single and factory

Learn binding types with single and factory.

single and factory

single and factory are the two Koin declarations that set a dependency's lifetime. single returns one instance; factory returns a new instance each time.

Example: single { Repository() } returns the same Repository throughout the app. factory { Service() } creates a new Service on each call. single suits stateful objects; factory suits lightweight, independent ones. The wrong choice can lead to memory leaks or unnecessary recreation.

When single, when factory?

If a dependency holds state and is shared across the app, choose single. For example, a database connection, session data.

For lightweight, independent, short lived objects, factory fits. For example, a fresh view model per request, a new request object per click. Using single wrongly lets dependencies affect each other; using factory wrongly creates memory overhead. The choice depends on the nature of the dependency.

Exercise Define Repository with single

Define val appModule = module { single { Repository() } }. Also define class Repository. Inside main, print println("Single ready").

Show example solution

class Repository

val appModule = module {
    single { Repository() }
}

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

Test yourself

Match single and factory

single

Singleton
factory

New each time
module

Dependency block
get

Another dependency
Which suits a database connection?
- factory
- single
- neither
- both
Show answer

single — A database connection is a singleton with single.

Complete the single call

val appModule = module {
    ____ { Repository() }
}

Show answer

val appModule = module {
    single { Repository() }
}

Next lesson: by inject

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