Binding the Cognitive Services Android Speech SDK - Part 1, binding the library

Jim Bennett | Sep 9, 2018

As part of the Microsoft Cognitive Services speech API, there is a native Java Android SDK available as an .aar file. I wanted to use this in a Xamarin app, so I created a binding project for it.

The code for this is available in my GitHub.

Binding an SDK is a four step process:

  • Create the binding project with the relevant jar or aar file
  • Make any necessary tweaks to the code or project to make it compile
  • Make any required amendments to the code to make it into idiomatic C#
  • Test it all out and fix up any issues

There are some great docs available on the basics for doing this at, but each library is different and can have it’s own unique challenges so I thought I’d write a few posts to highlight the steps I needed to take to bind the speech SDK.

In this first part, I’ll show how to create a binding project, add the speech SDK aar file, and make everything compile. In the second part I’ll show how to make the code more idiomatic C#, then in the third part I’ll show how to use it and fix up a nasty issue with the Android compiler and using jars created with the latest versions of Java.

Binding the SDK

The first step was to create a binding project and add the .aar file. I followed the instructions in the Xamarin docs, creating a new Android binding project and adding the client-sdk-0.6.0.aar file to the Jars folder.

A binding project with an aar file in the Jars folder

When you compile this project, the compile step will generate code to bind every Java class it finds. Each class in the generated code is a wrapper for a Java class - it doesn’t re-implement the Java code, it instead creates the same kind of thin wrapper that is used by the Xamarin Android SDK bindings. If you want to see this generated code, you can find it in the obj/${Congfiguration}/generated/src folder.

Making it work

After doing this, I compiled the library and hit a couple of compiler errors that I needed to fix up:

/.../Com.Microsoft.Cognitiveservices.Speech.Internal.StdMapWStringWString.cs(72,72): Error CS0738: 'StdMapWStringWString' does not implement interface member 'IIterable.Iterator()'. 'StdMapWStringWString.Iterator()' cannot implement 'IIterable.Iterator()' because it does not have the matching return type of 'IIterator'. (CS0738) (Speech)
/.../Com.Microsoft.Cognitiveservices.Speech.Internal.StdMapWStringWStringMapIterator.cs(83,83): Error CS0738: 'StdMapWStringWStringMapIterator' does not implement interface member 'IIterator.Next()'. 'StdMapWStringWStringMapIterator.Next()' cannot implement 'IIterator.Next()' because it does not have the matching return type of 'Object'. (CS0738) (Speech)

The reason for this is that the Xamarin Java SDK doesn’t contain the generic versions of IIterator and IIterable which are used by this library, instead it will default to using the non-generic versions, and the implementation of the generic interfaces doesn’t match the signature of the non-generic version. So - how can it be fixed?

Metadata.xml and Additions

Inside the binding project you can both alter the generated code and add new code.

  • Transforms/Metadata.xml - in this XML file you can alter the code that is generated. You can add entries to this file to remove some of the autogenerated code, either at class, method or property level. You can also change the generated code, for example changing the namespace - something especially useful to change from Java style namespaces to C# style.

  • Additions - In this folder you can add code that is compiled into the final dll. Each autogenerated class is declared as partial, so not only can you add new classes and code, you can also add new parts to a generated class.

Fixing the StdMapWStringWString code

The StdMapWStringWString class implements a generic version of the IIterable interface - IIterable<StdMapWStringWStringMapIterator>. The Xamarin Java SDK doesn’t contain the generic base interface, so the bound library defaults to implementing IIterable. The problem is this interface contains a method Iterator that returns a different type in the generic version to the non-generic version. The generated code implements this method returning a StdMapWStringWStringMapIterator, but the non-generic version expects a method returning IIterator, so you get a compiler error.

This is simple enough to fix - you just need to change the return type of the binding to be IIterator, and this can be done in the Metadata.xml file.

The Metadata.xml file is a file containing transforms that you want to make to the generated code - and can be adding new items, removing items or change the attributes of items such as the name or the return type.

Open the generated code from the obj/${Congfiguration}/generated/src/ folder - the file will be called Microsoft.Azure.CognitiveServices.Speech.Internal.StdMapWStringWString.cs. If you look at all the public items in this file (the class, public methods and public properties), you will see each one has a comment describing the Metadata.xml path:

// Metadata.xml XPath class reference: path="/api/package[@name='']/class[@name='StdMapWStringWStringMapIterator']"
public partial class StdMapWStringWStringMapIterator

This path is used to identify each item to the Metadata.xml file, so locate the Iterator() method and note the path.

