Should the new async boolean in main thread scheduler help recyclerview preserve scroll state on configChange?

[ursusursus]

Hi,
I am trying the new update, and was hoping it would solve the issue when emit already comes from main thread, so observeOn main thread doesnt wait for next frame.

Where I notice this the most is recylcerview. If stream of adapter data is kept synchnronous, on rotation - scroll state is kept / restored. If stream is async - scroll state is reset.

When using the new api, Id expect for it to be able to restore the scroll state on replayed, startWith, etc . emits. However this is not the case, even on a minimal example.

class FooActivity : AppCompatActivity() {
    private lateinit var disposable: Disposable

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.controller_workflow_jobs)

        val adapter = SimpleAdapter(...)

        val recyclerView = findViewById<RecyclerView>(R.id.recyclerView).apply {
            setHasFixedSize(true)
            this.layoutManager = LinearLayoutManager(this@FooActivity)
            this.adapter = adapter
        }

        disposable = Observable.fromCallable {
            listOf(
                    JobEntity("1", "Foo1", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo2", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo3", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo4", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo5", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo6", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo7", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo8", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo9", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo10", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo11", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo12", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo13", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo14", ItemState.REGULAR, "1"),
                    JobEntity("1", "Foo15", ItemState.REGULAR, "1")
            )
        }
 
                .observeOn(AndroidSchedulers.mainThread())
                .throwingSubscribe {
                    adapter.setItems(it)
                }
    }

    override fun onDestroy() {
        super.onDestroy()
        disposable.dispose()
    }
}

Shouldnt this work? Why there would be any delay? I.e. I get its async anyways but I am pretty sure it should not take more than 16ms to leak into next frame, or can it? (Nexus 5, Pixel 2) (If I comment out the observeOn it works)

[ursusursus]

Any info on this please?

[bonebizz21]

Thanks

[faezndeck]

no support video my acc tumblr

[JakeWharton]

This is not supported. Using a HandlerScheduler will always post and never short-circuit when already on the main thread. There are more details in #228. You even already asked about this behavior in #423.

The new async handler will still always post, it just doesn't wait for vsync. It's also not enabled by default. You have to opt in using the plugin system which you can read about here.