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Kodein

Kodein is a Dependency Injection library. It allows you to bind your business unit interfaces with their implementation and thus having each business unit being independent.

Kodein-DI allows you to:

• Lazily instantiate your dependencies when needed;
• Stop caring about dependency initialization order;
• Easily bind classes or interfaces to their instance, provider or factory;
• Easily debug your dependency bindings and recursions.

Kodein-DI is a good choice because:

• It is small, fast and optimized (makes extensive use of inline);
• It proposes a very simple and readable declarative DSL;
• It is not subject to type erasure (like Java);
• It integrates nicely with Android;
• It proposes a very kotlin-esque idiomatic API;
• It (also) can be used in plain Java.

Kodein is compatible with all platforms that the Kotlin language compiles to: JVM & compatible (Android), Javascript and all the Kotlin/Native targets.

Bindings

Declaration

In Kodein, bindings are declared in a DI Block. The syntax is quite simple:

val kodein = DI {
    bindProvider<Random> { SecureRandom() }
    bindSingleton<Database> { SQLiteDatabase() }
}

As you can see, Kodein offers a DSL (Domain Specific Language) that allows to very easily declare a binding. Kodein offers many bindings that can manage implementations: provider, singleton, factory, multiton, instance, and more.

Separation

When building large applications, there are often different modules, built by their own team, each defining their own business units.

Kodein allows you to define binding modules that can later be imported in a DI container:

val module = DI.Module {
    bindSingleton<Database>(tag = "local") { SQLiteDatabase() }
    bindProvider<Database>(tag = "remote") { DatabaseProxy() }
}

Importing a DI module:

val di = DI {
    import(module)
}

Injection & Retrieval

The container

All declarations are made in the constructor of a DI container. Think of the DI container as the glue that allows you to ask for dependency. Whatever dependency you need, if it was declared in the DI container constructor, you can get it by asking DI. This means that you always need to be within reach of the DI object. There are multiple ways of doing so:

• You can pass the DI object around, as a parameter to created objects;
• You can have the DI object being statically available (in Android, for example, it is common to use a property of the Application object).

Injection vs Retrieval

Kodein supports two different methods to allow a business unit to access its dependencies: injection and retrieval.

• When dependencies are injected, the class is provided its dependencies at construction;
• When dependencies are retrieved, the class is responsible for getting its own dependencies.

Injection

If you want your class to be injected, then you need to declare its dependencies at construction:

class Presenter(private val db: Database, private val rnd: Random) {}

Now you need to be able to create a new instance of this Presenter class. With Kodein-DI, this is very easy:

val presenter by di.newInstance { Presenter(instance(), instance()) }

For each argument of the Presenter constructor, you can simply use the instance() function, and Kodein-DI will actually pass the correct dependency.

Retrieval

When using retrieval, the class needs to be available to access a DI instance, either statically or by argument. In these examples, we’ll use the argument.

class Presenter(val di: DI) {
    private val db: Database by di.instance()
    private val rnd: Random by di.instance()
}

Note the usage of the by keyword. When using dependency retrieval, properties are retrieved lazily.

Kodein on Android

You can use Kodein as-is in your Android project or use the util library kodein-di-android.

Note. Kodein does work on Android as-is. The kodein-di-android-* extensions add multiple android-specific utilities to Kodein. Using or not using this extension really depends on your needs.

Install

How to use kodein-di-android:

1. Add this line in your dependencies block in your application build.gradle file:

   implementation 'org.kodein.di:kodein-di-framework-android-???:7.14.0'
   

2. Declare the dependency bindings in the Android Application, having it implements DIAware.

   class MyApp : Application(), DIAware {
    override val di by DI.lazy { 
        /* bindings */
    }
   }
   

3. In your Activities, Fragments, and other context aware android classes, retrieve the DI object with the di function.
4. Retrieve your dependencies!

View Models

The kodein-di-framework-android-x-viewmodel module simplifies injection and retrieval of View Models which inherit androidx.lifecycle.ViewModel, without requiring that the View Model implement DIAware.

Note. If your View Model uses a SavedStateHandle, use the View Model with SavedStateHandle module instead.

Create a binding for a ViewModel:

val di = DI {
    bindProvider { MyViewModel(
        myDependency = instance()
    ) }
}

Retrieve a plain ViewModel in an Activity or Fragment:

class MyActivity : AppCompatActivity(), DIAware {
    override val di: DI by closestDI()
    private val viewModel: MyViewModel by viewModel()
}

class MyFragment : Fragment(), DIAware {
    override val di: DI by closestDI()
    private val viewModel: MyViewModel by viewModel()
}

Links

Kodein

Kodein Open Source Initiative documentation

Further reading

Kotlin Dependency Injection with Kodein

credit goes to @swayangjit