Open the Metadata.xml file and add a new attr node inside the metadata node. Set the path attribute of this node to match the path in the comment for the Iterator() method. Then add an attribute called name with the value managedReturn to tell the transformations that this is a changed to the managed return type - so the type for the binding library only. This will treat the underlying return value as the original type which is what you want. The value for this attribute is set inside the node, and should be Java.Util.IIterator.

The full node is shown below:

  <attr path="/api/package[@name='']/class[@name='StdMapWStringWString']/method[@name='iterator' and count(parameter)=0]" name="managedReturn">Java.Util.IIterator</attr>

Now if you compile the project, one error will be gone. If you re-open the generated file you will see the new return type.

You can read more on the capabilities if this file in the docs.

Fixing the StdMapWStringWStringMapIterator code

The Xamarin Java SDK doesn’t contain the generic version of IIterator, so the bound code uses the non-generic version. This interface has a Next() method that returns the next item from the collection being iterated. In the non-generic version of this interface, Next() returns a Java.Lang.Object, whereas in the generic version it returns the generic arg, in this case a Java.Lang.String. This means the generated code uses the non-generic interface, but the implementation uses the generic method, causing a compiler error.

The fix for this is a little bit more work - you can’t just change the return type as it is used in a private method created by the binding. Instead, the fix for this is to remove the generic Next() method and replace it with a non-generic version. Re-writing binding methods is not easy as there is a lot of code in the binding, so for cases like this the best way is to copy the generated code, adjusting it to suit. If you open the file from the obj directory, and look at the Next() method you will see the implementation consists of not just the Next() method, but also an n_next() method, a GetNextHandler() method and a cb_next Delegate field. This is all the plumbing needed to create the binding method and call through to the underlying Java method.

Adding the new method

Lets’s start by adding the new method as this will mostly be a copy of the existing code. Add a new class to the Additions folder called StdMapWStringWStringMapIterator, mark the class as partial and change the namespace to match the generated file (these will be fixed up later to be more C#-like).

namespace Com.Microsoft.Cognitiveservices.Speech.Internal 
    public partial class StdMapWStringWStringMapIterator

Copy the Next(), GetNextHandler(), n_next() methods and the cb_next field from the generated code and paste them into the new class part. Strip out any unnecessary namespaces and use var everywhere - this will make it easier when you change the namespaces to be more C#-like in the next post.

Change the return type of Next to be Java.Lang.Object instead of Java.Lang.String. Leave the types in the Register attribute and __id fields as they are, as the underlying method that is called returns a Java.Lang.String, and you only need to change the return type for the binding wrapper.

In the n_next() method, change the return call to call ToString() on the result of the call to Next() to use the correct type. The final code will look like this:

namespace Com.Microsoft.Cognitiveservices.Speech.Internal
    public partial class StdMapWStringWStringMapIterator
        static Delegate cb_next;
# pragma warning disable 0169
        static Delegate GetNextHandler()
            if (cb_next == null)
                cb_next = JNINativeWrapper.CreateDelegate((Func<IntPtr, IntPtr, IntPtr>)n_Next);
            return cb_next;

        static IntPtr n_Next(IntPtr jnienv, IntPtr native__this)
            var __this = Object.GetObject<StdMapWStringWStringMapIterator>(jnienv, native__this, JniHandleOwnership.DoNotTransfer);
            return JNIEnv.NewString(__this.Next()?.ToString()); // ToString called on the object.
# pragma warning restore 0169

        [Register("next", "()Ljava/lang/String;", "GetNextHandler")]
        public virtual unsafe Java.Lang.Object Next() // Return type changed from string to object
            const string __id = "next.()Ljava/lang/String;";
                var __rm = _members.InstanceMethods.InvokeVirtualObjectMethod(__id, this, null);
                return JNIEnv.GetString(__rm.Handle, JniHandleOwnership.TransferLocalRef);

Removing the Next method

To remove a method from the autogenerated file, you add a remove-node entry to the Metadata.xml file with the path of the Next() method node you want to remove. To remove the Next() method, add this following to this file:

<remove-node path="/api/package[@name='']/class[@name='StdMapWStringWStringMapIterator']/method[@name='next' and count(parameter)=0]"/>

The syntax is <remove-node path="..."/> where the path comes from the comment in the generated code. Once this line has been added, compile the code and check the generated file in the obj directory. The compiler error about the missing Next() method will have gone, and the Next() method will be removed from the autogenerated file - when the library is built it will use the version in the file in the Additions folder.

You should now be able to compile this library successfully.

In the second part, I’ll show how you can make the code more C#-like. You can find the code for this in my GitHub, and you can read more on