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[controls] improve perf of "merged" ResourceDictionary lookups
Applies to: dotnet#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: * dotnet#21229 * dotnet#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 juse 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`.
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