Is there a Compose construct for handling back button presses? So far I've just been trying to use the Activity's

OnBackPressedDispatcher

, but wondering if there's a better way...

I've done it this way and it's worked well https://gist.github.com/adamp/62d13fe5bf0d6ddf9fcf58f8a6769523

Wow, this is great. Didn't think to use a static Ambient in this way. Thanks so much!!

Curious why you’re updating the

onBackPressed

property in an

onCommit

– I’ve seen a lot of internal Compose code update model properties like directly from the composable function, with no

onCommit

wrapper. It looks like an optimization, but I would think

onCommit

is actually more expensive than just setting a property. I’m guessing it has something to do with handling frames/threading correctly?

Yep, that's why. If you come across code that is updating non-

MutableState

properties of things that live beyond the current composition pass during composition, it's a bug. And yeah, we've got them here and there.

I hacked up a back press handler like this

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {

            val backPressed = state { false }

            onBackPressedDispatcher.addCallback(object : OnBackPressedCallback(true) {
                override fun handleOnBackPressed() {
                    backPressed.value = true
                }
            })

            MaterialTheme {
                Navigator(backPressed)
            }
        }
    }
}

that doesn't really work since it only handles one transition from false to true, and the ownership of who or what should set it back to false again is unclear. It's likely to lead to composition loops or non-idempotent operations

I sensed it was not right, but wanted a back press handler anyway. I think this is why AlertDialog wasn't closing https://kotlinlang.slack.com/archives/CJLTWPH7S/p1591295610338600?thread_ts=1591295610.338600&cid=CJLTWPH7S

constructs like that AlertDialog example should be fine since, "is the dialog showing?" is a state. The back button being pressed is an event; it happens instantaneously, it might mutate other state, but then it is done. It shouldn't persist as a, "the back button was pressed at some point in the past" state on its own.

But I am resetting the backpressed state when it is consumed, and it worked well for navigation between screens but not Exit dialog. I am yet to learn Ambient and onCommit stuff though. 😁

I expect that the consumption code for that back pressed state either has bugs, is error-prone, or both 🙂

I’m curious about how

AmbientBackPressedDispatcher

gets any value at all. Where is the value being implicitly being passed from given that we initialized it to null in the code example.