Problem with ControlTemplate and DataBinding set via Style

SimonHahn · April 2016

Hello,

there seems to be a problem with setting ControlTemplate via implicite Style from Application Resources. In this case content is designed like defined ControlTemplate, but all binded properties doesn't work.
Following example-code demonstrates explained behavior:

App.xaml:

<Application xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:ControlTemplateTest" x:Class="ControlTemplateTest.App">
    <Application.Resources>
        <ResourceDictionary>
            <ControlTemplate x:Key="tileControlTemplate">
                <Grid RowSpacing="0">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition/>
                    </Grid.RowDefinitions>
                    <BoxView Color="{TemplateBinding Path=BackgroundColor}" Grid.RowSpan="2"  VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand"/>
                    <Grid  BackgroundColor="#50000000" Padding="0,2">
                        <Label StyleId="tileHeader" Text="{TemplateBinding Path=Title}" FontAttributes="Bold" FontSize="Small" HorizontalOptions="Center"/>
                    </Grid>
                    <ContentPresenter Grid.Row="1" Padding="5"/>
                </Grid>
            </ControlTemplate>
            <Style TargetType="local:TileControl">
                <Setter Property="ControlTemplate" Value="{StaticResource tileControlTemplate}"/>
            </Style>
        </ResourceDictionary>
    </Application.Resources>
</Application>

DemoPage.xaml:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:ControlTemplateTest"
             x:Class="ControlTemplateTest.DemoPage">
    <ContentPage.BindingContext>
        <local:DemoPageViewModel/>
    </ContentPage.BindingContext>
    <StackLayout>
        <local:TileControl Title="Tile 1">
            <StackLayout>
                <Label Text="Name:"/>
                <Label Text="{Binding Name}"/>
            </StackLayout>
        </local:TileControl>
    </StackLayout>
</ContentPage>

Adding implicit style into DemoPage.Resources everything works fine.

Greetings

PhilippSumi · October 2016

I just stumbled over this one I think. It appears the the Content maintained by the ContentPresenterdoes not derive the BindingContext from the parent. I had to assign the binding context in code, which is bad hackery. If there's a better solution to this, I'll gladly see a correction here:

(setting up a binding may do the trick, too btw)

public class CardView : ContentView
{
    ...

    /// HACK Explicitly updates the binding context of the Content (doesn't seem to
    /// be derived implicitly.
    protected override void OnBindingContextChanged()
    {
        base.OnBindingContextChanged();

        if (Content != null)
        {
            Content.BindingContext = BindingContext;
        }
    }
}

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