Kotlin 不理解 ViewModelProviders.of(activity ?: fragment)

Question

Inside my Fragment I initialize a ViewModel using ViewModelProviders . I want it to take its Activity if not null , otherwise itself ( Fragment ).

private val viewModel: MainViewModel by lazy {
    ViewModelProviders.of(activity ?: this).get(MainViewModel::class.java)
}

None of the following functions can be called with the arguments supplied.

• of(Fragment) defined in androidx.lifecycle.ViewModelProviders
• of(FragmentActivity) defined in androidx.lifecycle.ViewModelPro

It seems the language does not allow me to invoke conflicting method signatures (between of(Activity) and of(Fragment) . (It might be understandable, maybe the compiler has to reference only one method and cannot link to both on the same line.) Is that so?

I now have to use

    activity?.let {
        ViewModelProviders.of(it).get(MainViewModel::class.java)
    } ?: run {
        ViewModelProviders.of(this).get(MainViewModel::class.java)
    }

Is there any better way of doing this?

Answer 1

Yes, it's compiler ambiguity , because you're passing activity & this (fragment instance) at the same time which has different implementations in ViewModelProviders .

Another approach to do this is by using when condition like below, (although your approach is also good) :

private val viewModel: MainViewModel by lazy {
    return@lazy when {
        activity != null -> {
            ViewModelProviders.of(activity as FragmentActivity).get(MainViewModel::class.java) // you can either pass activity object
        }
        else -> {
            ViewModelProviders.of(this).get(MainViewModel::class.java) // or pass fragment object, both are not possible at same time.
        }
    }
}

Answer 2

if you are using androidx so should add two lines for lifecycle :

 def lifecycle_version = "2.2.0-rc02"
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
    kapt "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"

and use it like this :

 val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)

Answer 3

In addition to Jeel's answer, I'd recommend that if you commonly need to use this pattern, you should define an extension function on Fragment for it to avoid repetition. For example:

fun Fragment.getViewModelProvider() =
    activity?.let(ViewModelProviders::of) ?: ViewModelProviders.of(this)

inline fun <reified T : ViewModel> Fragment.getViewModel() = 
    getViewModelProvider().get(T::class.java)

From there, within any Fragment you can just call either:

val viewModel: MainViewModel = getViewModel()
val viewModel = getViewModel<MainViewModel>()

It'll avoid both the boilerplate of fetching the provider as well as specifying the Java class specifically.