Java / Kotlin generics

Question

i am having a template / generics question.

This is the code i have at the moment

data class RealmWatcher<T>(
        val results: RealmResults<T>,
        val handler: (RealmResults<T>) -> Unit)

And and using this in an Android fragment to listen to specific results and execute actions based on the changes. So lets take this as an example

private val realmListener = arrayOf(
            RealmWatcher<Notification>(Realm.getDefaultInstance().where(Notification::class.java).equalTo("isNew", true).findAll(),
                                       { n: RealmResults<Notification> ->
                                           // DO STUFF
                                       })

I am doing this while start / stop of the fragment

override fun onResume() {
    super.onResume()
    // start listening and execute
    realmListener.forEach { it.handler(it.results) }
    realmListener.forEach { it.results.addChangeListener(it.handler) }
}

override fun onPause() {
    // stop listening
    realmListener.forEach { it.results.removeChangeListener(it.handler) }
    super.onPause()
}

It works only when i am using one type (like Notification above). How should I define that if i want to use different types in the form of

private val realmListener = arrayOf(
            RealmWatcher<Notification>(Realm.getDefaultInstance().where(Notification::class.java).equalTo("isNew", true).findAll(),
                                       { n: RealmResults<Notification> ->
                                       // TODO STUFF
                                       }),
            RealmWatcher<Project>(Realm.getDefaultInstance().where(Project::class.java).equalTo("isOdd", true).findAll(),
                                       { n: RealmResults<Project> ->
                                         // TODO STUFF
                                       })
                                       )

When mixing the types (Notification and Project) I will get a Type mismatch error.

And when defining the

private val realmListener:Array<RealmWatcher<out Any>>

I will also get Type mismatch errors

How can I define the Array to have several different RealmWatcher with different types T?

Put variance on your realm watcher class i.e. data class RealmWatcher<out T> — Alex
– Alex, Commented Nov 16, 2017 at 23:17
@SimY4 That won't work, as T is also used in in position in the code. See my answer for a working solution that also encapsulates the logic better. — Robin
– Robin, Commented Nov 20, 2017 at 8:11

Robin · Accepted Answer · 2017-11-17 16:21:19Z

Doing exactly what you want is going to be tricky, because I don't know of a way to tell the compiler that what you're doing is actually safe. Adding variance, like one of the comments suggests, won't work as T is used in both in and out positions.

However, you could take a different approach. Extend your RealmWatcher like this:

data class RealmWatcher<T>(
    val results: RealmResults<T>,
    val handler: (RealmResults<T>) -> Unit
) {
    fun attach() {
        handler(results)
        results.addChangeListener(handler)
    }
    fun detach() {
        results.removeChangeListener(handler)
    }
}

and then use those two additional functions in your lifecycle callbacks:

fun onResume() {
    super.onResume()
    // start listening and execute
    realmListener.forEach { it.attach() }
}

fun onPause() {
    // stop listening
    realmListener.forEach { it.detach() }
    super.onPause()
}

This will do what you need it to do without creating a problem because of the mixed generic parameters.

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