IValueConverter on iOS called to often

[Julian]

Hey guys, 

I am currently implementing the Telerik Listview into my Xamarin Forms application.

But I am running into a strange behaviour on iOS. Am using the following IValueConverter

public class ByteArrayToImageConverter : IValueConverter
   {
       public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           if (value != null)
           {
               return ImageSource.FromStream(() => new MemoryStream((byte[])value));
           }
           return null;
       }

       public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
       {
           throw new NotImplementedException();
       }
   }

 

My Listview looks like this:

<telerikDataControls:RadListView
    x:Name="LVUser "
    Grid.Column="0" Grid.Row="0">
      <telerikDataControls:RadListView.LayoutDefinition>
        <telerikListView:ListViewLinearLayout ItemLength="60" />
      </telerikDataControls:RadListView.LayoutDefinition>
      <telerikDataControls:RadListView.GroupHeaderTemplate>
        <DataTemplate>
          <ContentView Style="{StaticResource GroupingHeaderBar}">
            <Label
              Text="{Binding }"
              Style="{StaticResource GroupingHeaderLabel}" />
          </ContentView>
        </DataTemplate>
      </telerikDataControls:RadListView.GroupHeaderTemplate>
      <telerikDataControls:RadListView.ItemTemplate>
        <DataTemplate>
          <telerikListView:ListViewTemplateCell>
            <telerikListView:ListViewTemplateCell.View>
              <Grid Padding="5,1,0,1">
                <Grid.ColumnDefinitions>
                  <ColumnDefinition Width="48"/>
                  <ColumnDefinition Width="*"/>
                  <ColumnDefinition Width="10"/>
                </Grid.ColumnDefinitions>

                <ffimageloading:CachedImage
                  Grid.Column="0"
                  HeightRequest="48" WidthRequest="48"
                  x:Name="Picture"
                  Source="{Binding Picture, Converter={StaticResource ByteToImage}}"
                  HorizontalOptions="Start"
                />

                <Label
                  Text="{Binding Id, Converter={StaticResource NameConverter}}"
                  Style="{StaticResource MainLabel}"
                  Grid.Column="1"
                  VerticalOptions="CenterAndExpand"/>

                <FontAwesome:FontAwesomeIcon
                 x:Name="LblMore"
                 Text=""
                 Grid.Column="2"
                 Style="{StaticResource MainLabel}"
                 VerticalOptions="CenterAndExpand"
                 HorizontalOptions="EndAndExpand"
             />

              </Grid>
            </telerikListView:ListViewTemplateCell.View>
          </telerikListView:ListViewTemplateCell>
        </DataTemplate>
      </telerikDataControls:RadListView.ItemTemplate>
    </telerikDataControls:RadListView>

 

This runs perfectly on Android but on iOS the value converter get's called unlimited times. I am using the current stable nuget package.

The "NameConverter" get's called as often as the other one.

 

Can't I use IValueConverter's?

 

Best regards

Julian

[Sascha]

Hey Julian,

yes, i have a feeling that the iOS ListView implementation does too much. It slows down everything, and it could have something to do with calling the getter bindings too often.

 

You might try to not use the data binding here, but update the field when the binding context changes within your view.

[Julian]

Hey Sascha, 

I gues you are write and its calling the getter to often.

I removed my IValueConverter's and replaced them with getter's in my Model. Effect stays the same.

Best regards

Julian

[Julian]

Hey Sascha,

I guess you are right. I remoed my IValueConverter's and replaced them with getters in my model. The effect stays the same. 

Best regards

Julian

[Sascha]

Yes, check my checkin for your problem in my POC repository

https://github.com/esskar/TelerikListViewPoc/commit/58b01dc554f09e047872188c9fe20cfd083f1268

if you ran the example, you'll see that the getter is called once for all items in the list during initialisation, and then again for all items that are visible. Adding an item is then worse as it calls the getter for all items again and more.

[Julian]

Thanks for your example project Sascha.

Do you know if the Telerik team aware of this problem?

Best regards

Julian

[Nikolay Demirev]

Hello Julian,

I was able to reproduce the issue, and it is related to the virtualization of the native iOS control. I have logged a public item in our Feedback portal which you can follow in order to receive updates on the matter.

Regards,
Nikolay Demirev
Telerik by Progress