by inject

Learn to inject dependencies with by inject().

What is by inject?

by inject() is the Koin delegate that gives a class its dependency lazily. It requires the KoinComponent interface.

Example: class App : KoinComponent { val repo: Repository by inject() }. This class gets the Repository instance on first access of repo. Because it is lazy, the dependency is not created until first use. get() can replace inject but it loads eagerly (immediately). by inject() is more efficient.

Exercise KoinComponent with by inject

Define class Repository. Define class App : KoinComponent with val repo: Repository by inject() inside. In main, print println("Class ready").

Show example solution

class Repository

class App : KoinComponent {
    val repo: Repository by inject()
}

fun main() {
    println("Class ready")
}

The KoinComponent interface

For a class to use by inject() it must implement the KoinComponent interface. This interface gives the class access to Koin.

Example: class Service : KoinComponent { val repo: Repository by inject() }. Implementing KoinComponent lets the class call get() and inject() inside. This connects the class to Koin directly. Unlike Dagger, Koin has no annotations; you only implement the interface. This approach is idiomatic Kotlin.

Test yourself

When does by inject() create the dependency?
- When the class is created
- At app start
- On first access
- Never
Show answer

On first access — by inject() is lazy, so it creates on first access.

Complete the by inject call

class App : KoinComponent {
    val repo: Repository ____()
}

Show answer

class App : KoinComponent {
    val repo: Repository by inject()
}

Match by inject parts

by inject

Lazy delegate
KoinComponent

Interface
Repository

Dependency
App

Class

Next lesson: by viewModel

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies