Jul 17, 2019
Jan 20, 2020
Fabio Collini (fabioCollini)
Fabio Collini (fabioCollini)
Source code



Inversion simplifies the ServiceLoader usage to retrieve all the implementations of a certain interface. Using Inversion it's easy to use the dependency inversion in a multi module project.

Basic example

A first module defines an interface and a create field (annotated with InversionDef) to create the real implementation of the interface:

interface MyInterface {
    fun doSomething(): String

    companion object {
        val create by Inversion.of(MyInterface::class)

The create field is a () -> MyInterface lambda, it can be used to create a new instance. The first module doesn't contain any real implementation of MyInterface.

A second module defines the real implementation annotated with InversionImpl:

class MyImpl : MyInterface {
    override fun doSomething() = "Hello world!"

And that's all! Now the real implementation can be retrieved invoking create:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        val impl = MyInterface.create()
        findViewById<TextView>( = impl.doSomething()

Custom implementation creation

In case an extra logic is needed to create the real implementation, an InversionProvider annotated method can be used:

fun provideImpl(): MyInterface = MyImpl() 

If a parameter is needed the InversionDef annotated property can be defined as an extension property:

interface MyInterface {
    fun doSomething()

val Application.factory by Inversion.of(MyInterface::class)

In this example the Application instance can be used to create the implementation in the InversionProvider annotated method:

class MyImpl(val app: Application) : MyInterface {
    override fun doSomething() {

fun Application.provideImpl(): MyInterface = MyImpl(this)

Multi binding

Multiple implementations can be managed using the mapOf method instead of of:

val allValues by Inversion.mapOf(MyInterface::class)

In this way the allValues field is a () -> Map<String, MyInterface> that can be used to retrieve a map with all the implementations.

Multiple implementations can be defined specifying in the annotation a String that will be used as the key in the map:

fun MyClass.provideImplA(): MyInterface = MyImplA()

fun MyClass.provideImplB(): MyInterface = MyImplB()

A multi binding example is available in this plaid fork, here the commits that introduces Inversion and removes some reflection calls.

Internal implementation

Under the hood Inversion is an annotation processor that generates the configuration to simplify the ServiceLoader usage. ServiceLoader is a standard Java class that can be used to discover the implementations of an interface based on some config file. An explanation on how to use a ServiceLoader on Android is available in the post Patterns for accessing code from Dynamic Feature Modules, here there are the commits to introduce Inversion in the demo project.

ServiceLoader reads some resource files to retrieve the implementations to use, on Android the disk read can be avoided using R8. R8 removes the ServiceLoader calls and replaces it with the direct instantiations. Looking at the code of the last example in the apk optimized with R8 we can find the following code instead of the ServiceLoader invocation:

Iterator it = Arrays.asList(new MultiInstanceInterface_Factory[] {
    new MultiInstanceInterface_FactoryImpl_B(), 
    new MultiInstanceInterface_FactoryImpl_A()

MultiInstanceInterface_FactoryImpl_A and MultiInstanceInterface_FactoryImpl_B are two classes generated by Inversion.


The Inversion annotation processor executes some basic validations to raise a compilation error in case of any misconfiguration. A validate method in the Inversion object can be invoked to verify that there is an implementation defined for each definition. This method uses reflection calls so it's better to invoke it in a test or in the application startup only in the debug build.

JitPack configuration

Inversion is available on JitPack, you can use it adding the JitPack repository in your build.gradle (in top level dir):

repositories {
    maven { url "" }

and the dependencies in the build.gradle of the modules:

dependencies {
    kapt 'com.github.fabioCollini.inversion:inversionCodGen:0.3.1'
    implementation 'com.github.fabioCollini.inversion:inversionLib:0.3.1'


Copyright 2019 Fabio Collini

Licensed under the Apache License, Version 2.0 (the "License");
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