[Microsoft.Android.Ref] support for older targetSdkVersion

[jonathanpeppers]

Currently, if you build a .NET 5 project that has
targetSdkVersion="29" (older than 30):

error XA5207: Could not find android.jar for API level . This means the Android SDK platform for API level  is not installed. Either install it in the Android SDK Manager (Tools > Android > Android SDK Manager...), or change the Xamarin.Android project to target an API version that is installed. (C:\Users\jopepper\android-toolchain\sdk\platforms\android-\android.jar missing.)

A lot of string.Empty appears in the error messages...

This is because we only have one AndroidApiInfo.xml:

<AndroidApiInfo>
  <Id>30</Id>
  <Level>30</Level>
  <Name>R</Name>
  <Version>v11.0</Version>
  <Stable>True</Stable>
</AndroidApiInfo>

And so calls like MonoAndroidHelper.SupportedVersions.GetApiLevelFromId
won't work for API 29, because API 29 is unknown.

To fix this, we should ship all of the AndroidApiInfo.xml files in
the Microsoft.Android.Ref.nupkg, such as:

• ref\net5.0\Java.Interop.dll
• ref\net5.0\Mono.Android.dll
• ref\net5.0\v10.0\AndroidApiInfo.xml
• ref\net5.0\v11.0\AndroidApiInfo.xml

The code here will continue to work, because it searches recursively:

https://github.com/xamarin/xamarin-android-tools/blob/3974fc38c0f25f943b5d3bf0a4e174532a2a60ee/src/Xamarin.Android.Tools.AndroidSdk/AndroidVersions.cs#L45-L46

I added a test verifying app with targetSdkVersion="29" work under
.NET 5.

[pjcollins]

I think we should only support targeting a single Android binding version for a given pack? For instance, users who want to target API 29 would need to install and use 10.0.100 versioned Android packs. Is this the only way to improve the error messaging? I'm not too familiar with how we use these xml files to distinguish between "supported Android version" and "known Android version".

[jonathanpeppers]

I think we should only support targeting a single Android binding version for a given pack?

Should you be able to put android:targetSdkVersion="29" in your manifest and use Microsoft.Android.Sdk 11.0.x? This is independent of the binding for API 30.

Right now if we bumped the samples here:

https://github.com/xamarin/net6-samples

We would have to update every manifest to say targetSdkVersion="30". It seems like you shouldn't have to do that? (even if the error message was better)

[dellis1972]

The compileSdkVersion I believe should match the nuget we are referencing. Both min and target should in theory be independent of that I think. We probably need @jonpryor to chime in here.

[jonathanpeppers]

So this appears to only be broken for specifically API 29, because it isn't listed here:

https://github.com/xamarin/xamarin-android-tools/blob/2d3690e428c8523b3779f84e5c804d1fd3c0d6fe/src/Xamarin.Android.Tools.AndroidSdk/AndroidVersions.cs#L139-L176

So the question is, do we ship all the AndroidApiInfo.xml or update this list?

[pjcollins]

I had a slight misunderstanding of the original issue at hand. I would agree that we should continue to emit a warning rather than an error in cases where the targetSdkVersion in your manifest set to a value lower than the binding version.

As for the fix, I think we want to avoid shipping multiple AndroidApiInfo.xml files with a single Mono.Android.dll if possible. I am worried about running into potential issues with AndroidVersions.InstalledBindingVersions containing versions that aren't available in a one .NET context. I think we should be able to tell the difference between installed and known versions, though maintaining two separate areas of Android version information is a bit tedious...

[jonpryor]

Obligatory:

Do we need the AndroidApiInfo.xml files at all? If so, how many do we truly need?

The purpose behind the AndroidApiInfo.xml files, added in 8942eca, is to have a mapping between the $(TargetFrameworkVersion), API level, and API ID (which may differ from the API level during previews, e.g. ID "R" was API-30).

In a .NET 5+ environment, $(TargetFrameworkVersion) is completely different; it'll "always" be e.g. net5.0-android or net6.0-android, plus a new property for the OS version (I forget the name), e.g. net5.0-android12.0 is a plausible TFV. Our historical mapping will not be relevant.

That "narrows" the scope to API-Level & ID, e.g. we'll need a mapping from:

• net5.0-android11.0 to API-30
• net5.0-android12.0 to API-31 (level) & API-S (Id)

AndroidApiInfo.xml may be useful for that, which "seconds" @pjcollins comment:

we want to avoid shipping multiple AndroidApiInfo.xml files with a single Mono.Android.dll if possible

A .net5.0-android12.0 binding would be a separate assembly, and presumably a separate "package" (noun?).

This suggests that we shouldn't ship e.g. ref\net5.0\v10.0\AndroidApiInfo.xml.

Thus we return to the original "bug":

Currently, if you build a .NET 5 project that has targetSdkVersion="29" (older than 30):

error XA5207: Could not find android.jar for API level .
This means the Android SDK platform for API level  is not installed.
Either install it in the Android SDK Manager (Tools > Android > Android SDK Manager...),
or change the Xamarin.Android project to target an API version that is installed.
(C:\Users\jopepper\android-toolchain\sdk\platforms\android-\android.jar missing.)

The source of the XA5207: https://github.com/xamarin/xamarin-android/blob/f8eb265c3b91e90fa959364368e2e7d98d7f49a7/src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs#L609-L614

Is because AndroidSdkInfo.TryGetPlatformDirectoryFromApiLevel() requires that there be a known mapping from idOrApiLevel to an ID, which comes from AndroidApiInfo.xml files. As there is no file providing information about ID "29", this returns null.

What if instead it allowed idOrApiLevel to be an ID, without requiring additional mapping information?

diff --git a/src/Xamarin.Android.Tools.AndroidSdk/AndroidSdkInfo.cs b/src/Xamarin.Android.Tools.AndroidSdk/AndroidSdkInfo.cs
index 1297e51..d2d3b5f 100644
--- a/src/Xamarin.Android.Tools.AndroidSdk/AndroidSdkInfo.cs
+++ b/src/Xamarin.Android.Tools.AndroidSdk/AndroidSdkInfo.cs
@@ -98,11 +98,15 @@ namespace Xamarin.Android.Tools
 
 		public string? TryGetPlatformDirectoryFromApiLevel (string idOrApiLevel, AndroidVersions versions)
 		{
+			string? dir = GetPlatformDirectoryFromId (idOrApiLevel);
+			if (Directory.Exists (dir))
+				return dir;
+
 			var id  = versions.GetIdFromApiLevel (idOrApiLevel);
 			if (id == null)
 				return null;
 
-			string? dir = GetPlatformDirectoryFromId (id);
+			dir = GetPlatformDirectoryFromId (id);
 
 			if (Directory.Exists (dir))
 				return dir;

Other AndroidApiInfo methods may need to be likewise updated, but this should reduce the number of instances in which the AndroidApiInfo.xml files are required, which hopefully will in turn mean we don't need to ship "extra" such files.

[jonathanpeppers]

It looks like we should fix this here instead: dotnet/android-tools#90