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}")
    }
}

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?