jpobst/prototype.android.mavenbindings

Prototype of MSBuild tasks to facilitate binding of Java libraries from Maven repositories

prototype.android.mavenbindings's Issues

Proposal: Vaguely Typed Slim Bindings

Background

One of the key tenets of "slim bindings" is that you only bind the type(s)/method(s) that you need to use. Unfortunately this does not work for a normal binding project because your desired types likely derive from other unbound types or take unbound types as method parameters or return types.

Example:

public class MapView extends MapViewBase implements MappableComponent {
  public MapView (Activity activity, MapViewOptions options) { ... }

  public PinResult AddPin (string name, PinCoordinates coordinates) { ... }
}

Trying to bind just MapView would fail because it also needs the types: MapViewBase, MappableComponent, MapViewOptions, PinResult, and PinCoordinates. Binding each of those types may require further types, and you essentially end up needing to bind the entire library.

Slim Bindings

The way slim bindings fixes this is that you write a Java wrapper around the API you need that does not expose these custom types to C#. This may look something like:

public class MyMapView extends android.view.View {
  private MapView mv;

  public MyMapView (Activity activity, string option1, bool option2) {
    var options = new MapViewOptions (option1, option2);
    mv = new MapView (activity, options);
  }
  
  public bool AddPin (string name, long latitude, long longitude) {
    var result = mv.AddPin (name, new PinCoordinates (latitude, longitude);
    return result.Success;
  }
}

In effect, it performs "type erasure" of custom types in order to bind the class.

One potential downside to this is that users must write a Java wrapper in order to expose the correct API, and many users may not know Java or would prefer to remain in C#.

Flutter's New jingen Bindings

Either as a conscience choice or because it is very early in development, jnigen does not expose any custom types in their bindings. Every non-primitive type is exposed as jni.JObject. (Their ~equivalent of Java.Lang.Object.)

So their binding of the above class would look like this (in a C#-looking language rather than dart):

public class MyMapView : Java.Lang.Object {
  public MyMapView (Java.Lang.Object activity, Java.Lang.Object options) { ... }

  public Java.Lang.Object AddPin (string name, Java.Lang.Object coordinates) { ... }
}

Basically, they are performing the same "type erasure" that our slim bindings require.

Vaguely Typed Slim Bindings

Applying the automatic type erasure that jnigen does to our generator process would provide a nice "hybrid" approach:

• Bindings could be generated automatically without the user needing to write and maintain a Java wrapper.
• Binding everything as JLO would eliminate nearly all errors that prevent typed bindings from automatically working today:
  • Missing implementations of abstract base type methods
  • Missing implementations of interface methods
  • Covariant return type issues
  • Generics
  • Mismatched override signatures

The downside is that you lose all C#/compile time type checking and type checks are performed at runtime by Java. For this reason, it feels better to keep these as "slim bindings" that are used sparingly on a few types rather than binding an entire library without types.

Example:

// Works correctly:
var activity = my_activity;
var map_view = new MapView (activity, null);

// Compiles, but throws a Java exception like IncompatibleClassException or something at runtime
var context = my_activity.Context;
var map_view = new MapView (context, null);

We would need a way to specify which types to bind, perhaps in MSBuild?

<ItemGroup>
  <AndroidMavenLibrary Include="map.company:MyMaps" Version="1.0.0" />
  <AndroidBindType Include="map.company.MyMapView" />
</ItemGroup>

Theoretically this is all that should be required to create a vaguely typed binding for the MyMapView class.

Flutter also includes the ability to exclude problematic members if needed, which could be something like:

<AndroidBindExcludeMember Include="map.mycompany.PinResult map.mycompany.MyMapView.AddPin (string, map.mycompany.PinCoordinates)" />

Or it could use XPath.

Working With Vaguely Typed Slim Bindings

Inevitably you will need to work with some additional "real" types from a library. Consider the AddPin method, which requires a PinCoordinates parameter and returns a PinResult type.

Assuming they do not come from a typed binding of a dependency, we would need to additionally bind them:

<AndroidBindType Include="map.company.PinCoordinates" />
<AndroidBindType Include="map.company.PinResult" />

We can now create instances of these types, or use JavaCast<T> to convert JLO to a type:

public void DoMapStuff (MyMapView map)
{
  var coords = new PinCoordinates (0, 0);
  var result = map.AddPin ("Null Island", coords).JavaCast<PinResult> ();

  Console.WriteLine ("Adding pin was successful: " + result.Success.ToString ());
}

Support odd version strings

Hey,

I'm trying to use this with this: https://central.sonatype.com/artifact/org.jboss/jboss-vfs/3.2.17.Final but the ".Final" is causing the following:

