One of the best ways to investigate a problematic Xamarin.Android Binding is to first ensure you have the proper tooling available:
- Diagnostic MSBuild Output Enabled(Instructions)
- Java Decompiler(http://jd.benow.ca/)
- .NET Decompiler(https://www.jetbrains.com/decompiler/)
- Binding SDK Documentation
- Android API Level Documentation
- Optional: Beyond Compare(Or a similar tool for comparing files)
After you have all of your tools available, you can take a preliminary look at the problem at hand.
- Build the problematic Bindings project
- Get the full Diagnostic Build Log
- Examine the error (You may need to look through each Build Task to get a further idea of where the problem lies)
You now have the diagnostic build output, which should already give us clues to the problem at hand. Now we can do some investigation in the Android Library(.aar/.jar
) and various documentation.
First, let's decompile the Android Library
- If it's a
.jar
, simply drag/open the .jar in a Java Decompiler - If it's an
.aar
, you can extract/unzip the archive and find theclasses.jar
file to open in a Java Decompiler
Now it's time for an initial look. Do you see anything out of the ordinary or closely related to the error message at hand?
Here are a couple of things that come to mind:
-
Are there any classes that have characteristics of obfuscation? (only lowercase letters/numbers/$) EX:
a.class / a$.class
-
Are there any
import
statements of libraries not referenced? -
What respective versions of dependencies does the Binding SDK use?
-
What Android API level does the
.jar/.aar
support? -
What version of Java/JDK was this library compiled with?
The api.xml
file is typically found in the obj\Debug
folder of the Bindings project. This will be an XML definition of the API at hand. This is a great starting place to see what is currently being generated, and what can be missing. It also gives a reference to other generated classes/types that can help assist you when you're fixing Metadata.xml
.
- If the library already exists on NuGet, simply download the NuGet package to the Bindings project. (Support libraries / etc)
- Otherwise add the missing library to your bindings project as a
ReferenceJar
,EmbeddedReferenceJar
, orLibraryProjectZip
Sometimes types will not be generated or unexpected crashes may occur because you are using either a newer or older version of Java compared to what the library was compiled with. Ensure that the JDK Version is the same or compatible with the library.
/interface
EX:/interface[@name='AuthListener']
/class
EX:/class[@name='MapView']
/method
EX:/method[@name='setTileSource']
/method(with parameters)
EX:/method[@name=``onCreate``and count(parameter)=2 and parameter[1][@type=``com.my.CustomActivity``] and parameter[2][@type=``android.os.Bundle``]]"
/parameter
EX:/parameter[@name='p0']
/parameter(with type)
EX:parameter[1][@type=``com.my.CustomActivity``]
name="managedType"
- EX:Java.Lang.Object
name="obfuscated"
- Changes the obfuscation EX:true
/false
name="managedName"
- Changes the managed name EX:MyCSharpName
name="propertyName"
- Changes the property name EX:MyPropertyName
name="managedReturn"
- Changes the managed return type EX:Java.Lang.Object
name="argsType"
- changes the argument type EX:MyCustomErrorEventArgs
name="sender"
- Changes which parameter of a method should be the sender parameter when it's mapped to an event EX:true
/false
name="eventName"
- Changes the event name EX:MyEventName
Typically we will see characteristics of obfuscated types in our respective .jar/.aar
libraries and we must unobfuscate them for the Bindings Generator to generate the respective C# types.
<attr path="/api/package[@name='{package_name}']/class[@name='{name}']" name="obfuscated">false</attr>
See Common Paths for more types
Sometimes you'll run into duplicate managedNames
or you might need to normalize your generated C# classes for sanity reasons.
<attr path="/api/package[@name='{package_name}']/class[@name='{name}']" name="managedName">NewManagedName</attr>
See Common Paths for more types
Your class might not have the proper visibility for the Bindings Generator to traverse through as it does not generate bindings for non-public classes or derived classes. Typically switching the visibility to public
fixes this.
<attr path="/api/package[@name='{package_name}']/class[@name='{name}']" name="visibility">public</attr>
See Common Paths for more types
You can use <add-node>
to add just about anything to your binding which will generate in the api.xml
file. Typically you may want to add a class, change a constructor, or switch a generic type.
EX: (Creates a class with a constructor and field):
<add-node path="/api/package[@name='org.alljoyn.bus']">
<class abstract="false" deprecated="not deprecated" final="false" name="AuthListener.AuthRequest" static="true" visibility="public" extends="java.lang.Object">
<constructor deprecated="not deprecated" final="false" name="AuthListener.AuthRequest" static="false" type="org.alljoyn.bus.AuthListener.AuthRequest" visibility="public" />
<field name="p0" type="org.alljoyn.bus.AuthListener.Credentials" />
</class>
</add-node>
Typically it's easiest to just remove anything in a binding that we will not use. You can look at the class that you want to use and see everything it references to get a better idea of what you will need and what you will not.
<remove-node path="/api/package[@name='{package_name}']/class[@name='{name}']" />
See Common Paths for more types
https://gist.github.com/brendanzagaeski/c32d65c21e152799af69
https://gist.github.com/brendanzagaeski/6d1052a8b76f9067548e
https://gist.github.com/brendanzagaeski/69f490e31ca6a71136ff
https://gist.github.com/brendanzagaeski/3868e30b85a1feb1181b
https://gist.github.com/brendanzagaeski/9607158
- Be sure to
[Export]
the respective Method/Class/etc. - Also ensure you reference Mono.Android.Export in your Xamarin.Android Project
https://developer.xamarin.com/api/type/Java.Interop.ExportAttribute/
In computing, 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.
A C# type that wraps the respective Java type. Typically the operations are forwarded from the wrapper class to the Java type.
This typically means the C# object inherits Java.Lang.Object
When using a Build Action
such as EmbeddedJar
or EmbeddedReferenceJar
, it will embed the respective library into the .apk so it will be available at runtime.
Otherwise it is expected that either the Device or the application will provide the .jar at runtime. (I.E. It is already loaded on device or will be provided via a download/etc)
When using a Build Action
such as ReferenceJar
or EmbeddedReferenceJar
, it will not generate Manage Callable Wrappers(ACW) and will not be exposed to the client.
Because of this limitation, you will need to be aware of the respective generated C# code as there might be certain things that the languages handle differently.
EX: Java -> C#
- get/set methods -> properties
- fields -> properties
- listeners -> events
- static nested class -> nested class
- inner class -> nested class with an instance constructor
Although Xamarin.Android Binding errors might be confusing and the JNI might be intimidating, there is always a few ways to work around the issue at hand.
Documentation:
Creating Bindings Using Metadata
Naming Parameters With Javadoc
Xamarin Univeristy Course:
https://university.xamarin.com/classes/track/xamarin-android#and450-binding