Delphi DevExpress TcxDBTimeEdit控件空值（Null）处理方案问询

需求：使用DevExpress TcxDBTimeEdit控件，实现用户鼠标全选时间内容后按Delete键清空控件内容，同时将关联的数据库字段值设为Null。

尝试过的无效代码 #

曾在组件Exit事件中编写以下代码，但未达到预期效果：

if
   (cxDBTimeEdit1.Text = '00:00 AM')
   or (cxDBTimeEdit1.Text = '12:00 AM')
   or (cxDBTimeEdit1.Text = '')
then dmClaimLine.qryClaimLineTimeOfService.AsString:='';

开发环境 #

• Delphi 2007
• DevExpress 6.50（2009年4月版本）
• Devart SDAC 8.2.9连接MS SQL Server 2017数据库

最终实现代码 #

通过以下三个事件的配合代码，成功实现需求：

1. KeyDown事件处理Delete按键逻辑 #

procedure TfrmC420201ClaimLine.tedTimeOfServiceKeyDown(Sender: TObject;
      var Key: Word; Shift: TShiftState);
begin
  inherited;
  // 实现选中时间后按Delete键将数据库字段设为Null
  // 需先解绑DataField、切换焦点再重新绑定，此方式虽看似特殊但有效
  if Key = VK_DELETE then
  begin
    tedTimeOfService.DataBinding.DataField:='';
    with dmClaimLine do
    begin
        if (FMode <> 'Assess') then Exit;
        ClaimLineEditable;
        dmClaimLine.qryClaimLineTimeOfService.Clear;
        dmClaimLine.qryClaimLineTimeOfService.AsString:='';
    end;
    edtMediTimeDuration.SetFocus;
    tedTimeOfService.DataBinding.DataField:='TimeOfService';
  end;
end;

2. Enter事件同步控件与数据库值 #

procedure TfrmC420201ClaimLine.tedTimeOfServiceEnter(Sender: TObject);
begin
    inherited;

    with dmClaimLine do
    begin
        if (FMode <> 'Assess') then Exit;
        ClaimLineEditable;
        dmClaimLine.qryClaimLineTimeOfService.AsDateTime := tedTimeOfService.Time        
    end;
end;

3. PropertiesChange事件同步控件修改到数据库 #

procedure TfrmC420201ClaimLine.tedTimeOfServicePropertiesChange(Sender: TObject);
var EditValueStr, TimeStr: string;
begin
    inherited;
    if VarIsNull(tedTimeOfService.EditValue) then EditValueStr  := 'Null' else EditValueStr  := TimeToStr(tedTimeOfService.Time);
    if VarIsNull(tedTimeOfService.Time) then TimeStr := 'Null' else TimeStr := TimeToStr(tedTimeOfService.Time);

    with dmClaimLine do
    begin
        if (FMode <> 'Assess') then Exit;
        if (dmClaimLine.qryClaimLineTimeOfService.AsString = '')
        and (EditValueStr = 'Null') then exit;
        if dmClaimLine.qryClaimLineTimeOfService.AsDateTime <> tedTimeOfService.Time
        then
        begin
            ClaimLineEditable;
            dmClaimLine.qryClaimLineTimeOfService.AsDateTime := tedTimeOfService.Time
        end;
    end;
end;

内容的提问来源于stack exchange，提问作者Allan F