Koin brings a simple API to let you define instances that are tied to a limit lifetime.

What is a scope?

Scope is a fixed duration of time or method calls in which an object exists. Another way to look at this is to think of scope as the amount of time an object’s state persists. When the scope context ends, any objects bound under that scope cannot be injected again (they are dropped from the container).

Scope definition

By default in Koin, we have 3 kind of scopes:

  • single definition, create an object that persistent with the entire container lifetime (can't be dropped).
  • factory definition, create a new object each time. Short live. No persistence in the container (can't be shared).
  • scoped definition, create an object that persistent tied to the associated scope lifetime.

To declare a scoped definition, use the scoped function like follow. A scope gathers scoped definitions as a logical unit of time.

Declaring a scope for a given type, we need to use the scope keyword:

module {
scoped { Presenter() }
// ...

Scope Id & Scope Name

A Koin Scope is defined by its:

  • scope name - scope's qualifier
  • scope id - unique identifier of the scope instance

scope<A> { } is equivalent to scope(named<A>()){ } , but more convenient to write. Note that you can also use a string qualifier like: scope(named("SCOPE_NAME")) { }

From a Koin instance, you can access:

  • createScope(id : ScopeID, scopeName : Qualifier) - create a closed scope instance with given id and scopeName
  • getScope(id : ScopeID) - retrieve a previously created scope with given id
  • getOrCreateScope(id : ScopeID, scopeName : Qualifier) - create or retrieve if already created, the closed scope instance with given id and scopeName

Scope Component: Associate a scope to a component [2.2.0]

Koin has the concept of KoinScopeComponent to help bring a scope instance to its class:

class A : KoinScopeComponent {
override val scope: Scope by lazy { newScope() }
class B

The KoinScopeComponent interface brings several extensions:

  • newScope to create scope from current component's scope Id & name
  • get, inject - to resolve instances from scope (equivalent to scope.get() & scope.inject())

Let's define a scope for A, to resolve B:

module {
scope<A> {
scoped { B() } // Tied to A's scope

We can then resolve instance of B directly thanks to org.koin.core.scope get & inject extensions:

class A : KoinScopeComponent {
override val scope: Scope by lazy { newScope(this) }
// resolve B as inject
val b : B by inject() // inject from scope
// Resolve B
fun doSomething(){
val b = get<B>()
fun close(){
scope.close() // don't forget to close current scope

Resolving dependencies within a scope

To resolve a dependency using the scope's get & inject functions: val presenter = scope.get<Presenter>()

The interest of a scope is to define a common logical unit of time for scoped definitions. It's allow also to resolve definitions from within the given scope

// given the classes
class ComponentA
class ComponentB(val a : ComponentA)
// module with scope
module {
scope<A> {
scoped { ComponentA() }
// will resolve from current scope instance
scoped { ComponentB(get()) }

The dependency resolution is then straight forward:

// create scope
val myScope = koin.createScope<A>()
// from the same scope
val componentA = myScope.get<ComponentA>()
val componentB = myScope.get<ComponentB>()

By default, all scope fallback to resolve in main scope if no definition is found in the current scope

Close a scope

Once your scope instance is finished, just closed it with the close() function:

// from a KoinComponent
val scope = getKoin().createScope<A>()
// use it ...
// close it

Beware that you can't inject instances anymore from a closed scope.

Getting scope's source value [2.1.4]

Koin Scope API in 2.1.4 allow you to pass the original source of a scope, in a definition. Let's take an example below. Let's have a singleton instance A:

class A
class BofA(val a : A)
module {
single { A() }
scope<A> {
scoped { BofA(getSource() /* or even get() */) }

By creating A's scope, we can forward the reference of the scope's source (A instance), to underlying definitions of the scope: scoped { BofA(getSource()) } or even scoped { BofA(get()) }

This in order to avoid cascading parameter injection, and just retrieve our source value directly in scoped definition.

val a = koin.get<A>()
val b = a.scope.get<BofA>()
assertTrue(b.a == a)

Difference between getSource() and get(): getSource will directly get the source value. Get will try to resolve any definition, and fallback to source value if possible. getSource() is then more efficient in terms of performances.

Scope Linking [2.1.0]

Koin Scope API in 2.1 allow you to link a scope to another, and then allow to resolve joined definition space. Let's take an example. Here we are defining, 2 scopes spaces: a scope for A and a scope for B. In A's scope, we don't have access to C (defined in B's scope).

module {
single { A() }
scope<A> {
scoped { B() }
scope<B> {
scoped { C() }

With scope linking API, we can allow to resolve B's scope instance C, directly from A'scope. For this we use linkTo() on scope instance:

val a = koin.get<A>()
// let's get B from A's scope
val b = a.scope.get<B>()
// let's link A' scope to B's scope
// we got the same C instance from A or B scope
assertTrue(a.scope.get<C>() == b.scope.get<C>())