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lib/models/account.dart

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+import 'json_util.dart';
+
+/// 账户信息（对应后端 AccountResponse）
+///
+/// 语义说明（重要）：
+/// - [balance] 是后端计算出来的：`initialBalance + totalIncome - totalExpense`
+/// - [initialBalance] 才是可写入数据库的字段（后端 PUT /accounts/balance 改的是它）
+/// - [totalIncome] / [totalExpense] 是**全部账单**的汇总，只读
+class AccountInfo {
+  final int? id;
+  final String name;
+  final double initialBalance;
+  final double balance;
+  final double totalIncome;
+  final double totalExpense;
+
+  const AccountInfo({
+    this.id,
+    this.name = '默认账户',
+    required this.initialBalance,
+    required this.balance,
+    required this.totalIncome,
+    required this.totalExpense,
+  });
+
+  factory AccountInfo.fromJson(Map<String, dynamic> json) {
+    final initial = asDouble(json['initialBalance']);
+    final income = asDouble(json['totalIncome']);
+    final expense = asDouble(json['totalExpense']);
+    return AccountInfo(
+      id: json['id'] == null ? null : asInt(json['id']),
+      name: asNonEmpty(json['name'], '默认账户'),
+      initialBalance: initial,
+      // 后端没给 balance 时本地按同一公式兜底
+      balance: json['balance'] == null
+          ? initial + income - expense
+          : asDouble(json['balance']),
+      totalIncome: income,
+      totalExpense: expense,
+    );
+  }
+
+  static const empty = AccountInfo(
+    initialBalance: 0,
+    balance: 0,
+    totalIncome: 0,
+    totalExpense: 0,
+  );
+}
+
+/// 把"用户想要的目标余额"反算成后端可写入的期初余额。
+///
+/// 因为 balance = initialBalance + totalIncome - totalExpense，
+/// 所以要让余额显示为 target，期初余额必须设为：
+/// `target - totalIncome + totalExpense`
+double initialBalanceFor({
+  required double targetBalance,
+  required double totalIncome,
+  required double totalExpense,
+}) {
+  if (!targetBalance.isFinite) return 0;
+  return targetBalance - totalIncome + totalExpense;
+}