[Windows] Optimize getting default font size and font family values

[MartyIX]

Description of Change

I've been profiling my "complex grid" case again (see #21787) and I noticed that getting DefaultFontFamily and DefaultFontSize is costly on Windows because it requires interop operations on every access of FontManager.DefaultFontFamily and FontManager.DefaultFontSize.

Interestingly, on Android it's just a simple constant.

Moreover, on IOS the value is cached.

But on Windows, the values are read over and over again.

There are two paths possible for Windows here:

1. Cache the two values. This would be very easy but it might turn out to be wrong (this PR)
2. Cache DefaultFontFamily and DefaultFontSize but listen to changes (it appears ContentControlThemeFontFamily is basically static; see [Windows] Optimize getting default font size and font family values #22782 (comment)).

Performance impact

• main: Maui.Controls.Sample.Sandbox.exe_20240602_102503.speedscope.MAIN.json
• PR: Maui.Controls.Sample.Sandbox.exe_20240602_102749.speedscope.PR.json

-> ~%79 improvement

Issues Fixed

Contributes to #21787

[MartyIX]

(Perhaps @Youssef1313 would know if this PR can work given his recent work with RegisterPropertyChangedCallback. Sorry for pinging if not or not interested.)

[jsuarezruiz]

/azp run

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[Youssef1313]

(Perhaps @Youssef1313 would know if this PR can work given his recent work with RegisterPropertyChangedCallback. Sorry for pinging if not or not interested.)

Hi @MartyIX

I don't recall doing stuff related to RegisterPropertyChangedCallback in MAUI. So, I don't have enough context to review, unfortunately :(

[MartyIX]

@Youssef1313 Thank you for your reply! Yes, though you filed unoplatform/uno#15437 issue. This PR is actually more related to WinUI than to MAUI. But perhaps I got it wrong.

[Youssef1313]

@MartyIX Ah, in that case, it's not yet clear to us exactly why inheritance changes don't fire the callback. However, if there is nothing MAUI-specific here, I don't see how these RegisterPropertyChangedCallback can ever get called in this specific case. To my understanding, Application.Current.Resources.RegisterPropertyChangedCallback(Control.FontFamilyProperty, OnPropertyChanged); means, if FontFamilyProperty changed on Application.Current.Resources, then call OnPropertyChanged. However, ResourceDictionary isn't actually Control in the first place. So, I doubt the callback can get called.

[MartyIX]

@MartyIX [..] So, I doubt the callback can get called.

Yes, I could not make it fire so I think it's not correct. Would you have an idea of a different approach or how to make this work?

I tend to think that what I try to achieve here is not possible (at the moment).

[Youssef1313]

Yes I don't think WinUI exposes an API to listen for changes in ResourceDictionary. But maybe there could be a way to somehow reduce the number of calls to DefaultFontFamily / DefaultFontSize

[MartyIX]

Yes I don't think WinUI exposes an API to listen for changes in ResourceDictionary. But maybe there could be a way to somehow reduce the number of calls to DefaultFontFamily / DefaultFontSize

So I found https://github.com/microsoft/microsoft-ui-xaml/blob/2a60e27c591846556fa9ec4d8f305afdf0f96dc1/dxaml/xcp/dxaml/themes/generic.xaml#L14 and https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.media.fontfamily.xamlautofontfamily?view=winrt-22621.

Then one can see many instances of {ThemeResource ContentControlThemeFontFamily} in WinUI's source code.

ThemeResource changes with theme but otherwise it's static (AFAIK) (resolved once), not a dynamic resource.

It's hard to imagine that default font or font size changes with a theme change, so just caching the values once seems easy and correct. Fully correct would be to cache values after theme changes which seems reasonably easy to do as well.

Would you agree with the assessment or not?

[Youssef1313]

@MartyIX I guess users can do something like Application.Current.Resources["ContentControlThemeFontFamily"] = new FontFamily("CustomFont"); to change the resource value, but the question is "Is this a scenario that should be supported?"

[MartyIX]

@MartyIX I guess users can do something like Application.Current.Resources["ContentControlThemeFontFamily"] = new FontFamily("CustomFont"); to change the resource value, but the question is "Is this a scenario that should be supported?"

Good point.

Given my previous comment, it should not lead to UI changes because WinUI uses {ThemeResource ContentControlThemeFontFamily}. The dictionary value can still change but what would be the purpose of it (if I'm right in the analysis above it would be mostly ignored).

The good thing is that I can try this change in a sandbox project to see what would happen.

[Foda]

I think just caching the value is fine. If this becomes a big issue for people we can either introduce a flag to disable the cache, or revert the PR. But I think 99% of users will benefit from this so we should merge it.

[mattleibow]

/azp run

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