Xamarin Forms - How to write typeof() in XAML? - forms

Question

The xamarin forms class DataTemplate recieves a parameter with Type in ctor.
public class DataTemplate : ElementTemplate, IDataTemplateController
{
public DataTemplate(Type type);
...
}
How to pass the type of a specific class to a ctor in XAML? In c# I would write new DataTemplate(typeof(DeviceViewModel)). But I have to write this in XAML.
Pseudo code:
<DataTemplate>
<x:Arguments>
<typeof(viewModels:DeviceViewModel)/>
</x:Arguments>
<myControls:MyCustomControl/>
</DataTemplate>
EDIT
To make my goals more clear I created a derived example. Let's say there is following structure:
Picture
Music
Document
txt
pdf
xml
My List contains these elements. Every of this type has to be handled in own DataTemplate. In my case the items are quite complex so thats why I create a ViewModel for each Item. In code it could look like this:
public abstract class BaseViewModel
public class PictureViewModel : BaseViewModel
public class MusicViewModel : BaseViewModel
public class DocumentViewModel : BaseViewModel
/* My List full of different ViewModels*/
List<BaseViewModel> itemList;
Now I create a TemplateSelector which holds other TemplateSelector. It calls the right one based on view model type:
using System;
using Xamarin.Forms;
using MyApp.ViewModels;
namespace MyApp.TemplateSelectors
{
public class MyItemTemplateSelector : DataTemplateSelector
{
public DataTemplateSelector PictureTemplateSelector { get; set; }
public DataTemplateSelector MusicTemplateSelector { get; set; }
public DataTemplateSelector DocumentTemplateSelector { get; set; }
public DataTemplateSelector DefaultTemplate { get; set; }
protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
{
switch (item)
{
case PictureViewModel vm:
return PictureTemplateSelector.SelectTemplate(item, container);
case MusicViewModel vm:
return MusicTemplateSelector.SelectTemplate(item, container);
case DocumentViewModel vm:
return DocumentTemplateSelector.SelectTemplate(item, container);
default:
return DefaultTemplate.SelectTemplate(item, container);
}
}
}
}
In XAML all the assignment happens:
<DataTemplate x:Key="PictureDefaultTemplate">
<ViewCell>
<Image Source="{Binding FilePath}"/>
</ViewCell>
</DataTemplate>
<DataTemplate x:Key="MusicDefaultTemplate">
<ViewCell>
<Button Text="Play this!"/>
</ViewCell>
</DataTemplate>
<DataTemplate x:Key="DocumentDefaultTemplate">
<ViewCell>
<Label Text="show filename at least"/>
</ViewCell>
</DataTemplate>
<!-- The DocumentTemplateSelector handles file extensions differently -->
<DataTemplate x:Key="MyTxtTemplate">
<ViewCell>
<Label Text="This is a text file"/>
</ViewCell>
</DataTemplate>
<DataTemplate x:Key="MyPdfTemplate">
<ViewCell>
<Label Text="This is a pdf file"/>
</ViewCell>
</DataTemplate>
<DataTemplate x:Key="MyXmlTemplate">
<ViewCell>
<Label Text="This is a xml File"/>
</ViewCell>
</DataTemplate>
<!--provide specific template selectors with initiated datatemplates-->
<templateSelectors:PictureTemplateSelector x:Key="pictureTemplateSelector" DefaultTemplate="{StaticResource PictureDefaultTemplate}"/>
<templateSelectors:MusicTemplateSelector x:Key="musicTemplateSelector" DefaultTemplate="{StaticResource MusicDefaultTemplate}"/>
<templateSelectors:DocumentTemplateSelector x:Key="documentTemplateSelector" DefaultTemplate="{StaticResource DocumentDefaultTemplate}"
TxtTemplate="{StaticResource MyTxtTemplate}"
PdfTemplate="{StaticResource MyPdfTemplate}"
XmlTemplate="{StaticResource MyXmlTemplate}"/>
<!--provide superior template selector with other template selectors -->
<templateSelectors:MyItemTemplateSelector x:Key="myItemTemplateSelector"
PictureTemplateSelector="{StaticResource pictureTemplateSelector}"
MusicTemplateSelector="{StaticResource musicTemplateSelector}"
DocumentTemplateSelector="{StaticResource documentTemplateSelector}"/>
Last bind MyItems (objects of type BaseViewModel) and set superior template selector. This is where I set the CachingStrategy to RecycleElementAndDataTemplate:
<ListView ItemsSource="{Binding MyItems}"
ItemTemplate="{StaticResource myItemTemplateSelector}"
IsPullToRefreshEnabled="True"
CachingStrategy="RecycleElementAndDataTemplate"/>
The Exception is thrown in DocumentTemplateSelector.
using System;
using Xamarin.Forms;
using MyApp.ViewModels;
namespace MyApp.TemplateSelectors
{
public class DocumentTemplateSelector : DataTemplateSelector
{
public DataTemplate TxtTemplate { get; set; }
public DataTemplate PdfTemplate { get; set; }
public DataTemplate XmlTemplate { get; set; }
public DataTemplate DefaultTemplate { get; set; }
protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
{
var doc = (DocumentViewModel)item;
switch (doc.FileExtension)
{
case "txt":
return TxtTemplate; // Exception
case "pdf":
return PdfTemplate; // Exception
case "xml":
return XmlTemplate; // Exception
default:
return DefaultTemplate; // Exception
}
}
}
}
Following exception is thrown:
System.NotSupportedException: RecycleElementAndDataTemplate requires DataTemplate activated with ctor taking a type.
#IvanIčin #G.hakim I'm not worried that the default mechanism is not working but after studying the xamarin forms code from github I see no other solution to make it work without passing the type in DataTemplate ctor.
Update: new Xamarin Forms enhancement proposal
https://github.com/xamarin/Xamarin.Forms/issues/7060
Above enhancement would solve my request.