Managing Android Scopes

Working with the Android lifecycle

Android components are mainly managed by their lifecycle: we can't directly instantiate an Activity nor a Fragment. The system make all creation and management for us, and make callbacks on methods: onCreate, onStart...

That's why we can't describe our Activity/Fragment/Service in a Koin module. We need then to inject dependencies into properties and also respect the lifecycle: Components related to the UI parts must be released on soon as we don't need them anymore.

Then we have:

• long live components (Services, Data Repository ...) - used by several screens, never dropped
• medium live components (user sessions ...) - used by several screens, must be dropped after an amount of time
• short live components (views) - used by only one screen & must be dropped at the end of the screen

Long live components can be easily described as single definitions. For medium and short live components we can have several approaches.

In the case of MVP architecture style, the Presenter is a short live component to help/support the UI. The presenter must be created each time the screen is showing, and dropped once the screen is gone.

A new Presenter is created each time

class DetailActivity : AppCompatActivity() {

    // injected Presenter
    override val presenter : Presenter by inject()

We can describe it in a module:

• as factory - to produce a new instance each time the by inject() or get() is called

val androidModule = module {

    // Factory instance of Presenter
    factory { Presenter() }
}

• as scope - to produce an instance tied to a scope

val androidModule = module {

    scope<DetailActivity> {
        scoped { Presenter() }
    }
}

note

Most of Android memory leaks come from referencing a UI/Android component from a non Android component. The system keeps a reference on it and can't totally drop it via garbage collection.

Scope for Android Components (3.2.1 update)

Declare an Android Scope

To scope dependencies on an Android component, you have to declare a scope section with the scope block like follow:

class MyPresenter()
class MyAdapter(val presenter : MyPresenter)

module {
  // Declare scope for MyActivity
  scope<MyActivity> {
    // get MyPresenter instance from current scope 
    scoped { MyAdapter(get()) }
    scoped { MyPresenter() }
  }
}

Android Scope Classes

Koin offers ScopeActivity, RetainedScopeActivity and ScopeFragment classes to let you use directly a declared scope for Activity or Fragment:

class MyActivity : ScopeActivity() {
    
    // MyPresenter is resolved from MyActivity's scope 
    val presenter : MyPresenter by inject()
}

Under the hood, Android scopes needs to be used with AndroidScopeComponent interface to implement scope field like this:

abstract class ScopeActivity(
    @LayoutRes contentLayoutId: Int = 0,
) : AppCompatActivity(contentLayoutId), AndroidScopeComponent {

    override val scope: Scope by activityScope()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        checkNotNull(scope)
    }
}

We need to use the AndroidScopeComponent interface and implement the scope property. This will setup the default scope used by your class.

Android Scope API

To create a Koin scope bound to an Android component, just use the following functions:

• createActivityScope() - Create Scope for current Activity (scope section must be declared)
• createActivityRetainedScope() - Create a retained Scope (backed by ViewModel lifecycle) for current Activity (scope section must be declared)
• createFragmentScope() - Create Scope for current Fragment and link to parent Activity scope

Those functions are available as delegate, to implement different kind of scope:

• activityScope() - Create Scope for current Activity (scope section must be declared)
• activityRetainedScope() - Create a retained Scope (backed by ViewModel lifecycle) for current Activity (scope section must be declared)
• fragmentScope() - Create Scope for current Fragment and link to parent Activity scope

class MyActivity() : AppCompatActivity(contentLayoutId), AndroidScopeComponent {

    override val scope: Scope by activityScope()
    
}

We can also setup a retained scope (backed by a ViewModel lifecycle) with the following:

class MyActivity() : AppCompatActivity(contentLayoutId), AndroidScopeComponent {

    override val scope: Scope by activityRetainedScope()
}

note

If you don't want to use Android Scope classes, you can work with your own and use AndroidScopeComponent with the Scope creation API

Scope Links

Scope links allow to share instances between components with custom scopes.

In a more extended usage, you can use a Scope instance across components. For example, if we need to share a UserSession instance.

First declare a scope definition:

module {
    // Shared user session data
    scope(named("session")) {
        scoped { UserSession() }
    }
}

When needed to begin use a UserSession instance, create a scope for it:

val ourSession = getKoin().createScope("ourSession",named("session"))

// link ourSession scope to current `scope`, from ScopeActivity or ScopeFragment
scope.linkTo(ourSession)

Then use it anywhere you need it:

class MyActivity1 : ScopeActivity() {
    
    fun reuseSession(){
        val ourSession = getKoin().createScope("ourSession",named("session"))
        
        // link ourSession scope to current `scope`, from ScopeActivity or ScopeFragment
        scope.linkTo(ourSession)

        // will look at MyActivity1's Scope + ourSession scope to resolve
        val userSession = get<UserSession>()
    }
}
class MyActivity2 : ScopeActivity() {

    fun reuseSession(){
        val ourSession = getKoin().createScope("ourSession",named("session"))
        
        // link ourSession scope to current `scope`, from ScopeActivity or ScopeFragment
        scope.linkTo(ourSession)

        // will look at MyActivity2's Scope + ourSession scope to resolve
        val userSession = get<UserSession>()
    }
}