Creating a library binding in Android

1. Xamarin Android Binding
David Hauck
Software Engineer at Skyline Technologies
Xamarin Certified Developer

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4. August 8th – 10th 2016
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5. What is binding?
 Run an existing Java library in your C# application

6. Why should we use binding?
 Many existing java libraries for Android
 Use them in your app with little effort

7. How does binding work?
Picture from xamarin.com
https://developer.xamarin.com/guides/android/advanced_topics/java_integration_overview/binding-a-java-library/

8. Managed Callable Wrapper (MCW)
Java Class C# Code

9. Why MCWs?
 Encapsulate JNI use so that client code doesn't need to know about the
underlying complexity
 Permit sub-classing Java types and implementing Java interfaces

10. Under the hood
 JNI
 The Java Native Interface (JNI) is a programming framework that enables Java
code running in a Java Virtual Machine (JVM) to call and be called by native
applications (programs specific to a hardware and operating system platform) and
libraries written in other languages such as C, C++ and assembly.
 (Wikipedia)

11. Example: Constructor Binding
int value = 42;
IntPtr class_ref = JNIEnv.FindClass ("java/lang/Integer");
IntPtr id_ctor_I = JNIEnv.GetMethodID (class_ref, "<init>", "(I)V");
IntPtr lrefInstance = JNIEnv.NewObject (class_ref, id_ctor_I, new JValue (value));
// Dispose of lrefInstance, class_ref…

12. Creating MCWs for your Xamarin project
 Have a java library (jar)
 Create a Xamarin Binding project
 Insert jar into project
 Build the project
 Creates dll consisting of the MCW

13. Jar Build Actions
 Input Jar
 Will create the actual bindings for this jar
 EmbeddedReferenceJar

14. Creating MCWs for your Xamarin project
api.xml Transforms MCW

15. Api.xml
 {Project_Directory}objDebugapi.xml
 Xamarin parses the jar and creates this file
 Diagram of how the api is laid out
 Includes classes, methods, etc

16. Api.xml

17. Transforms
 EnumFields.xml
 Converts Constants to Enums
 EnumMethods.xml
 Changes parameters in methods to use Enums
 Metadata.xml
 Various add/remove/edit transforms

18. EnumFields.xml
<mapping jni-class="android/support/v4/app/FragmentActivity$FragmentTag"
clr-enum-type="Android.Support.V4.App.FragmentTagType">
<field jni-name="Fragment_name" clr-name="Name" value="0" />
<field jni-name="Fragment_id" clr-name="Id" value="1" />
<field jni-name="Fragment_tag" clr-name="Tag" value="2" />
</mapping>

19. EnumMethods.xml
<mapping jni-class="android/support/v4/app/Fragment.SavedState">
<method jni-name="writeToParcel" parameter="flags" clr-enum-
type="Android.OS.ParcelableWriteFlags" />
</mapping>

20. Metadata.xml
<attr
path="/api/package[@name='com.kontakt.sdk.android.common.model']/interface[@name='IModel']“
name="visibility">public</attr>
<remove-node
path="/api/package[@name='com.kontakt.sdk.android.common.util']/class[@name='Closeables']/me
thod[@name='closeQuietly' and count(parameter)=1 and
parameter[1][@type='java.io.OutputStream']]" />

21. Demo