[windows] improve memory usage of CollectionView

[jonathanpeppers]

Context: #16436
Context: https://github.com/symbiogenesis/Maui.DataGrid/tree/net8-memory-leak

A customer sample has a CollectionView with 150,000 data-bound rows.

Debugging what happens at runtime, I found we effectively were doing:

_itemTemplateContexts = new List<ItemTemplateContext>(capacity: 150_000);
for (int n = 0; n < 150_000; n++)
{
    _itemTemplateContexts.Add(null);
}

Then the items were created as you scroll each row into view:

if (_itemTemplateContexts[index] == null)
{
    _itemTemplateContexts[index] = context = new ItemTemplateContext(_itemTemplate, _itemsSource[index],
        _container, _itemHeight, _itemWidth, _itemSpacing, _mauiContext);
}
return _itemTemplateContexts[index];

This code accesses the indexer multiple times, into a List<T> with 150,000 null items.

To improve this:

• use a Dictionary<int, T> instead, just let it size dynamically.

• use TryGetValue(..., out var context), so each call accesses the indexer one less time than before.

• use either the bound collection's size or 64 (whichever is smaller) as a rough estimate of how many might fit on screen at a time.

Taking a memory snapshot of the app after startup:

Before:
Heap Size: 82,899.54 KB
After:
Heap Size: 81,768.76 KB

Which is saving about 1MB of memory on launch.

In this case, it feels better to just let the Dictionary size itself with an estimate of what capacity will be.

[symbiogenesis]

You said

use either the bound collection's size or 64 (whichever is smaller) as a rough estimate of how many might fit on screen at a time.

The code uses Math.Max. Based on your comment, perhaps Math.Min would be the right answer here?

[jonathanpeppers]

Yes, I tested w/ no capacity and just put the wrong one... Thanks.

[samhouts]

Also related? #15523

[rmarinho]

Also related? #15523

I don't think so, this doesn't really fix a leak, but just avoids create a large list with all the items (null) from the start. Saves memory.

Seems we still need a better repo to show the "leak" on #16436 , but yeah the report is similar to #15523.

[rmarinho]

We can also ignore the UITests are failing.