Android - A way to show/hide loader without writting false/true in every viewmodelScope.launch

Question

I have been writting a lot of code that looks like this

class LoginViewModel : ViewModel() {
 
 // A fragment observes this and shows/hides a progressbar
 val loader = MutableLiveData<Boolean>() 
 
 fun fetchSomeData() = viewModelScope.launch {
 loader.postValue(true)
 // ...
 loader.postValue(false)
 }
}

is there a way to do it in a more clean way? if I have to do 100 requests then there will be 200 lines of loader.postValue

• Simon Forsberg: Note that generally on Code Review Stack Exchange, we prefer code with more context rather than less. I think you have gotten a downvote because of the // ... part in your code. (You can leave it as-is for now as your question has already been answered). As an example of a question with more context, see codereview.stackexchange.com/q/279291/31562

Answer 1

If I interpret it correctly, you can basically wrap everything in a single method, and pass it a lambda:

fun launchWithPost(code: suspend () -> Unit) {
 viewModelScope.launch {
 loader.postValue(true)
 code.invoke()
 loader.postValue(false)
 }
}
fun fetchSomeData() = launchWithPost {
 // ...
}

• George Shalvashvili: This looks good, but it wont be reusable in another screen, which will have different ViewModel
• Simon Forsberg: @GeorgeShalvashvili You could make it an extention method: CoroutineScope.launchWithPost, or have both screens implement the same interface which specifies that viewModelScope and loader should both be available, and then have it as an extension method on that interface.

Answer 2

Here is what I came up with:

fun Job.withLoader(state: MutableStateFlow<Boolean>) {
 state.tryEmit(true)
 invokeOnCompletion { state.tryEmit(false) }
}
fun Job.withLoader(state: MutableLiveData<Boolean>) {
 state.postValue(true)
 invokeOnCompletion { state.postValue(false) }
}

The usage:

fun authenticate(name: String, password: String) = viewModelScope.launch() {
 authRepo.authenticate(
 username = name,
 password = password,
 )
}.withLoader(loading)

Answer 3

I would leave handling the MutableLiveData<Boolean> and launching a coroutine separate. That way it's not bound to ViewModel at all. The best extension to add it to is the MutableLiveData value itself. Also you are not doing any kind of error handling. Is it correct not to set the value back to false on error?

fun fetchSomeData() = viewModelScope.launch {
 loader.run {
 //do your magic
 }
}
suspend fun MutableLiveData<Boolean>.run(code: suspend () -> Unit) {
 postValue(true)
 try {
 code.invoke()
 } finally {
 postValue(false)
 }
}

• Tenfour04: Good call setting false in a finally block. I'd suggest a different name than run because it shadows the name of standard library top-level function, impacting readability. Another possible improvement is to replace postValue() with withContext(Dispatchers.Main.immediate){ value = ... } to avoid unnecessary deferring of the UI change by a frame.