Add support for objects in `@JavaImplementation`

[madsodgaard]

Currently the @JavaImplementation macro does not support native functions with Java classes as parameters or return objects.

If we added support for this, we could for example write a OkHttpClientCallback using native functions that allows us to implement a Callback interface in Swift:

public abstract class OkHttpCallback implements Callback {
    @Override
    public void onFailure(Call call, IOException e) {
        onFailure(e.getMessage());
    }

    @Override
    public void onResponse(Call call, Response response) throws IOException {
        onResponse(response);
    }

    public native void onFailure(String message);
    public native void onResponse(Response response);
}

@JavaClass("com.madsodgaard.swiftjava.OkHttpCallback", implements: Callback.self)
open class OkHttpCallback: JavaObject {
    private let continuation: CheckedContinuation<Response, any Error>

    public init(
        continuation: CheckedContinuation<Response, any Error>
    ) {
        self.continuation = continuation
        super.init()
    }
}

@JavaImplementation("com.madsodgaard.swiftjava.OkHttpCallback")
extension OkHttpCallback {
    @JavaMethod
    func onFailure(message: String) {
        continuation.resume(throwing: OkHttpClientTransport.Error.networkError(message))
    }

    @JavaMethod
    func onResponse(response: Response) {
        continuation.resume(returning: response)
    }
}

The above currently does not compile with

'JNIType' is not a member type of class 'OkHttp.Response'

[madsodgaard]

I missed this from the docs:

Because Java has implicitly nullability of references, AnyJavaObject types do not directly conform to JavaValue: rather, optionals of AnyJavaObject-conforming type conform to JavaValue. This requires Swift code to deal with the optionality
at interface boundaries rather than invite implicit NULL pointer dereferences.

So basically changing Response to Response?, made it work.

[ktoso]

My bad, forgot about this -- thanks for resolving