Using java interop in your xamarin.android apps

Using Java interop in your

Xamarin.Androidapps

@WMeints

AgendaJava interop… wait what!?

Beyond basic interop

Building 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 APIsAndroidBindings

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

– JavaMono 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 pointerificdepending 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 project

2. 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!


Using java interop in your xamarin.android apps

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