Using Java interop in your Xamarin.Android apps

Using Java interop in your

Xamarin.Android apps

@WMeints

AgendaJava interop… wait what!?Beyond basic interopBuilding your own interop components

JAVA INTEROP, WAIT WHAT?!

Java interop, what what?!Building an Android app in C#

namespace TaskTracker.Client.Android{ [Activity(Label = "TaskTracker.Client.Android", MainLauncher = true, Icon = "@drawable/icon")] public class Activity1 : ListActivity { private TasksOfflineContext _context;

protected override void OnCreate(Bundle bundle) { base.OnCreate(bundle);

_context = new TasksOfflineContext("sync", new Uri("http://10.0.2.2:15314/taskssyncservice.svc/"));

_context.CacheController.ControllerBehavior.SerializationFormat = SerializationFormat.ODataJSON;

_context.CacheController.RefreshCompleted += OnRefreshCompleted; _context.CacheController.RefreshAsync(); }

Java interop, what what?!Meanwhile in Java country…

Cross Platform Mobile Development - Android

public class Activity1 extends android.app.ListActivity implements mono.android.IGCUserPeer{

static final String __md_methods;static {

__md_methods = "n_onCreate:(Landroid/os/Bundle;)V:GetOnCreate_Landroid_os_Bundle_Handler\n" +"";

mono.android.Runtime.register ("TaskTracker.Client.Android.Activity1, “ + “TaskTracker.Client.Android, Version=1.0.0.0, Culture=neutral,

PublicKeyToken=null", Activity1.class, __md_methods);

}

public Activity1 (){

super ();if (getClass () == Activity1.class)

mono.android.TypeManager.Activate ("TaskTracker.Client.Android.Activity1, “ + “TaskTracker.Client.Android, Version=1.0.0.0,

Culture=neutral, “ …

Java interop, what what?!

Linux Kernel

Mono (.NET Runtime) Dalvik (Java Runtime)

.NET APIs Android Bindings

Android.* Java.*MCW

ACW

Android callable wrappers

Mono callable wrappers

Java interop, wait what?!What you are essentially doing is building apps that rely heavily upon interop.

In fact: Everything you build on Android will at some point talk to a piece Java code.


Java interop, wait what?!So why not take advantage of the situation?

– You can extend your app with Java code– Found a cool Android library? Bind it!


BEYOND BASIC INTEROP

Beyond basic interopThe two-way traffic between Android and Mono is done through two endpoints:

– Mono Java Mono Callable Wrappers– Java Mono Android Callable Wrappers


Beyond basic interop - MCWs

Linux Kernel



Android.* Java.*MCW

ACW

Beyond basic interop – MCWsTalking to Java from .NET is done through the Java Native Interface

– The next bit is going to get pointy or pointerific depending on what you like…


MCW INTERNALSDemo


Beyond basic interop – MCWsSteps to create an instance of a Java class– Find the handle to the type– Marshal constructor arguments– Invoke the constructor– Safe the instance pointer!


Beyond basic interop – MCWsSteps to invoke a Java method:– Find the handle to the method– Marshal the arguments– Invoke the method


Beyond basic interop – MCWsNotice the TransferOwnership settings

– Important, who is the owner of the handle?– Care must be taken when transferring handles– If two objects own a handle, the app will be

unstable!


Beyond basic interop – MCWsImportant to know:

– When invoking JNI, native handles are used– Has effect on garbage collection, so clean it up!– Please, Reduce the amount of memory copy

actions, it will improve the performance.


Beyond basic interop - ACWs

Linux Kernel



Android.* Java.*MCW

ACW

ACW INTERNALSDemo


Beyond basic interop - ACWsAndroid callable wrappers are the least of your problems.

– Generated by the Mono compiler– Don’t touch or you will break them!


Performance considerationsA few things you need to know:

– Value types are copied between Java and Mono– For reference types pointers are exchanged– 4 bytes in .NET != 4 bytes in Java, sometimes


Beware Bitmap users!

BUILDING YOUR OWN INTEROP

Building your own interopExplicit use of interop is possible from your own app through these methods:

– Add Java source files to your project– Create bindings for an existing library


Adding Java sourcesAdd a .java file to your project for

– You found an activity or service that you don’t want to translate to .NET code

– You have a single component, but not enough to create a library (Please keep it to one file).


ADDING JAVA SOURCESDemo


Binding Java librariesThis is the real stuff, the big one, the goodest.

– Allows you to use existing libraries– Automatically generates wrappers for Java classes

based on the settings you provide.


Binding Java librariesThe steps for binding a Java library:

1. Add the Java library to the binding project2. Customize the transformations 3. Extend the library with your own goodies





BINDING A JAVA LIBRARYDemo





CUSTOMIZING TRANSFORMATIONSDemo





EXTENDING BINDINGSDemo


Final thoughtsJava interop is all around you in Xamarin.Android

– Don’t worry too much about it in your day-to-day Android development.

– Use it when you need it, to get more power!


Documents

Using Java interop in your Xamarin.Android apps