Android shared preference observer using kotlin coroutines (1.3.0)

PreferenceExtensions.kt

inline fun <reified T> SharedPreferences.observeKey(key: String, default: T): Flow<T> {
    val flow = MutableStateFlow(getItem(key, default))

    val listener = SharedPreferences.OnSharedPreferenceChangeListener { _, k ->
        if (key == k) {
            flow.value = getItem(key, default)!!
        }
    }
    registerOnSharedPreferenceChangeListener(listener)

    return flow
        .onCompletion { unregisterOnSharedPreferenceChangeListener(listener) }
}

inline fun <reified T> SharedPreferences.getItem(key: String, default: T): T {
    @Suppress("UNCHECKED_CAST")
    return when (default){
        is String -> getString(key, default) as T
        is Int -> getInt(key, default) as T
        is Long -> getLong(key, default) as T
        is Boolean -> getBoolean(key, default) as T
        is Float -> getFloat(key, default) as T
        is Set<*> -> getStringSet(key, default as Set<String>) as T
        is MutableSet<*> -> getStringSet(key, default as MutableSet<String>) as T
        else -> throw IllegalArgumentException("generic type not handle ${T::class.java.name}")
    }
}

What if getItem is suspend function?

implementation updated using MutableStateFlow instead of Channel

Can I use parts of this code (giving credit of course) to support a class I'm teaching on migrating from Java to Kotlin?

@neuhoffm yeah sure feel free to use any gist from my profile and good luck 😄

Awesome, thank you!

Thanks for the idea!
Are you sure flow.onCompletion part works as expected? According to the docs, StateFlow never completes.

Also, registerOnSharedPreferenceChangeListener has a warning that the listener might be garbage-collected prematurely.

Hi @gmk57! onCompletion seems to be called correctly, and I think that's because it's creating internally an unsafeFlow, but you can use a channelFlow as well

regarding the listener being garbage collected I never seen that warning, which version are you using? 🤔

Indeed, onCompletion is triggered by the collector's cancellation:

@Test
fun testOnCompletion() = runBlocking {
    var onCompleteCalled = false
    MutableStateFlow("abc")
        .onCompletion { onCompleteCalled = true }
        .take(1).collect()
    assertTrue(onCompleteCalled)
}

This quirk is mentioned in the docs: "Unlike catch, this operator reports exception that occur both upstream and downstream and observe exceptions that are thrown to cancel the flow". I would say that onCompletion name is somewhat misleading. ;)

As for listener, I've seen it in the javadocs, at least for API 29-32.

inline fun <reified T> SharedPreferences.observeKey(key: String, default: T?): Flow<T?> {
    val flow = MutableStateFlow(getItem(key, default))

    val listener = SharedPreferences.OnSharedPreferenceChangeListener { _, k ->
        if (key == k) {
            try {
                flow.value = getItem(key, default)!!
            } catch (e: Exception) {
                flow.value = null
            }
        }
    }

    return flow
        .onCompletion { unregisterOnSharedPreferenceChangeListener(listener) }
        .onStart { registerOnSharedPreferenceChangeListener(listener) }
}

inline fun <reified T> SharedPreferences.getItem(key: String, default: T?): T? {
    @Suppress("UNCHECKED_CAST")
    return when (default) {
        is String? -> getString(key, default) as T?
        is Int -> getInt(key, default) as T
        is Long -> getLong(key, default) as T
        is Boolean -> getBoolean(key, default) as T
        is Float -> getFloat(key, default) as T
        is Set<*> -> getStringSet(key, default as Set<String>) as T
        else -> throw IllegalArgumentException("generic type not handle ${T::class.java.name}")
    }
}

Updated to take into account the null value (if no value is stored for this key)
Add register in the 'onStart' otherwise the flow was cancelled every recomposition :)

Hmm, @tkubasik-luna your version always produces Flow<T?> while the original one returned Flow<T>. We provide the default value, so the latter makes more sense to me. And it worked fine when no value is stored for the key, thanks to the default passed to getString, getInt, etc.

try/catch could potentially catch the "stored type does not match expected type" kind of errors, but in this case we fail earlier, on line 2. And it's probably good, because this type mismatch is clearly a programmer's mistake which should be fixed.

As for cancellation, in my testing both variants cancel/restart on every recomposition when called directly inside composable. I suppose it's inevitable since we're creating a new flow each time. But this is easily solved by wrapping prefs.observeKey() in remember, or better by moving it to a more stable place, e.g. ViewModel or Repository. 😃

I came with this solution:

    private fun SharedPreferences.userIdFlow(key: String, defValue: String?): Flow<String?> = callbackFlow {
        var currentValue = getString(key, defValue)

        val listener = SharedPreferences.OnSharedPreferenceChangeListener { _, k ->
            if (k == key) {
                val newValue = getString(key, defValue)
                if (newValue != currentValue) {
                    currentValue = newValue
                    trySend(newValue).isSuccess
                }
            }
        }

        registerOnSharedPreferenceChangeListener(listener)
        if (currentValue != null && currentValue!!.isNotEmpty()) {
            trySend(currentValue).isSuccess // emit current value immediately if not null and not empty
        }

        awaitClose { unregisterOnSharedPreferenceChangeListener(listener) }
    }

What's your analysis on it compared to yours?