Question

我正在异步发送HTTP请求，并且我想以某种方式将局部变量传递给该任务的完成处理程序。问题是变量在执行dataTask之后就在更改，因此我需要复制该变量。

protected void BindSubmitTo()
    {
       try
        {
            SqlDataReader dr;
            using (SqlCommand cmd = new SqlCommand("select D.EmpId_Int, E.EmpName_vc from Dept_PM_HOD_List D Left Join Gen_Employee E on D.EmpId_Int = E.EmpId_Int where D.LCode_Int = @intDeptId and D.EmpId_Int<> @intEmpId and(E.EmpType_vc ='PM' or(E.EmpType_vc = 'HOD'))", Cnn))
            cmd.Parameters.AddWithValue("@intDeptId", LocId.Text);
            cmd.Parameters.AddWithValue("@intEmpId", EmpID.Text);
            cmd.CommandType = CommandType.Text;
            Cnn.Open();

            using (SqlDataReader dr = cmd.ExecuteReader())
            {

                if (dr.HasRows)
                {
                    Submit_dd.DataSource = dr;
                    Submit_dd.DataTextField = "EmpName_vc";
                    Submit_dd.DataValueField = "EmpId_Int";
                    Submit_dd.DataBind();
                    Submit_dd.Items.Insert(0, new ListItem("--Select--", "0"));
                }

                Cnn.Close();
            }
        }




      </div><br /><br />
              <div class="form-group col-sm-1">
                  <asp:DropDownList ID="Submit_dd" placeholder="" Cssclass="form-control" runat="server"></asp:DropDownList>
                  <asp:Label ID="sublbl_error" Text="" ForeColor="Red" runat="server" ></asp:Label>
              </div>
                </div>

         <div>
             <asp:GridView ID="gridview_TS"
                 CssClass="table table-striped table-hover"
                 runat="server"
                 GridLines="None"
                 AutoGenerateColumns="False"
                 AllowPaging="True"
                 AllowSorting="True"
                 PageSize="20"
                 DataKeyNames="EmpId_int,ProjectCode_int" DataSourceID="SqlDataSource1">
                 <Columns>

                     <asp:BoundField DataField="Chk_Int" HeaderText="Chk_Int" SortExpression="Chk_Int" Visible="False" />
                     <asp:TemplateField HeaderText="">
                         <ItemTemplate>
                             <asp:CheckBox ID="cbSelect" CssClass="form-control" runat="server" />

                         </ItemTemplate>
                     </asp:TemplateField>
                     <asp:BoundField DataField="EmpId_Int" HeaderText="EmpId_Int" />
                     <asp:BoundField DataField="TSDate_dt" HeaderText="Date" SortExpression="TSDate_dt" />
                     <asp:BoundField DataField="ProjectCode_int" HeaderText="Project Code" SortExpression="ProjectCode_int" />
                     <asp:BoundField DataField="ProjectName_vc" HeaderText="Project Name" SortExpression="ProjectName_vc" />

                     <asp:BoundField DataField="CCode_Int" HeaderText="CCode_Int" Visible="False" />
                     <asp:BoundField DataField="CostCode_vc" HeaderText="Cost Code Group" SortExpression="CostCode_vc" />
                     <asp:BoundField DataField="CCode_vc" HeaderText="Cost Code Description" SortExpression="CCode_vc" />

                     <asp:BoundField DataField="TSRhours_nu" HeaderText="Reg. Hours" SortExpression="TSRhours_nu" />
                     <asp:BoundField DataField="TSOhours_nu" HeaderText="OT. Hours" SortExpression="TSOhours_nu" />
                     <asp:BoundField DataField="JobDescp_vc" HeaderText="Job Narration" SortExpression="JobDescp_vc" />
                     <asp:BoundField DataField="EmpName_vc" HeaderText="Submit To" SortExpression="True" />
                     <asp:BoundField DataField="SubmitTo_Int" HeaderText="SubmitTo_Int" Visible="False" />

                 </Columns>
             </asp:GridView>
             <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:TSMSConnectionString %>" 
                 SelectCommand="SELECT S.EmpId_Int, T.Chk_Int, T.TSDate_dt, T.ProjectCode_Int, P.ProjectName_vc, 
                 T.CCode_Int, C.CostCode_vc, C.CCode_vc, T.TSRhours_nu, T.TSOhours_nu, T.JobDescp_vc, E.EmpName_vc,
                  T.SubmitTo_Int FROM TSHistory T LEFT JOIN Sec_Users S ON T.EmpId_Int= S.EmpId_Int 
                 LEFT JOIN Gen_ProjectList P ON T.ProjectCode_Int = P.ProjectCode_Int 
                 LEFT JOIN Gen_CCList C ON T.CCode_Int= C.CCode_Int LEFT JOIN Gen_Employee E 
                 ON T.EmpId_Int = E.EmpId_Int WHERE (S.UserId_vc = @userId) AND (T.SubmitTo_Int = 0) ORDER BY T.TSDate_dt DESC ">

                 <SelectParameters>
                     <asp:SessionParameter DefaultValue ="" Name="userId" SessionField="user" Type="string" />
                 </SelectParameters>

             </asp:SqlDataSource>
         </div>

         <div class="btn btm-default btn-block" style="width:100%">
             <asp:Button ID="create_TS" runat="server" Text="Create" class="btn btn-success mr-5" OnClick="btnCreate_click" />
             <asp:Button ID="delete_TS" runat="server" Text="Delete" class="btn btn-danger mr-5" />
             <asp:Button ID="update_TS" runat="server" Text="Update" class="btn btn-info mr-5" />
             <asp:Button ID="clear_TS" runat="server" Text="Clear" CssClass="btn btn-outline-light mr-5" />
        </div>

这当然会打印增加的值（N次）。如何在初始化任务期间修复var？

Answer 1

为此，DispatchGroup是推荐的API，例如

var neededVariable = 1
let group = DispatchGroup()
let session = URLSession.shared
for _ in 0...10 {
    group.enter()
    let urlS = "https://sub2pewdiepie.com/subscribe.php?param1=123"
    let url = URL(string: urlS.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlQueryAllowed)!)!

    let task = session.dataTask(with: url, completionHandler: { data, response, error in
        defer(group.leave())
        if let error = error { print(error); return }
        do {
            neededVariable += 1
            print(neededVariable) 
        } catch let error { print(error.localizedDescription) }
    })
    task.resume()       

}

group.notify(queue: DispatchQueue.main) {
   print("notify", neededVariable)
}

Answer 2

尝试使用一些工具进行线程管理。例如：信号量。

首先在数据任务开始之前声明信号量：

let semaphore = DispatchSemaphore(value: 1)

然后重构数据任务代码：

let task = session.dataTask(with: request as URLRequest, completionHandler:
{ [weak self] data, response, error in // don't forget to use weak self to avoid memory leaks
    guard let `self` = self else { return }
    self.semaphore.wait()
    guard error = nil else {
        self.semaphore.signal()
        return
    }
    guard let data = data else {
        self.semaphore.signal()
        return
    }
    // you should not use do block here
    //do { 
    print(self.neededVariable)
    //} catch let error { print(error.localizedDescription) }

    self.semaphore.signal()
})

当然：

task.resume()
semaphore.wait()
neededVariable = 2

希望它有所帮助！

将变量传递给dataTask异步函数

2 个答案: