Multiple Koin Modules in Android

By using Koin, you describe definitions in modules. In this section we will see how to declare, organize & link your modules.

Using several modules

Components doesn't have to be necessarily in the same module. A module is a logical space to help you organize your definitions, and can depend on definitions from other module. Definitions are lazy, and then are resolved only when a a component is requesting it.

Let's take an example, with linked components in separate modules:

// ComponentB <- ComponentA
class ComponentA()
class ComponentB(val componentA : ComponentA)

val moduleA = module {
    // Singleton ComponentA
    single { ComponentA() }
}

val moduleB = module {
    // Singleton ComponentB with linked instance ComponentA
    single { ComponentB(get()) }
}

We just have to declare list of used modules when we start our Koin container:

class MainApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        startKoin {
            // ...

            // Load modules
            modules(moduleA, moduleB)
        }
        
    }
}

Up to you to organise your self per Gradle module, and gather several Koin modules.

Check Koin Modules Section for more details

Module Includes (since 3.2)

A new function includes() is available in the Module class, which lets you compose a module by including other modules in an organized and structured way.

The two prominent use cases of the new feature are:

• Split large modules into smaller and more focused ones.
• In modularized projects, it allows you more fine control over a module visibility (see examples below).

How does it work? Let's take some modules, and we include modules in parentModule:

// `:feature` module
val childModule1 = module {
    /* Other definitions here. */
}
val childModule2 = module {
    /* Other definitions here. */
}
val parentModule = module {
    includes(childModule1, childModule2)
}

// `:app` module
startKoin { modules(parentModule) }

Notice we do not need to set up all modules explicitly: by including parentModule, all the modules declared in the includes will be automatically loaded (childModule1 and childModule2). In other words, Koin is effectively loading: parentModule, childModule1 and childModule2.

An important detail to observe is that you can use includes to add internal and private modules too - that gives you flexibility over what to expose in a modularized project.

info

Module loading is now optimized to flatten all your module graphs and avoid duplicated definitions of modules.

Finally, you can include multiple nested or duplicates modules, and Koin will flatten all the included modules removing duplicates:

// :feature module
val dataModule = module {
    /* Other definitions here. */
}
val domainModule = module {
    /* Other definitions here. */
}
val featureModule1 = module {
    includes(domainModule, dataModule)
}
val featureModule2 = module {
    includes(domainModule, dataModule)
}

// `:app` module
class MainApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        startKoin {
            // ...

            // Load modules
             modules(featureModule1, featureModule2)
        }
        
    }
}

Notice that all modules will be included only once: dataModule, domainModule, featureModule1, featureModule2.