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Passing Parameters - Injected Parameters

In any definition, you can use injection parameters: parameters that will be injected and used by your definition.

Passing values to inject​

Given a definition, you can pass parameters to that definition:

class Presenter(val a : A, val b : B)

val myModule = module {
    single { params -> Presenter(a = params.get(), b = params.get()) }
}

Parameters are sent to your definition with the parametersOf() function (each value separated by comma):

class MyComponent : View, KoinComponent {

    val a : A ...
    val b : B ... 

    // inject this as View value
    val presenter : Presenter by inject { parametersOf(a, b) }
}

Defining an "injected parameter"​

Below is an example of injection parameters. We established that we need a view parameter to build of Presenter class. We use the params function argument to help retrieve our injected parameters:

class Presenter(val view : View)

val myModule = module {
    single { params -> Presenter(view = params.get()) }
}

You can also write your injected parameters directly with the parameters object, as destructured declaration:

class Presenter(val view : View)

val myModule = module {
    single { (view : View) -> Presenter(view) }
}
caution

Even if the "destructured" declaration is more convenient and readable, it's not type safe. Kotlin won't detect that passed type are in good orders if you have several values

Resolving injected parameters in order​

Instead of using get() to resovle a parameter, if you have several parameters of the same type you can use the index as follow get(index) (also same as [ ] operator):

class Presenter(val view : View)

val myModule = module {
    
    single { p -> Presenter(p[0],p[1]) }
}

Resolving injected parameters from graph​

Koin graph resolution (main tree of resolution of all definitions) also let you find your injected parameter. Just use the usual get() function:

class Presenter(val view : View)

val myModule = module {
    single { Presenter(get()) }
}

Injected parameters: indexed values or set (3.4.3)​

In addition to parametersOf, the following API are accessible:

  • parameterArrayOf: to use an array of value, and data will be use by its index
val params = parameterArrayOf(1,2,3)
params.get<Int>() == 1
params.get<Int>() == 2
params.get<Int>() == 3
params.get<Int>() == 3
  • parameterSetOf: to use a set of values, with different kinds. Doesn't use index to scroll values.
val params = parameterSetOf("a_string", 42)
params.get<Int>() == 42
params.get<String>() == "a_string"
params.get<Int>() == 42
params.get<String>() == "a_string"

The default function parametersOf is working with both index & set of values:

val params = parametersOf(1,2,"a_string")
params.get<String>() == "a_string"
params.get<Int>() == 1
params.get<Int>() == 2
params.get<Int>() == 2
params.get<String>() == "a_string"
note

You can "cascade" parameter injection with parametersOf or parameterArrayOf, to consume value based on index. Or use parametersOf or parameterSetOf to cascading based on type to resolve.