I have code that should change SharedPreferences into obsarvable storage with flow so I've code like this
internal val onKeyValueChange: Flow<String> = channelFlow {
val callback = SharedPreferences.OnSharedPreferenceChangeListener { _, key ->
coroutineScope.launch {
//send(key)
offer(key)
}
}
sharedPreferences.registerOnSharedPreferenceChangeListener(callback)
awaitClose {
sharedPreferences.unregisterOnSharedPreferenceChangeListener(callback)
}
}
or this
internal val onKeyValueChange: Flow<String> = callbackFlow {
val callback = SharedPreferences.OnSharedPreferenceChangeListener { _, key ->
coroutineScope.launch {
send(key)
//offer(key)
}
}
sharedPreferences.registerOnSharedPreferenceChangeListener(callback)
awaitClose {
sharedPreferences.unregisterOnSharedPreferenceChangeListener(callback)
}
}
Then I observe this preferences for token, userId, companyId and then log into but there is odd thing as I need to build app three times like changing token not causes tokenFlow to emit anything, then second time new userId not causes userIdFlow to emit anything, then after 3rd login I can logout/login and it works. On logout I am clearing all 3 properties stores in prefs token, userId, companyId.
For callbackFlow:
You cannot use emit() as the simple Flow (because it's a suspend function) inside a callback. Therefore the callbackFlow offers you a synchronized way to do it with the trySend() option.
Example:
fun observeData() = flow {
myAwesomeInterface.addListener{ result ->
emit(result) // NOT ALLOWED
}
}
So, coroutines offer you the option of callbackFlow:
fun observeData() = callbackFlow {
myAwesomeInterface.addListener{ result ->
trySend(result) // ALLOWED
}
awaitClose{ myAwesomeInterface.removeListener() }
}
For channelFlow:
The main difference with it and the basic Flow is described in the documentation:
A channel with the default buffer size is used. Use the buffer operator on the resulting flow to specify a user-defined value and to control what happens when data is produced faster than consumed, i.e. to control the back-pressure behavior.
The trySend() still stands for the same thing. It's just a synchronized way (a non suspending way) for emit() or send()
I suggest you to check Romans Elizarov blog for more detailed information especially this post.
Regarding your code, for callbackFlow you wont' be needing a coroutine launch:
coroutineScope.launch {
send(key)
//trySend(key)
}
Just use trySend()