Kotlin does not understand ViewModelProviders.of(activity ?: fragment)

问题

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?

回答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.
        }
    }
}

回答2:

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.

回答3:

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)

来源：https://stackoverflow.com/questions/52811685/kotlin-does-not-understand-viewmodelproviders-ofactivity-fragment

标签

android

kotlin

android-viewmodel

viewmodelproviders