Feature Request: Support Provider-like API for Instance Creation

[leinardi]

Use Provider<T> to enable flexible and explicit creation of new instances of a dependency directly within the injection target. This avoids boilerplate factory definitions in the component and allows injection targets to control when and how many instances of a dependency are created.

Currently, kotlin-inject supports creating new instances through factory methods in @Component but, as far as I know, there is no built-in equivalent for a Provider<T>-like API directly available in injection targets.

---

Proposed Solution

Introduce a Provider<T>-like interface into kotlin-inject that can be injected into classes. This would allow injection targets to request new instances of a dependency explicitly and on demand.

The proposed interface:

interface Provider<T> {
    fun get(): T
}

The kotlin-inject compiler would generate providers automatically for any class or dependency annotated with @Inject. The Provider<T> would invoke the binding logic for T each time get() is called.

---

Example: New Instance Creation in Injection Target

Here’s an example adapted to kotlin-inject:

Target Class

import me.tatarka.inject.annotations.Inject

@Inject
class BigCoffeeMaker(private val filterProvider: Provider<Filter>) {
    fun brew(numberOfPots: Int) {
        repeat(numberOfPots) {
            val filter = filterProvider.get() // New filter instance every time
            // Use the filter
        }
    }
}

Filter Class

@Inject
class Filter {
    // Implementation of Filter
}

AppComponent

@Component
abstract class AppComponent {
    abstract val bigCoffeeMaker: BigCoffeeMaker
}

Main Function

fun main() {
    val appComponent = AppComponent::class.create()
    val coffeeMaker = appComponent.bigCoffeeMaker
    coffeeMaker.brew(5)
}

---

Benefits

Eliminates the need for repetitive factory definitions in components or manual object creation in injection targets.

[leinardi]

Mmm, maybe this feature request is not necessary, am I wrong or I can just create a new instance using a lambda?

class BigCoffeeMaker(private val filterProvider: () -> Filter) {
}

[evant]

Yes you can use a lambda

[leinardi]

Nice, is this documented anywhere? I thought it was a missing feature because I couldn't find how to do it

[evant]

It's under https://github.com/evant/kotlin-inject?tab=readme-ov-file#function-support--assisted-injection

I do understand the documentation can be improved, feel free to add your feedback to #389

[leinardi]

Sure, done: #389 (comment)