what does the ::MainViewModel mean when pass to the singleArgViewModelFactory

Question

in kotlin coroutines lab smaple, https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#6

it creates the viewModel by passing the MainViewModel.FACTORY(repository)

val viewModel = ViewModelProviders
    .of(this, MainViewModel.FACTORY(repository))
    .get(MainViewModel::class.java)

the MainViewModel is as below, not understand what syntax is the ::MainViewModel used in the

val FACTORY = singleArgViewModelFactory(::MainViewModel)

the singleArgViewModelFactory has constructor:

singleArgViewModelFactory(constructor: (A) -> T)

which taking a function (A) -> T, what does the ::MainViewModel in the singleArgViewModelFactory(::MainViewModel) mean?

class MainViewModel(private val repository: TitleRepository) : ViewModel() {

    companion object {
        /**
         * Factory for creating [MainViewModel]
         *
         * @param arg the repository to pass to [MainViewModel]
         */
        val FACTORY = singleArgViewModelFactory(::MainViewModel)
    }


    ......
}



fun <T : ViewModel, A> singleArgViewModelFactory(constructor: (A) -> T):
        (A) -> ViewModelProvider.NewInstanceFactory {
    return { arg: A ->
        object : ViewModelProvider.NewInstanceFactory() {
            @Suppress("UNCHECKED_CAST")
            override fun <V : ViewModel> create(modelClass: Class<V>): V {
                return constructor(arg) as V
            }
        }
    }
}

For more, see https://kotlinlang.org/docs/reference/reflection.html#function-references — CommonsWare, May 15 '19 at 14:14
It is most likely a function reference to the constructor of the class. — EpicPandaForce, May 15 '19 at 14:24
@CommonsWare, and EpicPandaForce, thank you all for the help to quickly point to the explanation! — lannyf, May 15 '19 at 14:44

Roland · Accepted Answer · 2019-05-15T14:18:06.417

::MainViewModel is a function reference. For a parameter of type (A) -> T it basically is a reference to a function (constructor in this case) that accepts a parameter of type A and delivers T (which in this case is the MainViewModel itself).

Comparing reference with its actual counterpart:

val ref : (A) -> T = ::MainViewModel
val ref : (A) -> T = { MainViewModel(it) } // or: = { anA : A -> MainViewModel(anA) }

what does the ::MainViewModel mean when pass to the singleArgViewModelFactory

1 Answers1