Using java interop in your xamarin.android apps

Using Java
interop in your
Xamarin.Android
apps
@WMeints

Agenda
Java interop… wait what!?
Beyond basic interop
Building your own interop components

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();
}

Meanwhile in Java country…
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_Handlern" +
"";
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, “ …

Cross Platform Mobile Development - Android


Mono callable wrappers

Android
.NET APIs MCW Android.* Java.*
Bindings

Mono (.NET Runtime) ACW Dalvik (Java Runtime)

Linux Kernel
Android 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
The 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

Android
Bindings


Linux Kernel

Beyond basic interop – MCWs
Talking 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…


Demo

MCW INTERNALS


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


Steps to invoke a Java method:
– Find the handle to the method
– Marshal the arguments
– Invoke the method


Notice 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!


Important 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

Android
Bindings


Linux Kernel

Demo

ACW INTERNALS


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

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


Performance considerations
A 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
Explicit 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 sources
Add 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).


Demo

ADDING JAVA SOURCES


Binding Java libraries
This 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 libraries
The steps for binding a Java library:

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


Demo

BINDING A JAVA LIBRARY


Demo

CUSTOMIZING TRANSFORMATIONS


Demo

EXTENDING BINDINGS


Final thoughts
Java 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!


Using java interop in your xamarin.android apps

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