Nullability assignment of generic type in kotlin methods

January 3, 2023

I’ve created this extension method for an androidx.lifecycle.LiveData:

inline fun <T> LiveData<T>.observeNotNull(crossinline observer: (T) -> Unit) {
    this.observe(viewLifecycleOwner, Observer { value ->
        value?.let { observer(it) }
    })
}

As you can see, the value passed to the observer parameter (it) can never be null. But when I call this method, the value is technically nullable, as shown in this example:

I’d like to do something like crossinline observer: (T!) -> Unit, is this possible?

EDIT: the extension should work on both nullable and non-nullable livedata types. Preferably combined in 1 extension method, but ifit’s only possible when using 2 separate extension methods I’d still like to hear.

>Solution :

It depends on the concrete type that is represented by type variable T if it is nullable. If there is no further information to infer the concrete type, it is Any?, and thus it can be null. You have to define T: Any if you want to ensure that T represents a non-nullable type:

inline fun <T: Any> LiveData<T>.observeNotNull(crossinline observer: (T) -> Unit) {
    this.observe(viewLifecycleOwner, Observer { value ->
        value?.let { observer(it) }
    })
}

If you want to allow all types for T, but not for the argument of observer, then this should be possible to achieve with definitely non-nullable types in Kotlin 1.7 using notation T & Any:

inline fun <T> LiveData<T>.observeNotNull(crossinline observer: (T & Any) -> Unit) {
    this.observe(viewLifecycleOwner, Observer { value ->
        value?.let { observer(it) }
    })
}