引用飞镖中的另一个类

Question

我正在学习“颤抖” \“飞镖”，在我的课程中，讲师创建了如下模型和数据。我注意到类别类别中的列表费用。它是指费用类别吗？我们什么时候引用外部课程？这类似于SQL中的联接吗？

费用模型

class Expense {
  final String name;
  final double cost;

  Expense({this.name, this.cost});
}

类别模型

class Category {
  final String name;
  final double maxAmount;
  final List<Expense> expenses;

  Category({this.name, this.maxAmount, this.expenses});
}

数据

import 'dart:math';

import 'package:flutter_budget_ui/models/category_model.dart';
import 'package:flutter_budget_ui/models/expense_model.dart';

final rand = Random();

final List<double> weeklySpending = [
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
  rand.nextDouble() * 100,
];

_generateExpenses() {
  List<Expense> expenses = [
    Expense(name: 'Item 0', cost: rand.nextDouble() * 90),
    Expense(name: 'Item 1', cost: rand.nextDouble() * 90),
    Expense(name: 'Item 2', cost: rand.nextDouble() * 90),
    Expense(name: 'Item 3', cost: rand.nextDouble() * 90),
    Expense(name: 'Item 4', cost: rand.nextDouble() * 90),
    Expense(name: 'Item 5', cost: rand.nextDouble() * 90),
  ];
  return expenses;
}

List<Category> categories = [
  Category(name: 'Food', maxAmount: 450, expenses: _generateExpenses()),
  Category(name: 'Clothing', maxAmount: 500, expenses: _generateExpenses()),
  Category(name: 'Utilities', maxAmount: 600, expenses: _generateExpenses()),
  Category(
      name: 'Entertainment', maxAmount: 330, expenses: _generateExpenses()),
  Category(
      name: 'Transportation', maxAmount: 500, expenses: _generateExpenses()),
  Category(name: 'Housing', maxAmount: 1000, expenses: _generateExpenses()),
];