Every `ViewModel` instance has a `viewModelScope: ...

# android

[min]

Every ViewModel instance has a viewModelScope: CoroutineScope. The docs say that i might want to consider adding my own CoroutineScope parameter:

class MyViewModel(private val coroutineScope: CoroutineScope): ViewModel() {
    override fun onCleared() { this.coroutineScope.cancel() }
}

For better dependency injection during testing, and so on. Cancelling the scope might have unintended consequences though if it’s shared. What’s the best way to go about structuring concurrency in ViewModel instances? Take a context instead and construct a unique child scope for each instance?

[Ciaran Sloan]

Pass the coroutineScope into the ViewModel abstract class and the cancellation will be handled for you. You can then continue to use viewModelScope as it will be set as the provided scope:

class MyViewModel(coroutineScope: CoroutineScope): ViewModel(coroutineScope) {
    fun doSomeWork() {
        viewModelScope.launch {}
    }
}

[min]

@Ciaran Sloan Wouldn’t that just delegate the cancellation on onCleared to the superclass?

val scope = CoroutineScope(SupervisorJob())
val models = listOf(
    MyViewModel(scope),
    MyViewModel(scope),
    MyViewModel(scope),
)

Clearing one still invalidates the scope for all three. How do I deal with this problem?

[Ciaran Sloan]

Creating multiple instances of a view model is not the same thing as sharing a view model. If you're creating multiple instances you should also create a scope for that instance. Not attempt to share a single scope across multiple instances of the VM