Explore using System.OperatingSystem for API checks

[hartez]

We've created some nicer APIs for checking for API levels, OS versions, and API support in MAUI. But those are ported from our Forms APIs. It's possible we should be leveraging System.OperatingSystem, and that NativeVersion should be wrapping the stuff in that namespace.

[mattleibow]

Not only that, the new APIs are linker friendly and will remove impossible code paths. This is probably something we should do sooner than later for linker things - such as app size.

@jonathanpeppers I know you are working on linker things and trimming fixes, have you seen many cases of actual places to benefit?

[jonathanpeppers]

Yes, supposedly if you have something like:

if (OperatingSystem.IsAndroidAtLeast(24))
{
    //New APIs
}
else
{
    //Old APIs
}

If your app has SupportedOSPlatformVersion=24, then the linker will remove the else block. Same for all the platforms.

We've got to get all the assemblies trimmable first, though. Otherwise, the linker doesn't trim any Maui assemblies.

Also we can avoid some JNI calls, if we don't use Build.VERSION.SdkInt.

[hartez]

We need to verify the minimum versions for each platform. Probably 10 for iOS?

[marek-safar]

I didn't see the APIs but this sounds like the duplicated effort. There are APIs and tooling included in .NET nowadays that help developers to work with platform-specific APIs, including versioning support on the same platform. You can read about it for example at https://github.com/dotnet/designs/blob/main/accepted/2020/platform-checks/platform-checks.md

[hartez]

@marek-safar The point of this issue is to have MAUI use the APIs discussed in the doc you linked.

[pjcollins]

@jonathanpeppers you may have already done most (if not all) of this work for Android already? I'm working on some related changes for iOS so I'll add myself to this for now.

[jonathanpeppers]

@pjcollins yes, Android should be good after we got: 34e83cf

Might be worth checking if any new usage of Build.VERSION.SdkInt appeared, hopefully not?