Xamarin Forms elements internal reuse

Question

Xamarin Forms elements internal reuse

Nir Dil 31

While working on a bug in a Xamarin Forms app I've find out that elements in a CollectionView are constantly reused.
Namely, if let's say I have a CollectionView with 30 elements arranged in a list, Xamarin will usually create ~10 elements and then reuse them as you scroll through the collection, by setting the BindingContext each time to the relevant model.
Only after figuring this out I managed to solve the bug as I realized I need to reset any locally changed property on BindingContext changes.
Would love any clarifications about this behavior and whether it can be tweaked as the number of created elements is very unpredictable

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Answer 1

JarvanZhang 23,971

Hello,

Welcome to our Microsoft Q&A platform!

Xamarin.Forms.CollectionView doesn't provide a property to set the caching mode, it automatically uses the virtualization provided by the underlying native controls. The reuse rule follows the native control, please check the correspond control of native platform to get the working mechanism. For example, CollectionView is implemented as RecyclerView on Android. RecyclerView has a view pool which reuses the item view when the items scroll out or scroll into the screen.

Best Regards,

Jarvan Zhang

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Nir Dil 31 Reputation points

2021-06-24T12:24:06.47+00:00

Thanks! that worked.
I managed to create a custom CollectionView renderer on Android and set the ItemViewCacheSize to a bigger size.
Do you know if there's also a way to pre-render forward looking items, that have not been scrolled yet into view?
Also, do you know if there's a similar setting that can be tweaked on UWP?
JarvanZhang 23,971 Reputation points

2021-06-25T06:23:18.75+00:00

if there's also a way to pre-render forward looking items, that have not been scrolled yet into vie

The render is achieved in source code, it doesn't provide the related method to touch that.

if there's a similar setting that can be tweaked on UWP

CollectionView corresponds to ListViewBase on UWP, the view provides CacheMode property to manager the caching.
Check the doc: https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.listviewbase?view=winrt-20348
Nir Dil 31 Reputation points

2021-06-25T10:19:56.983+00:00

I think CacheMode has a different purpose, but CacheLength did the trick

Answer 2

For UWP I ended up overriding the native control ItemsPanel and changed the default CacheLength.
Something like this under OnElementChanged:

 if (e.NewElement is CollectionView cv &&
      Control is Windows.UI.Xaml.Controls.ItemsControl ic)
 {
      var vertical = cv.ItemsLayout is LinearItemsLayout linearLayout && linearLayout.Orientation == ItemsLayoutOrientation.Vertical;
      string xaml = "<ItemsPanelTemplate xmlns='...' xmlns:x='...'>" +
                    (Control is Windows.UI.Xaml.Controls.ListView ? "<ItemsStackPanel" : "<ItemsWrapGrid") +
                    " Orientation=" + (vertical? "'Vertical'" : "'Horizontal'") +
                    " CacheLength='10.0' />" +
                    "</ItemsPanelTemplate>";
      if (Windows.UI.Xaml.Markup.XamlReader.Load(xaml) is ItemsPanelTemplate t)
      {
           ic.ItemsPanel = t;
      }
 }

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Xamarin Forms elements internal reuse

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