如何在wpf中显示组合框加载消息

Question

我们在RadComboBox中加载RadGridView。当我们点击组合框时，需要花费时间。我认为由于RadGridView需要时间。 RadGridView在数据绑定中花费时间，因此组合框不会立即填充。我们想要如果尚未填充项目源，则弹出一个框“请等待产品列表加载......”。如果可能，请在该点加载列表，然后关闭该框。

<telerik:RadComboBox x:Name="partIdRadComboBox" Grid.Column="1" Height="32" Width="70" Margin="0,10,4,4" VerticalAlignment="Top"
                                             EmptyText="-" Text="{Binding Text, Mode=TwoWay}" IsEditable="True" AllowDrop="False"
                                             IsTextSearchEnabled="True" OpenDropDownOnFocus="True" FontSize="15" InputScope="Number">
    <telerik:RadComboBox.Items>
        <telerik:RadComboBoxItem>
            <telerik:RadComboBoxItem.Template>
                <ControlTemplate TargetType="telerik:RadComboBoxItem">
                    <telerik:RadGridView x:Name="RadGridView1" ShowGroupPanel="False" CanUserFreezeColumns="False"
                                            RowIndicatorVisibility="Collapsed" IsReadOnly="True" AutoGenerateColumns="False"
                                            IsFilteringAllowed="False" ItemsSource="{Binding Items}" IsSynchronizedWithCurrentItem="False"
                                            Height="150" SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
                                            RowStyle="{StaticResource ComboboxRowStyle}" HorizontalContentAlignment="Left">
                        <telerik:RadGridView.Columns>
                            <telerik:GridViewDataColumn DataMemberBinding="{Binding PartID}" Header="Part ID" />
                            <telerik:GridViewDataColumn DataMemberBinding="{Binding ShopID}" Header="Shop ID" />
                            <telerik:GridViewDataColumn DataMemberBinding="{Binding UnitRate}" Header="Base $" />
                            <telerik:GridViewDataColumn DataMemberBinding="{Binding BookUnitRental}" Header="Base Rent $" />
                            <telerik:GridViewDataColumn DataMemberBinding="{Binding Description}" Header="Description" />
                        </telerik:RadGridView.Columns>
                    </telerik:RadGridView>
                </ControlTemplate>
            </telerik:RadComboBoxItem.Template>
        </telerik:RadComboBoxItem>
    </telerik:RadComboBox.Items>
    <ie:Interaction.Triggers>
        <ie:EventTrigger EventName="PreviewKeyDown">
            <prism:InvokeCommandAction Command="{Binding SelectHighlightedProductCommand}"/>
        </ie:EventTrigger>
    </ie:Interaction.Triggers>
</telerik:RadComboBox>