Hello, I have a RadListView bound to an ObservableCollection inside RadSideDrawer.DrawerContent and my ListView doesn't show the items until the device changes orientation. I'm actually using the Recipes source code provided in the Telerik UI for Xamarin Andriod App but I can't get the list to show the items. The only code that I don't have to fully run the sample would be the code in the OnAppearing override in Example.xaml.cs. Any help would be much appreciated.

Hello Mario,

I have replied to this question in the Support Ticket you opened with the same question. To make sure there is no miscommunication while we work to resolve this issue, I'll keep the communication in the Support Ticket.

If you want to, you can come back to this forum post and update the community with the results once the case has been resolved.

Regards,
Lance | Tech Support Engineer, Sr.
Telerik by Progress

Hey Lance, was there a resolution to this problem? I am now facing the same issue?

 

Thanks,

Ben.

Hello Ben,

We were not able to replicate the behavior that Marko has reported so no further investigation has been conducted. Can you please share more information on your scenario? Do you test the behavior with our samples? I have attached a sample project for your reference which is based on the "Recipes" project. It would be great if you can modify the sample so that the undesired behavior is observed. Eventually, we can have a more detailed look and provide more concrete information.

Have a great rest of the week.

Regards,
Stefan Nenchev
Progress Telerik

Hey Stephan, I am no longer having the issue either, I think it was to do with custom observable collection I was using.

I am having a similar issue however my scenario differs from your test scenario as I have a asynchronous call to an external api to load data. 

There fore I am forced to call this method from the on appearing method and not the constructor like in your example.

My theory is that the UI is already generated before the data is loaded and therefore my list view appears empty. 

When my task returns with the collection i add them to the observable collection but this does not update the UI for some reason.

Is there a refresh method we could use?

Hi John,

The primary purpose of ObservableCollection is that when items are added and removed, it fires off a CollectionChanged event and anything bound to it will be notified of changes.

A common mistake that occurs in this scenario is that the items are not being added to the same OC that is bound to the RadListView because it was replaced without PropChanged.

What you can do to ensure you're not accidentally replacing the collection is make it read-only, and add the items from your async work. Here's an example:



If you're still not seeing the UI update, then the problem lies somewhere else and we'll need to see your code in order to debug it. Please reply back with your View and ViewModel so that I can replicate it.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

That seems to have done the trick.

 

Thanks for the assistance Lance

I'm also seeing this issue with the radlist but not seeing the issue with a regular listview accessing the same collection.  I'm using an observablecollection that has elements added to it when on the screen load (i'm not creating the collection at that time, as I understand that creating the object will mess up the onchanged event from firing).  I'm using v2017.2.626.3.  

Hi Lyndon,

Since you didn't share your code, I'll need to make an assumption based on your described scenario. A common mistake that happens is:  instead of using Add/Remove on the ObservableCollection (O.C.), you replace the entire collection when your O.C. doesn't have PropertyChanged wired up.

For example, looking at the code I provided in my last reply, if you did the following you would no longer see items added:



In order to fix this, you need to either:

1 - Only add items from the work result (like you see in the example I sent John)
2 - Make sure your O.C. also has PropertyChanged wired up

Here's an example of #2:


Now, if you replaced the instance of the ObservableCollection, anything bound to it will be notified and items added/removed will appear in the RadListView.


If you're still having trouble, please open a support ticket here and attach the relevant code form your application so we can investigate further.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

Hello Lance.  I am confident I didn't reinitialize the collection as I understand that it will lose its binding with the XAML.  This is the case with all MVVM and not just your controls.  My code works fine with Xamarins Listview but not your Radlist.  Its odd because I'm using your Listview control in other parts of the application without this problem.

I created an unlisted youtube video illustrating the scenario.  https://www.youtube.com/watch?v=VTYyTvO86Ho

My VM code is as follows:

 

A couple of things to note:

1.  I'm using an observableRangeCollection. I've tried it with an ObservableCollection also, just to debug the situation

2. The keyboard click you hear before the screen on simulator clicks is the me simply saving the xaml file without making any changes.  This save, forced the simulator screen refresh.  I'm using a tool called LiveXaml that helps with displaying Xaml output. Of course, I've removed the LiveXaml nuget and tried things.  It is not having an effect on the display issue.  I've also tried running this page on a physical device.  I'm still experiencing the issue.

Please let me know if there is anything you need from me.

My environment:

Telerik UI Xamarin v2017.2.626.3
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.
Microsoft Visual Studio Enterprise 2017 
Version 15.3.3
VisualStudio.15.Release/15.3.3+26730.12
Microsoft .NET Framework
Version 4.7.02053
Xamarin   4.6.0.299 (b63523e27)
Xamarin.Android SDK   7.4.0.21 (2851083)
Xamarin.Android Reference Assemblies and MSBuild support.
Xamarin.iOS and Xamarin.Mac SDK   10.12.0.20 (80b8487)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Hi Lyndon,

Thank you for explaining your implementation further. To test if this is a timing issue, let use the base class's Add method and not the specialized ReplaceRange or AddRange methods.

For example, update your OnNavTo code to insert the items one at a time so we get notifications for each item.



Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

I changed the code as you suggested, but still no luck.  I'm now doing a simple for loop to add the items to the collection.  The output of "[0:] Folder item being added to FileList" is displaying in the output and the xamarin listview is displaying data, but the Radlist is not displaying data.

I will update my the version of Telerik.UI to 2017.2.818.1 to see if it helps.

Also, I'm logged in as a regular user, but I should be logged in under my Enterprise login where I have an Ultimate license with Ultimate support.  Should I create a ticket and reference this item or should I keep responding here?

 

Hi Lyndon,

Yes, please open a ticket so that we can investigate this further. The ticket will allow you to attached a reproducible application. I'll also be able to escalate the ticket to the RadListView development team, if necessary.

Go here to create a new ticket.

About your license, the account you're using to respond to this thread is your gmail email address, it only has an expired trial license for UI for Xamarin.  You'll need to log out and log back in with the account you have a license for UI for Xamarin in order to submit a ticket.

In the ticket please provide us with the following items:

- The complete XAML of the view containing the RadListView (including code-behind)
- Any supporting code I'll need (e.g. the model of the item, DocumentHelper class, etc)

Once you've created the ticket, reply back here with the ticket ID number and I'll assign myself and investigate further. If you're not able to open a ticket, please reply back here with those items, pasted in code blocks.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

Hello Lyndon,

I have built out a sample application using full MVVM, find it attached. I used ReplaceRange on the RadListView's items source and it works as expected (go to StartPageViewModel at line 67).

Here's a screenshot at runtime (pressing button calls Refresh which increments the numbers after the emails):




As a rule, using ReplaceRange on an empty collection will not insert any items unless it finds a matching range. If your collection is empty, try using AddRange instead.

Can you update my attached demo so that it replicates the problem and we'll investigate further (note, you many need to add in your navigation system or PageBaseViewModel) framework. If your demo can repro the issue and I cannot find a fix, I'll escalate it to the UI for Xamairn development team.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

Thanks Lance.  I've created a new project and I'm working on recreating the issue in this new project.  I will open a ticket if the issue remains and I will update this thread either way.       

Hi all

I copy / pasted the xaml code from Telerik's Getting Started page for the ListView.  I then spent the best part of a day trying to get it to update when the ObservableCollection changed.  I had to remove the telerikDataControls:RadListView.BindingContext section (possibly because I'm using FreshMvvm) as well as set the VerticalOption to FillAndExpand.

Cheers
Craig