Hi, I have a class which contains a map of accumul...

# coroutines

[Joshua Hansen]

Hi, I have a class which contains a map of accumulator values, like this:

private val accumulators = mutableMapOf(
    "foo" to 0,
    "bar" to 0,
)

Then later on in an init block, I'm doing a heavy IO operation to count all the values. I'm walking a large file tree:

init {
    MyScope(Dispatchers.IO).launch {
        // Walk file tree and accumulate values in map
    }
}

finally I have them declared in the class as delegates:

val foo by accumulators
val bar by accumulators

How can I ensure that the init block runs before foo and bar are accessed? I'm using Swing, and when the UI loads, the user can click to display information. This operation inside init can take a few seconds, and it's possible the user requests the information during accumulation. Is there some sort of locking mechanism I can use so that when the user requests the info, it will wait in the backgorund and then show on the UI?

[Joshua Hansen]

Overall I'm having trouble with figuring out the best practice to asynchronously create an instance of a class, and then to know exactly when that class will be ready to be accessed by the UI

[uli]

makeing accumulators Deferred<Map<String,Int>> would do the trick. But then you have to await the value which needs a suspend function.
As properties can not have supsend getters, you’d need to write a regular suspend fun:

private val accumulators : Deferred<Map<String,Int>>

init {
  val completableDeferred = CompletableDeferred<Map<String,Int>>
  accumulators = completableDeferred
  val map = mutableMapOf(
    "foo" to 0,
    "bar" to 0,
  )
  MyScope(Dispatchers.IO).launch {
    // Walk file tree and accumulate values in map
    completableDeferred.compete(map)
  }
}

suspend fun foo() = accumulators.await().get("foo")

[uli]

1. Disclaimer: Above code is untested (i.e. written in slack)
2. Good to know. Awaiting an already completed deferrable will return it’s value imediately

[Joshua Hansen]

This is perfect! My example wasn't quite what I was doing. foo and bar are using a PropertyDelegateProvider which takes a suspend lamda as its delegate getValue funtion, and inside that lambda is where I get the map's value, so it ends up being exactly what I need.

[Joshua Hansen]

Thanks for the input!

[uli]

I don’t get it. Maybe I can learn here. I thought properties could not be suspend

[Joshua Hansen]

Ah, I left out a detail. The properties are a simple wrapper class I have - not a primitive value - which has a suspend getValue() function.

[uli]

ah, perfect

[Joshua Hansen]

Would be nice syntactically to have suspend getters though!

[uli]

Absolutely:
https://discuss.kotlinlang.org/t/suspend-variables-suspend-val-suspend-var/13142/6