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test/account_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:qiangbao_accounting/models/account.dart';
+
+void main() {
+  group('余额反算 initialBalanceFor', () {
+    // 后端公式：balance = initialBalance + totalIncome - totalExpense
+    test('无收支时目标余额就是期初余额', () {
+      expect(
+        initialBalanceFor(targetBalance: 5000, totalIncome: 0, totalExpense: 0),
+        5000,
+      );
+    });
+
+    test('有收入时需扣除收入', () {
+      // 收入 8000，想让余额显示 5000 → 期初 = 5000 - 8000 = -3000
+      expect(
+        initialBalanceFor(targetBalance: 5000, totalIncome: 8000, totalExpense: 0),
+        -3000,
+      );
+    });
+
+    test('有支出时需加回支出', () {
+      // 支出 1200，想让余额显示 5000 → 期初 = 5000 + 1200 = 6200
+      expect(
+        initialBalanceFor(targetBalance: 5000, totalIncome: 0, totalExpense: 1200),
+        6200,
+      );
+    });
+
+    test('收支都有时综合换算', () {
+      // 收 8000 支 1200，目标 5000 → 5000 - 8000 + 1200 = -1800
+      expect(
+        initialBalanceFor(targetBalance: 5000, totalIncome: 8000, totalExpense: 1200),
+        -1800,
+      );
+    });
+
+    test('换算后余额能被后端公式还原（往返一致）', () {
+      const target = 12345.67;
+      const income = 8000.0;
+      const expense = 3456.78;
+      final initial = initialBalanceFor(
+        targetBalance: target,
+        totalIncome: income,
+        totalExpense: expense,
+      );
+      // 模拟后端计算
+      final balance = initial + income - expense;
+      expect(balance, closeTo(target, 0.001));
+    });
+
+    test('目标余额非法时返回 0', () {
+      expect(
+        initialBalanceFor(targetBalance: double.nan, totalIncome: 100, totalExpense: 0),
+        0,
+      );
+      expect(
+        initialBalanceFor(targetBalance: double.infinity, totalIncome: 0, totalExpense: 0),
+        0,
+      );
+    });
+  });
+
+  group('AccountInfo.fromJson 防御性', () {
+    test('字段缺失时不抛异常并按公式兜底算余额', () {
+      final a = AccountInfo.fromJson({
+        'id': '3',
+        'initialBalance': '1000',
+        'totalIncome': 500,
+        'totalExpense': '200.5',
+        // name / balance 缺失
+      });
+      expect(a.id, 3);
+      expect(a.name, '默认账户');
+      expect(a.initialBalance, 1000);
+      expect(a.totalIncome, 500);
+      expect(a.totalExpense, 200.5);
+      expect(a.balance, 1000 + 500 - 200.5); // 本地兜底计算
+    });
+
+    test('后端给了 balance 时优先使用后端值', () {
+      final a = AccountInfo.fromJson({
+        'initialBalance': 1000,
+        'totalIncome': 500,
+        'totalExpense': 200,
+        'balance': 9999, // 故意与公式不一致
+      });
+      expect(a.balance, 9999);
+    });
+
+    test('完全空对象返回零值账户', () {
+      final a = AccountInfo.fromJson({});
+      expect(a.balance, 0);
+      expect(a.totalIncome, 0);
+      expect(a.totalExpense, 0);
+    });
+  });
+}