses.items[index].id, expenses: expenses, 
                                 )
                              ));
                           }, 
                           leading: Icon(Icons.monetization_on), 
                           trailing: Icon(Icons.keyboard_arrow_right), 
                           title: Text(expenses.items[index].category + ": " + 
                           expenses.items[index].amount.toString() + " \nspent on " + 
                           expenses.items[index].formattedDate, 
                           style: TextStyle(fontSize: 18, fontStyle: FontStyle.italic),))
                        );
                     }
                  }, 
                  separatorBuilder: (context, index) {
                     return Divider(); 
                  },
               ); 
            },
         ),
         floatingActionButton: ScopedModelDescendant<ExpenseListModel>(
            builder: (context, child, expenses) {
               return FloatingActionButton(
                  onPressed: () {
                     Navigator.push(
                        context, MaterialPageRoute(
                           builder: (context)
                           => ScopedModelDescendant<ExpenseListModel>(
                              builder: (context, child, expenses) { 
                                 return FormPage( id: 0, expenses: expenses, ); 
                              }
                           )
                        )
                     );
                     // expenses.add(
                        new Expense(
                           // 2, 1000, DateTime.parse('2019-04-01 11:00:00'), 'Food'
                        )
                     );
                     // print(expenses.items.length); 
                  },
                  tooltip: 'Increment', child: Icon(Icons.add), 
               );
            }
         )
      );
   } 
}
class FormPage extends StatefulWidget {
   FormPage({Key key, this.id, this.expenses}) : super(key: key); 
   final int id; 
   final ExpenseListModel expenses; 
   
   @override 
   _FormPageState createState() => _FormPageState(id: id, expenses: expenses); 
}
class _FormPageState extends State<FormPage> {
   _FormPageState({Key key, this.id, this.expenses}); 
   final int id; 
   final ExpenseListModel expenses; 
   final scaffoldKey = GlobalKey<ScaffoldState>(); 
   final formKey = GlobalKey<FormState>(); 
   double _amount; DateTime _date; 
   String _category;
   void _submit() {
      final form = formKey.currentState; 
      if (form.validate()) {
         form.save(); 
         if (this.id == 0) expenses.add(Expense(0, _amount, _date, _category)); 
         else expenses.update(Expense(this.id, _amount, _date, _category)); 
         Navigator.pop(context); 
      } 
   } 
   @override 
   Widget build(BuildContext context) {
      return Scaffold(
         key: scaffoldKey, appBar: AppBar( 
            title: Text('Enter expense details'), 
         ), 
         body: Padding(
            padding: const EdgeInsets.all(16.0),
            child: Form(
               key: formKey, child: Column(
                  children: [
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration( 
                           icon: const Icon(Icons.monetization_on), 
                           labelText: 'Amount', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        validator: (val) {
                           Pattern pattern = r'^[1-9]\d*(\.\d+)?$'; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) return 'Enter a valid number'; 
                           else return null; 
                        },
                        initialValue: id == 0 ? '' 
                        : expenses.byId(id).amount.toString(), 
                        onSaved: (val) => _amount = double.parse(val), 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.calendar_today), 
                           hintText: 'Enter date', 
                           labelText: 'Date', 
                           labelStyle: TextStyle(fontSize: 18), 
                        ),
                        validator: (val) {
                           Pattern pattern = r'^((?:19|20)\d\d)[- /.]
                           (0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])$'; 
                           RegExp regex = new RegExp(pattern); 
                           if (!regex.hasMatch(val)) return 'Enter a valid date'; 
                           else return null; 
                        },
                        onSaved: (val) => _date = DateTime.parse(val), 
                        initialValue: id == 0 ? '' : expenses.byId(id).formattedDate, 
                        keyboardType: TextInputType.datetime, 
                     ),
                     TextFormField(
                        style: TextStyle(fontSize: 22), 
                        decoration: const InputDecoration(
                           icon: const Icon(Icons.category), 
                           labelText: 'Category', 
                           labelStyle: TextStyle(fontSize: 18)
                        ), 
                        onSaved: (val) => _category = val, 
                        initialValue: id == 0 ? '' : expenses.byId(id).category.toString(), 
                     ),
                     RaisedButton(
                        onPressed: _submit, 
                        child: new Text('Submit'), 
                     ),
                  ],
               ),
            ),
         ),
      );
   }
}

現在，執行應用程式。
使用浮動按鈕新增新的支出。
透過點選支出條目來編輯現有支出。
透過向任一方向滑動支出條目來刪除現有支出。

應用程式的一些螢幕截圖如下：

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