Default implementations

A member that has a body in the interface does not need a binding. Its builder parameter is a nullable binding that defaults to null, meaning "keep the interface’s own implementation".

GreetingModule.kt

@DIModule
interface GreetingModule {
    val name: String
    val greeting: String get() = "Hello, $name"
    fun shout(): String = "$greeting!!!"
}

Only the abstract member must be bound:

val module = GreetingModule(name = Binding.value("World"))
module.greeting // "Hello, World"
module.shout()  // "Hello, World!!!"

Overriding a default

Any default can still be bound, which replaces the interface’s body:

val module = GreetingModule(
    name = Binding.value("World"),
    greeting = Binding.provider { "Hi $name" },
)
module.shout() // "Hi World!!!" (1)

1	The default body of `shout()` reads the overridden `greeting`.

Default bodies run against the module that was actually built. A default that reads another member therefore observes whatever that member was bound to.

Defaults and lifecycles

A default body behaves like a provider: it runs on every access, since it is ordinary interface code. To memoize a member that has a sensible default, bind it explicitly:

GreetingModule(
    name = Binding.value("World"),
    greeting = Binding.single { "Hello, $name" },
)

Default implementations are a good place for derived values, and for convenience functions that combine other members of the module.