Question

我仍在学习Dart和OOP，不能完全正确。问题是由于查询的延迟，_batch.addAll()在从内部ForEach填充_invDetails之前执行。我该如何正确组织？

Future<Response> exportAccpac() async {
    final Map<String, dynamic> _inv = {'inv': []};
    final Map<String, dynamic> _invDetails = {'invDet': []};
    final Map<String, dynamic> _batch = {};

    final _qExpenses = Query<UserCompany>(context);
    _qExpenses.where((e) => e.company.id).equalTo(companyId);
    _qExpenses
        .join(object: (e) => e.user)
        .join(set: (u) => u.expense)
        .where((d) => d.date)
        .lessThanEqualTo(date);

    final _expenseList = await _qExpenses.fetch();

    _expenseList.forEach((e) {
      _inv['inv'].add(
        {
         // a bunch of stuff here using e
        },
      );

      e.user.expense.forEach((userExpense) async {

        final _qGlCode = Query<GlCode>(context)
          ..where((g) => g.user_type.id).equalTo(_userTypeId)
          ..where((g) => g.company.id).equalTo(companyId)
          ..where((g) => g.expense_type.id).equalTo(userExpense.expense_type.id);

        final _glCodes = await _qGlCode.fetch();

        _invDetails['invDet'].add({
        // a bunch of stuff here using glCodes
        });
      });
    });

    _batch..addAll(_inv)..addAll(_invDetails);
  }

Answer 1

通常避免使用forEach-尤其是async。重新组织代码，如下所示：

  for (var userExpense in e.user.expense) {
    final _qGlCode = Query<GlCode>(context)
      ..where((g) => g.user_type.id).equalTo(_userTypeId)
      ..where((g) => g.company.id).equalTo(companyId)
      ..where((g) => g.expense_type.id).equalTo(userExpense.expense_type.id);

    final _glCodes = await _qGlCode.fetch();

    _invDetails['invDet'].add({
    // a bunch of stuff here using glCodes
    });
  }

这具有按顺序执行GL代码查询的效果，而以前您是并行启动它们的，但是没有办法等待它们完成。（为此，您应该使用Future.wait，但这在这里不需要。）

您也可以将外部forEach替换为for。

Answer 2

您可以将forEach置于异步函数下，并返回_inv最终变为：

final _expenseList = await _qExpenses.fetch();
final _inv = await _yourNewFunction();
_batch..addAll(_inv)..addAll(_invDetails);

如何正确构造此Dart异步代码？

2 个答案: