如何在绑定的对象上切换停靠面板是null和not null，我猜UpdateSourceTrigger不工作

Question

我的TabItem DataContext绑定了一个视图模型。它显示了该停靠面板中包含的所有UIContols (假设停靠面板的名称为DoockpanelWithdata)。我想要做的是，当我的Viewmodel对象的值等于null时，我想显示另一个Dockpanel，告诉它没有数据(假设是DoockpanelWithOutData)。

我的尝试是：

当VM==null和not null时如何切换这些停靠面板？

 <TabControl>
            <TabItem Name="Tab1" Cursor="Hand">                   
                <ListView Name="lZ" ItemsSource="{Binding UObj}" SelectedItem="{Binding SelectedItem ,Mode=TwoWay}"  >//On selected item i bind the the tabitem below which show different dockpanels on GM=null and not null 
              //columns here
                </ListView>
            </TabItem>
            <TabItem Name="TabGraph"  DataContext="{Binding GM , UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" Cursor="Hand">                  
                <DockPanel Name="DoockpanelWithOutData">
                    <DockPanel.Style>
                        <Style TargetType="DockPanel">
                            <Setter Property="Visibility" Value="Collapsed" />
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding GM, UpdateSourceTrigger=PropertyChanged}" Value="{x:Null}">
                                    <Setter Property="Visibility" Value="Visible" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </DockPanel.Style>
                    <TextBlock>There IS NO DATA</TextBlock>
                </DockPanel>
                <DockPanel  Name="DoockpanelWithdata">
                    <DockPanel.Style>
                        <Style TargetType="DockPanel">
                            <Setter Property="Visibility" Value="Visible" />
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding GraphVM, UpdateSourceTrigger=PropertyChanged}" Value="{x:Null}">
                                    <Setter Property="Visibility" Value="Collapsed" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </DockPanel.Style>
                    //Some more data
                </DockPanel>
            </TabItem>
        </TabControl>

在视图模型(UR1R2_VM.cs)中，我有：

private ObservableCollection<UModel> uObj; //binded to itemsource of another TabItem
 public ObservableCollection<UModel> UObj
        {
            get { return uObj; }
            set { uObj= value; OnPropertyChanged("UObj"); }
        }
        private UModel selectedItem; //Binded to listView selecteditem  of another tabitem        
    public UModel SelectedItem //HERE INSTANCE IS CREATED
    {
        get { return selectedItem; }
        set
        {
            selectedItem = value;
            GM = selectedItem != null ? new GM(selectedItem.Nom) : null; //HERE INSTANCE IS CREATED
            OnPropertyChanged("SelectedItem");                
        }
    }     


    private GM gM;
    public GM GM
    {
        get { return gM; }
        set
        {
            gM = value;
            OnPropertyChanged("GM");
        }
    }

Answer 1

给你..。

只要确保您的GraphVM属性在更改(即设置为空)时调用RaisePropertyChanged即可。为了设置触发器，这一点很重要。

<DockPanel Name="DoockpanelWithOutData">
  <DockPanel.Style>
    <Style TargetType="DockPanel">
      <Setter Property="Visibility" Value="Collapsed" />
      <Style.Triggers>
        <DataTrigger Binding="{Binding DataContext, UpdateSourceTrigger=PropertyChanged}" Value="{x:Null}">
            <Setter Property="Visibility" Value="Visible" />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </DockPanel.Style>
  <TextBlock>There IS NO DATA</TextBlock>
</DockPanel>
<DockPanel  Name="DoockpanelWithdata">  
  <DockPanel.Style>
    <Style TargetType="DockPanel">
      <Setter Property="Visibility" Value="Visible" />
      <Style.Triggers>
        <DataTrigger Binding="{Binding DataContext, UpdateSourceTrigger=PropertyChanged}" Value="{x:Null}">
            <Setter Property="Visibility" Value="Collapsed" />
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </DockPanel.Style>   
  //Some more data
</DockPanel>