Android Dependency Injection With Koin

Dependency injection is a programming technique that makes a class independent of its dependencies. This is made possible by decoupling the usage of an object from its creation. Many Android developers are familiar with Java-based dependency injection frameworks such as Dagger and Guice. <!--more--> However, some frameworks are written completely in Kotlin for Kotlin. These frameworks include Koin and Kodein. This article goes through how to manage dependencies in Android using the new dependency injection framework - Koin.

Prerequisites #

To follow through with this tutorial, you will need to:

1. Have Android Studio installed.
2. Have a basic knowledge of building Android applications.
3. Have a basic understanding of Kotlin programming language.

Let's get started!

What exactly is dependency injection #

Being a software developer, you must, or might have heard about dependency injection but didn't really understand what it is. You might even be asking yourself, why should I use this technique?

How important is it? Have I ever used it? Well, the answer to the last question is pretty simple. You have but without any dependency injections framework.

Consider the following scenario. You have two classes, A and B. Class B requires an instance of A for it to perform a specific task. It is correct to say that class B directly depends on class A.

Therefore, we often find ourselves creating an instance of the dependent class before using it or pass it as a parameter. All these are forms of dependency injection. This is fine for a small project.

However, as the project scales, maintenance and testing become challenging. This is where dependency injection frameworks come into play.

A dependency injection framework helps us to create and manage dependencies. As mentioned earlier, there are many Java-based Android dependency injection frameworks. However, with the increased adoption of Kotlin on Android, the demand for libraries written completely in Kotlin for Kotlin is rising.

Koin is a dependency injection framework that conforms to this need. It is a lightweight framework, easy to learn, and does not have much boilerplate code. Let's see how we can use this framework to manage dependencies in our Android applications.

Getting started with Koin #

Koin is fairly simple. All we need to do is create our classes, tell Koin how to create the dependencies, then we can call them whenever/wherever we need them.

Before we move any further, create an empty activity project and give it a name of your choice. Wait for the project build to complete then add the following dependency on your app-level build.gradle file.

implementation "org.koin:koin-android:1.0.2"

Let's go ahead and create some classes.

Creating project components #

We are going to need some components that depend on each other for demonstration. Let's create a scenario. Create a Kotlin file and in it add the following code.

class Student(private val course: Course, private val friend: Friend) {

    fun doWork(): String =
        course.study() + "\n" + friend.play()
}

class Friend {
    fun play(): String = "I am playing with my friend"
}

class Course {
    fun study(): String = "I am studying"
}

The first class is a Student class, a student has a course of study and a friend to play with. Therefore, the Student class depends on those classes.

It needs them to call or access the study() and play() functions contained in the Course and Friend classes respectively. To solve this dependency we add the two classes as parameters.

This means that the Student class can not be instantiated without instances of Course and Friend classes.

Open the MainActivity file and replace the onCreate function with the following code.

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val course = Course()
        val friend = Friend()
        val student = Student(course, friend)

        binding.textView.text = student.doWork()
    }

The application uses View binding to access the default TextView with an id of text_view. Visit this link to learn more about View binding.

The code creates instances of all the classes.

Upon running the application, the text view will display:

I am studying
I am playing with my friend

There is nothing wrong with the application as it is. But, as we mentioned earlier, depending on this kind of dependency injection will make the application hard to maintain as it scales. Let's see how Koin can help us manage these dependencies.

Creating a Koin module #

Koin uses modules to create dependencies. Create a new Kotlin file with the name 'module' and add the following code.

val appModule = module {
    single { Course() }
    factory { Friend() }
    factory { Student(get(), get()) }
}

The module function as the name suggests is used to create a module. single is used to create a singleton instance. This means that Koin will return the same instance of the class when the dependency is needed.

We have made Course a singleton since we assume that all the students do the same course. factory, on the other hand, is used when we want a new instance of the class every time we call it.

We have used the factory for both Friend and Student classes since we want a new instance whenever we call them. The get function is used to get the required dependency only if it has been specified in the module.

It detects the type of dependency we need and fetches it for us. That's all we need for the module. Let's go ahead and start Koin.

Starting Koin #

Starting Koin is fairly simple. Create an application class with the name of your choice and add the following code to start Koin.

startKoin(this, listOf(appModule))

We use the startKoin function to start Koin.

And that's it! Koin is fully set up. Now, let's use Koin in our MainActivity.

Replace the onCreate method of the MainActivity with this.

val binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)

val student: Student by inject()

binding.textView.text = student.doWork()

The reduction of lines of code is noticeable. by inject delegate is used to lazily inject dependencies. We can also use the get function to get the dependency non-lazily.

Run the application again.

The code works fine, but, this time we have employed a dependency injection framework that helps a lot in ensuring our project is maintainable and can be easily tested.

Conclusion #

In this article, we have gone through dependency injection and how to manage dependencies using a Kotlin dependency injection known as Koin. We have seen how Koin is easy to set up and work with.

We have also seen how dependency injection helps us in making our applications maintainable. I hope this tutorial gives you the basics you need to start using this great and lightweight framework.

Happy coding!

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Peer Review Contributions by: Wanja Mike