[controls] improve perf of "merged" ResourceDictionary lookups

[jonathanpeppers]

Applies to: #18505
Context: https://github.com/dotnet/maui/files/13251041/MauiCollectionView.zip

I profiled the above sample with dotnet-trace with the following PRs applied locally:

• [C] use ResourcesChanged to propagate Theme #21229
• [android] avoid OnLayout() for Label #21291

While scrolling, a lot of time is spent in ResourceDictionary lookups on an Android Pixel 5 device:

2.0% Microsoft.Maui.Controls!Microsoft.Maui.Controls.ResourceDictionary.TryGetValue(string,object&)

Drilling in, I can see System.Linq's Reverse() method:

0.56% System.Linq!System.Linq.Enumerable.ReverseIterator<TSource_REF>.MoveNext()
0.14% System.Linq!System.Linq.Enumerable.Reverse(System.Collections.Generic.IEnumerable`1<TSource_REF>)
0.04% System.Linq!System.Linq.Enumerable.ReverseIterator<TSource_REF>..ctor(System.Collections.Generic.IEnumerable`1<TSource_REF>)
0.04% System.Linq!System.Linq.Enumerable.ReverseIterator<TSource_REF>.Dispose()

Reverse() can be problematic as it can sometimes create a copy of the entire collection, in order to sort in reverse. We can just use a reverse for-loop instead.

The indexer, we can also avoid a double-lookup:

if (dict.ContainsKey(index))
    return dict[index];

And instead do:

if (dict.TryGetValue(index, out var value))
    return value;

The MAUI project template seems to setup a few "merged" ResourceDictionary as it contains Styles.xaml, so this is why this code path is being hit.

I wrote a BenchmarkDotNet benchmark, and it indicates the collection is being copied, as the 872 bytes of allocation occur:

Method	key	Mean	Error	StdDev	Gen0	Allocated
TryGetValue	key0	11.45 ns	0.026 ns	0.023 ns	-	-
Indexer	key0	24.72 ns	0.133 ns	0.118 ns	-	-
TryGetValue	merged99,99	117.06 ns	2.334 ns	2.497 ns	0.1042	872 B
Indexer	merged99,99	145.60 ns	2.737 ns	2.286 ns	0.1042	872 B

With these changes in place, I see less time spent inside:

0.91% Microsoft.Maui.Controls!Microsoft.Maui.Controls.ResourceDictionary.TryGetValue(string,object&)

The benchmark no longer allocates either:

Method	key	Mean	Error	StdDev	Allocated
TryGetValue	key0	11.92 ns	0.094 ns	0.084 ns	-
Indexer	merged99,99	23.12 ns	0.418 ns	0.391 ns	-
Indexer	key0	24.20 ns	0.485 ns	0.453 ns	-
TryGetValue	merged99,99	29.09 ns	0.296 ns	0.262 ns	-

This should improve the performance "parenting" of any MAUI view on all platforms -- as well as scrolling CollectionView.