The "MavenDependencyVerifierTask" task failed unexpectedly.
System.ArgumentException: '3.2.17.Final' is not a valid version string.
Parameter name: value
   at NuGet.Versioning.NuGetVersion.Parse(String value)
   at MavenNet.MavenVersionRange.Satisfies(String version)
   at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
   at Prototype.Android.MavenBinding.Tasks.DependencyResolver.IsDependencySatisfied(Dependency dependency, MicrosoftNuGetPackageFinder packages, LogWrapper log)
   at Prototype.Android.MavenBinding.Tasks.MavenDependencyVerifierTask.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()	Tesseract.Binding.Droid	C:\Users\kfrancis\.nuget\packages\xamprototype.android.mavenbinding.tasks\0.0.7\build\XamPrototype.Android.MavenBinding.Tasks.targets

They seem to include that in all their versions: https://central.sonatype.com/artifact/org.jboss/jboss-vfs/3.2.17.Final/versions

Proposal: Untyped Slim Bindings

Summary

Another potential way to implement "bind only what you need" would be to remain fully in C# and place the burden on the user to provide the metadata needed to perform JNI interop. While this can be cumbersome and tricky for lots of API, for a few small uses it might be acceptable.

Downsides:

• User must manually provide the metadata needed by JNI
• Only supports C# -> Java invocations, users cannot override methods or listen for events

Open questions:

• Can we determine the method signature from the parameters the user gives us? Or would they need to provide the signature as well?

Proposed API

// Create a new "com.googleplay.services.ApiClient" type instance
var api_client = new UntypedJavaObject ("com.googleplay.services", "ApiClient");

// or with constructor parameters
var api_client = new UntypedJavaObject ("com.googleplay.services", "ApiClient", "my_api_key");

// Invoke a method with 'void' return
api_client.InvokeMethod ("setApiKey", "my_api_key");

// Invoke a method with 'int' return
var quota = api_client.InvokeMethod<int> ("getRemainingQuota");

// Get an 'int' field
var quota = api_client.GetField<int> ("API_VERSION");

// Set an 'int' field
api_client.SetField ("API_VERSION", 34);

Would also need a static version:

var api_client = new UntypedStaticJavaObject ("com.googleplay.services", "ApiClient");
var quota = api_client.GetField<int> ("API_VERSION");

Interop with Classic Java.Lang.Object

Use a JLO as an "untyped" parameter type

var view = FindViewById (Resources.layout);
var map = new UntypedJavaObject ("com.mapbox.maps", "MapView");

map.InvokeMethod ("attachToView", view);

Return a JLO from an "untyped" return type

var map = new UntypedJavaObject ("com.mapbox.maps", "MapView");
var view = map.InvokeMethod<Android.Views.View> ("getAttachedView");

Cast a JLO that was bound as Object to an "untyped" type

var view = FindViewById (Resources.layout);
var unknown_jlo = view.GetThing ();

var untyped = UntypedJavaObject.FromObject (view);

Cast an "untyped" type to a known JLO

var map = new UntypedJavaObject ("com.mapbox.maps", "MapView");
var untyped = map.InvokeMethod<UntypedJavaObject> ("getAttachedView");

var view= untyped.JavaCast<Android.Views.View> ();

Execute an unbound method on a JLO

var view = FindViewById (Resources.layout);
var untyped = UntypedJavaObject.FromObject (view);

untyped.InvokeMethod ("newUnboundMethod");

Sample

Converting the sample from https://github.com/Redth/MapboxSlimBindingDemo/tree/main:

public class MainActivity : Activity
{
    protected override void OnCreate (Bundle? savedInstanceState)
    {
        base.OnCreate (savedInstanceState);
        
        // Create new MapView ('this' is 'Android.Content.Context')
        var mapview = new UntypedJavaObject ("com.mapbox.maps", "MapView", this);

        // MapView inherits Android.Views.View
        var view = mapview.JavaCast<Android.Views.View> ();

        // Set as our view
        SetContentView (view);
    }
}

Parent POM not understood

There seems to be an additional way to specify a parent POM we need to investigate:
https://repo1.maven.org/maven2/com/squareup/wire/wire-runtime/4.7.1/wire-runtime-4.7.1.pom

That is, this POM does not specify a <parent> section like our previous examples did:
https://repo1.maven.org/maven2/com/google/guava/guava/31.1-android/guava-31.1-android.pom

<parent>
  <groupId>com.google.guava</groupId>
  <artifactId>guava-parent</artifactId>
  <version>31.1-android</version>
</parent>

Instead, it specifies this which I think we need to add support for:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>com.squareup.okio</groupId>
      <artifactId>okio-bom</artifactId>
      <version>3.3.0</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>

This probably allows multiple parent POMs to be specified?

Intercom

So, I've long been searching for a way to create bindings without the annoying step of interacting with maven to get the aar file, so this is AMAZING.

