[net6] Infer Xcode without VSMac being installed

[Redth]

Originally posted in dotnet/maui-samples#126

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I have multiple Xcode versions installed:

-rw-r--r--@  1 alklig  admin  836 Feb  8 15:06 Xcode
lrwxr-xr-x   1 root    admin   28 Feb  8 15:08 Xcode.app -> /Applications/Xcode_12.4.app
drwxr-xr-x   3 alklig  staff   96 Jun  4  2018 Xcode94.app
drwxr-xr-x   3 alklig  staff   96 Oct  8  2020 Xcode_12.1.app
drwxr-xr-x   4 alklig  staff  128 Feb  8 15:07 Xcode_12.4.app

dotnet build HelloiOS produces

/usr/local/share/dotnet/packs/Microsoft.iOS.Sdk/14.4.100-ci.main.1192/tools/msbuild/iOS/Xamarin.Shared.targets(891,3): error : Could not find a valid Xcode app bundle at '/Applications/Xcode_12.3.app'. Please update your Apple SDK location in Visual Studio's preferences (Projects > SDK Locations > Apple > Apple SDK). [/Users/alklig/work/net6-mobile-samples/HelloiOS/HelloiOS.csproj]
/usr/local/share/dotnet/packs/Microsoft.iOS.Sdk/14.4.100-ci.main.1192/tools/msbuild/iOS/Xamarin.Shared.targets(891,3): error :          [/Users/alklig/work/net6-mobile-samples/HelloiOS/HelloiOS.csproj]

Build FAILED.

because in VS for Mac I had the Apple SDK setting pointing to /Applications/Xcode_12.3.app (which I deleted).

the CLI builds should use the result of xcode-select -p for the SDK, not the VS for Mac setting.

[spouliot]

A tricky part is that dotnet msbuild files are re-using / sharing code with legacy ones. We do not want (and must take care) to change the existing behaviour.

Also xcode-select -p is not really the best way as it can change as you build something (ask me how I know).

We should be using xcrun everywhere [1]. That's almost identical to xcode-select -p (by default) but you can override it with DEVELOPER_DIR so, in theory [2], more than one build can happen simultaneously with different Xcode. In practice you have, at least, a better chance to have a complete build with the same tool chain :-)

That override also means VSfM could use its Xcode location to set that variable - and nothing more should be needed.

For legacy: msbuild needs to check DEVELOPER_DIRand, if not present, go back to check with VSfM config (current check).

For dotnet: use DEVELOPER_DIR and, if not present, define it with xcode-select -p making the whole build consistent.

[1] there are still (even if many were fixed) msbuild tasks that call tools directly with their own logic.

[2] some tools (like ibtool) starts a short-lived daemon to avoid respawing the tool many time consecutively so YMMV...

[rolfbjarne]

One point to have in mind here is that to change xcode-select -p you need sudo, while modifying the VSMac setting doesn't, it's a per-user setting.

[rolfbjarne]

Just for reference, this is how we find Xcode:

1. If the MD_APPLE_SDK_ROOT environment variable is set, use that.
2. If the ~/Library/Preferences/Xamarin/Settings.plist file exists, use that.
3. Check xcode-select --print-path.
4. Use /Applications/Xcode.app.

This happens in this file: https://github.com/xamarin/Xamarin.MacDev/blob/14d53612d4624459a7ae617141c45e940ada6df5/Xamarin.MacDev/AppleSdkSettings.cs