I have a Xamarin BindingContext quandary

Question

I've created a custom CheckBox control to which I have added a Command/CommandParameter capability. I tried this using a Behavior and ran into the same problem. Here is my XAML:

`<ContentPage.BindingContext> <vm:ShoppingListViewModel /> </ContentPage.BindingContext>

<ContentPage.Content>
    <CollectionView ItemsSource="{Binding MyShoppingList}" 
                    SelectionMode="None"
                    EmptyView="No items in your Shopping List">
        <CollectionView.ItemTemplate>
            <DataTemplate>
                <Grid Padding="0" ColumnDefinitions="30, *, 200">
                    <controls:CheckBoxEx x:Name="CompletedCheckBox"
                                IsChecked="{Binding Completed, Mode=TwoWay}" 
                                Command="{Binding ToggledCommand}"                                    
                                CommandParameter="{Binding .}"
                                HorizontalOptions="Center" 
                                VerticalOptions="Center" />
                    <Label Text="{Binding ItemName}"  Grid.Column="1" HorizontalOptions="StartAndExpand" VerticalOptions="Center"/>
                    <Label Text="{Binding StoreName}" Grid.Column="2" HorizontalOptions="EndAndExpand" VerticalOptions="Center"/>
                </Grid>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>
</ContentPage.Content>

`

The BindingContext for my CollectionView is clearly my ViewModel; MyShoppingList is an ObservableCollection created in the ViewModel. The CheckBox IsChecked value is located in my Model as the Completed property. The Command, however, is not found because CollectionView apparently changes the BindingContext from the ViewModel to the Model, hence the Completed, ItemName, and StoreName properties are properly bound. How can I associate the Command with the ViewModel so that the Binding occurs correctly? The purpose of the Command is to trigger a database update. Here is the runtime error I see:

[0:] Binding: 'ToggledCommand' property not found on 'ShoppingList.Models.ShoppingListItem', target property: 'GLLCustomControls.CheckBoxEx.Command'

Jason Jason · Accepted Answer · 2020-07-21T21:53:09

something like this should work

Command="{Binding Source={RelativeSource AncestorType={x:Type vm:ShoppingListViewModel}}, Path=ToggledCommand}"

the Xamarin docs have multiple examples of more complex binding scenarios

I have a Xamarin BindingContext quandary

1 Answers