That being said, try creating one for intercom. The SDK Base, for example, has a very large set of runtime dependencies so it's quite tedious setting that up: https://mvnrepository.com/artifact/io.intercom.android/intercom-sdk-base/14.2.0

So far, the base csproj is starting to look like this and I'm curious if I'm doing this right:

<ItemGroup>
	<PackageReference Include="GoogleGson" Version="2.10.1.1" />
	<PackageReference Include="Square.OkHttp3" Version="4.10.0.1" />
	<PackageReference Include="Xamarin.AndroidX.Annotation" Version="1.6.0" />
	<PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.6.1" />
	<PackageReference Include="Xamarin.AndroidX.ConstraintLayout" Version="2.1.4.3" />
	<PackageReference Include="Xamarin.AndroidX.Core.Core.Ktx" Version="1.9.0.2" />
	<PackageReference Include="Xamarin.AndroidX.DataBinding.ViewBinding" Version="7.4.2" />
	<PackageReference Include="Xamarin.AndroidX.ExifInterface" Version="1.3.6" />
	<PackageReference Include="Xamarin.AndroidX.Fragment.Ktx" Version="1.5.6" />
	<PackageReference Include="Xamarin.AndroidX.RecyclerView" Version="1.3.0" />
	<PackageReference Include="Xamarin.AndroidX.VectorDrawable" Version="1.1.0.16" />
	<PackageReference Include="Xamarin.AndroidX.VectorDrawable.Animated" Version="1.1.0.16" />
	<PackageReference Include="Xamarin.AndroidX.WebKit" Version="1.6.1" />
	<PackageReference Include="Xamarin.Google.Android.Material" Version="1.8.0" />
	<PackageReference Include="Xamarin.Kotlin.StdLib" Version="1.8.20" />
	<PackageReference Include="Xamarin.KotlinX.Coroutines.Core" Version="1.6.4.2" />
	<PackageReference Include="XamPrototype.Android.MavenBinding.Tasks" Version="0.0.7" />
</ItemGroup>
<ItemGroup>
	<AndroidMavenLibrary Include="io.intercom.android:intercom-sdk-base" Version="14.2.0" />
	<AndroidMavenLibrary Include="io.intercom.android:intercom-sdk-ui" Version="14.2.0" />
	<AndroidMavenLibrary Include="com.squareup.okhttp3:okhttp" Version="4.9.3" Bind="false" />
	<AndroidMavenLibrary Include="org.jetbrains.kotlin:kotlin-parcelize-runtime" Version="1.8.20" Bind="false" />
	<AndroidMavenLibrary Include="org.jetbrains.kotlin:kotlin-android-extensions-runtime" Version="1.5.20" Bind="false" />
</ItemGroup>
<ItemGroup>
  <ProjectReference Include="..\Intercom.Droid.Commons\Intercom.Droid.Commons.csproj" JavaArtifact="com.intercom:android-commons" JavaVersion="2.0.0" />
  <ProjectReference Include="..\Intercom.Droid.Composer.Gallery\Intercom.Droid.Composer.Gallery.csproj" JavaArtifact="com.intercom:android-composer-gallery" JavaVersion="3.3.3" />
  <ProjectReference Include="..\Intercom.Droid.Composer\Intercom.Droid.Composer.csproj" JavaArtifact="com.intercom:android-composer" JavaVersion="3.3.3" />
</ItemGroup>

For example, if I add one that it's complaining about:

Severity	Code	Description	Project	File	Line	Suppression State	Priority
Error		Maven dependency 'com.google.android.flexbox:flexbox' version '3.0.0' is not satisfied.	Intercom.Droid.SDK.Base	C:\Users\kfrancis\.nuget\packages\xamprototype.android.mavenbinding.tasks\0.0.7\build\XamPrototype.Android.MavenBinding.Tasks.targets	28		Normal

as <AndroidMavenLibrary Include="com.google.android.flexbox:flexbox" Version="3.0.0" Bind="false" /> then I get the error:

Severity	Code	Description	Project	File	Line	Suppression State	Priority
Error		Cannot download artifact 'com.google.android.flexbox:flexbox'.
- com.google.android.flexbox_flexbox.jar: Response status code does not indicate success: 404 (Not Found).
- com.google.android.flexbox_flexbox.aar: Response status code does not indicate success: 404 (Not Found).	Intercom.Droid.SDK.Base	C:\Users\kfrancis\.nuget\packages\xamprototype.android.mavenbinding.tasks\0.0.7\build\XamPrototype.Android.MavenBinding.Tasks.targets	22		Normal

AFAIK, that entry should be correct based on the maven information: https://mvnrepository.com/artifact/com.google.android.flexbox/flexbox/3.0.0