Xamarin.Forms example

[raV720]

I am trying to implement dynamic loading items to ListView in Xamarin.Forms, but I cannot get it working. ListView loads whole collection instead of first page only. The method GetItemsAt of IPagedSourceProvider is called many times, until the end of data source.
Maybe someone can provide working example for Xamarin.Forms (best would be for WinRT platform)?

my code is:

public class PagedCollectionProvider<T> : AlphaChiTech.Virtualization.IPagedSourceProvider<T>
{
    private IEnumerable<T> Collection;

    public PagedCollectionProvider(IEnumerable<T> Collection)
    {
        this.Collection = Collection;
    }

    public int Count
    {
        get
        {
            var coll = Collection as ICollection<T>;
            if (coll != null)
                return coll.Count;

            return Collection.Count();
        }
    }

    public PagedSourceItemsPacket<T> GetItemsAt(int pageoffset, int count, bool usePlaceholder)
    {
        return new PagedSourceItemsPacket<T>()
        {
            LoadedAt = DateTime.Now,
            Items = Collection.Skip(pageoffset).Take(count)
        };
    }

    public int IndexOf(T item)
    {
        var list = Collection as IList<T>;
        if (list != null)
            return list.IndexOf(item);

        return Collection.IndexOf(item);
    }

    public void OnReset(int count)
    {

    }
}

[anagram4wander]

The problem i believe is with the virtualization of the listview in forms.. Could you check that is virtualizing ? If not post the ctor properties for the control, either xaml or code..

Cheers

On Jun 22, 2016, at 2:49 PM, raV720 notifications@github.com wrote:

I am trying to implement dynamic loading items to ListView in Xamarin.Forms, but I cannot get it working. ListView loads whole collection instead of first page only. The method GetItemsAt of IPagedSourceProvider is called many times, until the end of data source.
Maybe someone can provide working example for Xamarin.Forms (best would be for WinRT platform)?

my code is:

public class PagedCollectionProvider : AlphaChiTech.Virtualization.IPagedSourceProvider
{
private IEnumerable Collection;

public PagedCollectionProvider(IEnumerable<T> Collection)
{
    this.Collection = Collection;
}

public int Count
{
    get
    {
        var coll = Collection as ICollection<T>;
        if (coll != null)
            return coll.Count;

        return Collection.Count();
    }
}

public PagedSourceItemsPacket<T> GetItemsAt(int pageoffset, int count, bool usePlaceholder)
{
    return new PagedSourceItemsPacket<T>()
    {
        LoadedAt = DateTime.Now,
        Items = Collection.Skip(pageoffset).Take(count)
    };
}

public int IndexOf(T item)
{
    var list = Collection as IList<T>;
    if (list != null)
        return list.IndexOf(item);

    return Collection.IndexOf(item);
}

public void OnReset(int count)
{

}

}
—
You are receiving this because you are subscribed to this thread.
Reply to this email directly, view it on GitHub, or mute the thread.

[raV720]

thanks for reply!
Full source is here: https://bitbucket.org/equo/virtualizingxamarinformslistview.git

The most important parts are:

XAML VIEW:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MZMT.View.TestPage">

  <ListView Grid.Column="1" x:Name="listView">

    <ListView.ItemTemplate>
      <DataTemplate>
        <ViewCell>
          <ViewCell.View>
            <StackLayout Orientation="Vertical">

              <Grid>
                <Grid.ColumnDefinitions>
                  <ColumnDefinition Width="130"/>
                  <ColumnDefinition Width="auto"/>
                </Grid.ColumnDefinitions>


                <Frame Grid.Column="0" OutlineColor="Red"  VerticalOptions="Center" HorizontalOptions="Fill" HeightRequest="70">
                  <Label Text="d" HorizontalOptions="Center" VerticalOptions="Center" />
                </Frame>

                <StackLayout Grid.Column="1"
               Orientation="Vertical"
               Spacing="10"
               Padding="10, 10, 10, 10">
                  <Label Text="{Binding a}" FontSize="Medium"/>
                  <Label Text="{Binding b}" FontSize="Medium"/>
                  <Label Text="{Binding c}" FontSize="Medium"/>
                </StackLayout>

              </Grid>
            </StackLayout>
          </ViewCell.View>
        </ViewCell>
      </DataTemplate>
    </ListView.ItemTemplate>

  </ListView>

</ContentPage>

CODE BEHIND:

public partial class TestPage : ContentPage
    {
        public TestPage ()
        {
            InitializeComponent ();

            listView.ItemsSource = new EObservableCollection<TestObject>(CreateSource());
        }

        private void Btn_Clicked(object sender, EventArgs e)
        {
            Navigation.PopAsync();
        }

        private IEnumerable<TestObject> CreateSource()
        {
            var list = new List<TestObject>();

            for(int i = 0; i<2000; i++)
            {
                list.Add(new TestObject { a=i.ToString(), b="bbbb", c="cccc", d="dddddddd"});
            }

            return list;
        }

        private class TestObject
        {
            public string a { get; set; }
            public string b { get; set; }
            public string c { get; set; }
            public string d { get; set; }
        }
    }

AND WRAPPER:

public class EObservableCollection<T> : VirtualizingObservableCollection<T>
    {
        public EObservableCollection(IEnumerable<T> Collection)
            : base(new PaginationManager<T>(new PagedCollectionProvider<T>(Collection)))
        { }
    }

@raV720 Did you get this working? I've got it working nicely with Xamarin.Forms - if you want some help, let me know.

[ghuntley]

@james-lavery write up a blog post/ship some code on GitHub and I'll put it in the Weekly Xamarin newsletter with prominent placement - (http://www.weeklyxamarin.com/) / email inbox@weeklyxamarin.com

The RxUI team was just talking about how it's kind of ridiculous how bad the state of the art is in this area is. Lots of room for improvement. The world needs more examples of virtualization/paging of data when a page based API is present using Rx.Net (Observables/Observer) or with the most excellent https://github.com/RolandPheasant/DynamicData library.

@ghuntley Good idea - I'll try to get something together.

@ghuntley I've added a first cut demo project here

I'll write a blog for it in the next couple of days - it would probably be good to wait until I've got the blog written before including in the Xamarin Newsletter.

[ghuntley]

@james-lavery deal - send me an email when it's ready to go.

[imgen]

The demo seems not yet started.