当我调整窗口大小时，带有数据选择器的 UWP Treeview 在 App.g.cs 中生成错误

Question

Team, this is something very strange and I do not understand how to debug it. I have a tree view. Within the tree view I have 2 types: Customer Visit

ALL root elements are Customers ALL child elements are Visits

I've done some selectors to show the correct icons for the level:

Now when I open the leafs IF the list does NOT exceed the end of the tree view it works:

But if I open a leaf that EXCEEDS the window side:

I get this error:

Sender: the App Exception: {"Specified cast is not valid."}

Message:

"System.InvalidCastException: Unable to cast object of type 'TitoDoc2020.Models.MTreeViewVisit' to type 'TitoDoc2020.Models.MTreeViewPaz'.\r\n   at TitoDoc2020.Views.TitoDocPage.TitoDocPage_obj107_Bindings.SetDataRoot(Object newDataRoot)\r\n   at TitoDoc2020.Views.TitoDocPage.TitoDocPage_obj107_Bindings.ProcessBindings(Object item, Int32 itemIndex, Int32 phase, Int32& nextPhase)"

BUT I'm NOT casting that! I really do not understand where it could happen.

Complete project visible here.

Answer 1

According to the description of the error message, the reason for this is on the type conversion of the binding and has nothing to do with whether it is out of the window.

Please check your binding data to see if you have incorrectly bound MTreeViewVisit to MTreeViewPaz .

---

Update

The initial speculation may be that the adjustment of the window caused the change in the visible area, which in turn caused some TreeView entry loading problems.

I noticed that in the project you will create a TitoDocContext whenever it comes to loading data, which is not recommended.

You should only create a TitoDocContext , and pass the reference where it is needed, not use the using statement to release the Context. Because the frequent creation of DbContext will bring huge resource overhead, this may also be related to your TreeView entry loading exception. In fact, I created a List with 1000 entries and after getting rid of SqlServer, it worked well.

---

Update

I reproduced your question. From the point of view of the actual problem, it should be due to the reuse of entries within the TreeView . In order to reduce the overhead of system resources, TreeView (including ListView and GridView ) will reuse previously rendered templates. Because you used ItemTemplateSelector , the actual data type and the reused template are inconsistent, causing an error.

My suggestion is to create a UserControl to replace ItemTemplateSelector .

1. Extract the Visits collection and create a base class

public class MTreeViewBase
{
    public ObservableCollection<MTreeViewVisit> Visits { get; } = new ObservableCollection<MTreeViewVisit>();
}

public class MTreeViewVisit : MTreeViewBase { }
public class MTreeViewPaz : MTreeViewBase { }

2. Create a UserControl

TestControl.xaml

<UserControl.Resources>
    <DataTemplate x:Name="PAZTemplate" x:DataType="model:MTreeViewPaz">
        <StackPanel Orientation="Horizontal">
            <FontIcon
                    Margin="{StaticResource XXSmallTopRightBottomMargin}"
                    FontFamily="{StaticResource SymbolThemeFontFamily}"
                    Glyph="&#xE77B;" />
            <TextBlock
                    Margin="{StaticResource XXSmallTopRightBottomMargin}"
                    VerticalAlignment="Center"
                    Text="{x:Bind Name}" />
        </StackPanel>
    </DataTemplate>
    <DataTemplate x:Name="VisitTemplate" x:DataType="model:MTreeViewVisit">
        <StackPanel Orientation="Horizontal">
            <FontIcon
                    Margin="{StaticResource XXSmallTopRightBottomMargin}"
                    FontFamily="{StaticResource SymbolThemeFontFamily}"
                    Foreground="{x:Bind ImageColor}"
                    Glyph="{x:Bind ImageSrc}" />
            <TextBlock
                    Margin="{StaticResource XXSmallTopRightBottomMargin}"
                    VerticalAlignment="Center"
                    Text="{x:Bind VisitDescr}" />
        </StackPanel>
    </DataTemplate>
</UserControl.Resources>
<Grid>
    <ContentControl x:Name="MainContent"/>
</Grid>

3. Create dependent attributes for internal conversion

TestControl.xaml.cs

public MTreeViewBase Data
{
    get { return (MTreeViewBase)GetValue(DataProperty); }
    set { SetValue(DataProperty, value); }
}

// Using a DependencyProperty as the backing store for Data.  This enables animation, styling, binding, etc...
public static readonly DependencyProperty DataProperty =
    DependencyProperty.Register("Data", typeof(MTreeViewBase), typeof(TestControl), new PropertyMetadata(null,new PropertyChangedCallback(Data_Changed)));

private static void Data_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (e.NewValue != null)
    {
        var instance = d as TestControl;
        if(e.NewValue is MTreeViewPaz)
        {
            instance.MainContent.ContentTemplate = instance.PAZTemplate;
        }
        else
        {
            instance.MainContent.ContentTemplate = instance.VisitTemplate;
        }
    }
}

4. Modify your TreeView and ItemTemplate

<DataTemplate x:Key="BaseTemplate" x:DataType="model:MTreeViewBase">
    <winui:TreeViewItem
        IsExpanded="False"
        ItemsSource="{x:Bind Visits}">
        <controls1:TestControl Data="{Binding}"/>
    </winui:TreeViewItem>
</DataTemplate>

<winui:TreeView
    ...
    ItemTemplate="{StaticResource BaseTemplate}"
    ... />

This can ensure that the two types have the same entrance, and the conversion is done internally, which can effectively solve this problem.

Thanks.