[Java.Interop] Add JniRuntime.JniValueManager.TryCreatePeer()

[jonpryor]

Context: 78d5937
Context: dotnet/android@7a772f0
Context: dotnet/android#9716
Context: dotnet/android@694e975

dotnet/android@7a772f03 added the beginnings of a NativeAOT sample to dotnet/android which built a ".NET for Android" app using NativeAOT, which "rhymed with" the Hello-NativeAOTFromAndroid sample in 78d5937.

Further work on the sample showed that it was lacking support for Android.App.Application subclasses. dotnet/android#9716 began fixing that oversight, but in the process was triggering a stack overflow because when it needed to create a "proxy" peer around the my.MainApplication Java type, which subclassed
android.app.Application, instead of creating an instance of the expected MainApplication C# type, it instead created an instance of Android.App.Application. This was visible from the logs:

Created PeerReference=0x2d06/G IdentityHashCode=0x8edcb07 Instance=0x957d2a Instance.Type=Android.App.Application, Java.Type=my/MainApplication

Note that Instance.Type is Android.App.Application, not the in-sample MainApplication C# type.

Because the runtime type was Android.App.Application, when we later attempted to dispatch the Application.OnCreate() override, this resulted in a virtual invocation of the Java Application.onCreate() method instead of a non-virtual invocation of
Application.onCreate(). This virtual invocation was the root of a recursive loop which eventually resulted in a stack overflow.

The fix in dotnet/android@694e975e was to fix
NativeAotTypeManager.CreatePeer() so that it properly checked for a binding of the runtime type of the Java instance before using the "fallback" type provided to Object.GetObject<T>() in the Application.n_OnCreate() method:

partial class Application {
    static void n_OnCreate (IntPtr jnienv, IntPtr native__this)
    {
        // …
        var __this = global::Java.Lang.Object.GetObject<
	    Android.App.Application     // This is the "fallback" NativeAotTypeManager
	> (jnienv, native__this, JniHandleOwnership.DoNotTransfer)!;
        __this.OnCreate ();
        // …
    }
}

All well and good.

The problem is that NativeAotTypeManager in dotnet/android needs to support both dotnet/java-interop "activation constructors" with a signature of (ref JniObjectReference, JniObjectReferenceOptions), and the .NET for Android signature of (IntPtr, JniHandleOwnership). Trying to support both constructors resulted in the need to copy all of JniRuntime.JniValueManager.CreatePeer() and dependencies, which felt a bit excessive.

Add a new JniRuntime.JniValueManager.TryCreatePeer() method, which will invoke the activation constructor to create an IJavaPeerable:

partial class JniRuntime {
  partial class JniValueManager {
    protected virtual IJavaPeerable? TryCreatePeer (
        ref JniObjectReference reference,
        JniObjectReferenceOptions options,
        Type targetType);
  }
}

If the activation constructor is not found, then TryCreatePeer() shall return null, allowing CreatePeerInstance() to try for a base type or, ultimately, the fallback type.

This will allow a future dotnet/android PR to remove NativeAotTypeManager.CreatePeer() and its dependencies entirely, and instead do:

partial class NativeAotTypeManager {
    const   BindingFlags        ActivationConstructorBindingFlags   = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
    static  readonly    Type[]  XAConstructorSignature  = new Type [] { typeof (IntPtr), typeof (JniHandleOwnership) };
    protected override IJavaPeerable TryCreatePeer (ref JniObjectReference reference, JniObjectReferenceOptions options, Type type)
    {
        var c = type.GetConstructor (ActivationConstructorBindingFlags, null, XAConstructorSignature, null);
        if (c != null) {
            var args = new object[] {
                reference.Handle,
                JniHandleOwnership.DoNotTransfer,
            };
            var p       = (IJavaPeerable) c.Invoke (args);
            JniObjectReference.Dispose (ref reference, options);
            return p;
        }
        return base.TryCreatePeer (ref reference, options, type);
    }
}

vastly reducing the code it needs to care about.

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• src/Java.Interop/PublicAPI.Unshipped.txt: Language not supported

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src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs:393

• Ensure that the TryCreatePeer method correctly handles the case where the constructor is not found and returns null.

protected virtual IJavaPeerable? TryCreatePeer (

[jonpryor]

/azp run

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