ActivityResult API is an improvement over the traditional onActivityResult() method. But it can get much better with Kotlin, let’s see how?

Traditionally we are used to the catch Activity results in onActivityResult() method overrides that Activity/Fragment has, with Jetpack Activity v1.2.0-alpha02, ActivityResultRegistry was introduced which allowed us to register for activity result & made handling the result much efficiently.

The working is pretty simple, ActivityResultRegistry maintains a map of key: String to a ActivityResultCallback and invokes it whenever ComponentActivity’s onActivityResult() is called by first verifying the contract which was used to execute the required input & then invokes the appropriate ActivityResultCallback from the map.

Limitations

You can only register before onStart, in other words with each call to registerForActivityResult() a LifecycleOwner is passed which is used to check whether you are calling this method before onStart, if not it throws an IllegalStateException.

This limitation can cause various problems where if you register the callback after a button is clicked it would simply not work. Although this limitation is technically needed to ensure that the API works properly, we’ll see it later.

Approach to solution (novice way)

Note: Do not try this in production.

Jetpack compose does it differently, if you look at the code of rememberLauncherForActivityResult() you’ll see it makes use of ComponentActivity’s ActivityResultRegistery to register the callback when the composition is started & unregister when the composition is disposed/abandoned (DisposableEffect).

We can apply this logic to construct something similar,

fun <I, O> ComponentActivity.launchWithResult(
    input: I,
    contract: ActivityResultContract<I, O>,
    onResult: (O) -> Unit
) {
    val contractKey = "contract_${UUID.randomUUID()}"
    var launcher: ActivityResultLauncher<I?>? = null
    launcher = activityResultRegistry.register(contractKey, contract) { result ->
        launcher?.unregister()
        onResult(result)
    }
    launcher.launch(input)
}

What are we doing here is simply registering a contract with a unique key & unregistering the contract when the result is obtained, thus making this a one shot listener for Activity Result.

We can built another extension function on top of it to listen for onActivityResult(),

fun ComponentActivity.startActivityWithResult(
    input: Intent,
    onActivityResult: (ActivityResult) -> Unit
) : Unit = launchWithResult(input, ActivityResultContracts.StartActivityForResult(), onActivityResult)

Now using this is pretty simple as you just’ve to call it wherever you want to listen for activity result for an intent maybe on a button click or something else,

class MainActivity : ComponentActivity() {
    override fun onCreate(...) {
        ...
        binding.btnLogin.setOnClickListener {
            val intent: Intent = ...
            startActivityWithResult(intent) { result ->  // <--
                // do something with the result.
            }
        }
    }
}

Configuration change & Process death

The above code will not survive configuration & process death, in other words if you call startActivityWithResult() after a button click (or similar case where manual trigger is required) where an Activity is displayed & suppose you rotate the device (which will recreate all the activities in backstack including the current one), then you’ll not get the required result on the callback function you’ve passed as a parameter to the method.

Why? Since during configuration or process death all your current as well as previous activity gets recreated, the callback lambda which you pass get’s cleared from the memory hence you don’t recieve a call to it.

The idea is to register it in OnCreate() i.e in any of Activity/Fragment methods & then execute the input,

class MainActivity : Fragment() {
    private lateinit var launcher: ActivityResultLauncher<Intent> // <--
    override fun onViewCreated(...) {
        ...
        launcher = requireActivity().activityResultRegistry.register("a-key", contract) { result -> // <--
            // do something with result,
        }
        binding.btnLogin.setOnClickListener {
            val intent: Intent = ...
            launcher.launch(intent)
        }
    }
    // In an Activity you don't need to unregister as it will happen automatically,
    override fun onDestroyView(...) {
        launcher.unregister() // <--
    }
}

Registeration must be done through a unique key which should survive configuration or process death, hence UUID.randomUUID() will not work. Now whenever configuration change happens or process death happens, after restarting Activity/Fragment it will re-register the callback with the same key we’ve used where we’ll safely get the result in our callback.

Did you see what we’ve done? In a way we’ve implemented registerForActivityResult(), but the only difference here is we are registering using a custom key. But why not use registerForActivityResult() if we’re doing the same thing? The thing is we can’t register multiple callbacks for the same contract when using registerForActivityResult(), hence we directly use ComponentActivity’s ActivityResultRegistry to do so. If your requirement is to have a single callback for a contract then sure go with registerForActivityResult() as it’ll automatically unregister itself when the LifecycleOwner’s lifecycle is destroyed.

Conclusion

android  kotlin