android - How to build list from async tasks properly in Kotlin - Stack Overflow

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I have function like this

fun getUsersList(userIds: List<String>): List<User> {

    val list = mutableListOf<User>()

    userIds.forEach { id ->
        getUserFromDatabase(id) { user ->
            list.add(user)
        }
    }

    return list
}

Ofcourse in this case method return empty list cos it call return list before async tasks in cycle is completed.
So how to make it wait until cycle will be completed before return?
I learned coroutine basics but still have no idea how to handle this, that's why i'm using callbacks to update viewModel.

UPD: To clarify my situation, I get (for example) List<Type1> from database and from this list get userId's and after that get User's from database. I need to build Map<Type1,User> and put it in viewModel. For that I use:

list<Type1>.forEach { type1 -> 
        getUserFromDatabase(type1.userId) { user ->
            map.put(type1, user)
        }
    }

Actually question was about returning result instead of callbacks and I got the answer. Further I'll handle it myself. Thank you all.

I have function like this

fun getUsersList(userIds: List<String>): List<User> {

    val list = mutableListOf<User>()

    userIds.forEach { id ->
        getUserFromDatabase(id) { user ->
            list.add(user)
        }
    }

    return list
}

Ofcourse in this case method return empty list cos it call return list before async tasks in cycle is completed.
So how to make it wait until cycle will be completed before return?
I learned coroutine basics but still have no idea how to handle this, that's why i'm using callbacks to update viewModel.

UPD: To clarify my situation, I get (for example) List<Type1> from database and from this list get userId's and after that get User's from database. I need to build Map<Type1,User> and put it in viewModel. For that I use:

list<Type1>.forEach { type1 -> 
        getUserFromDatabase(type1.userId) { user ->
            map.put(type1, user)
        }
    }

Actually question was about returning result instead of callbacks and I got the answer. Further I'll handle it myself. Thank you all.

• android
• kotlin
• kotlin-coroutines

Share Improve this question Follow edited Mar 28 at 18:40 Exicode asked Mar 28 at 16:59 ExicodeExicode 611010 bronze badges 3

• 1 From the code it seems the best option would be to fetch all users with a single database query. Something like SELECT * FROM users WHERE id IN (<list of ids>). That's more performant and directly returns a List<User>. You won't have to bother with how to parallelize (if at all) the different calls and merge the results afterwards. You could write a new question if you need more help with the database part. – tyg Commented Mar 28 at 18:18
• 1 The question shows lack of understanding of DB design, the solution should be either SQL query with JOIN or if you have 2 separate DBs - it should be done with query mentioned by @tyg – Viktor Yakunin Commented Mar 28 at 18:52
• @ViktorYakunin, tyg, You were right, I just didn't know how to create complex request to the database. Nevertheless, the question wasn't useless, I still had to find out how to return the value in the case of asynchronous tasks. – Exicode Commented Mar 28 at 23:43

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1 Answer 1

Sorted by: Reset to default 1

Build a List<Deferred<User>> and then call awaitAll() on it to get the resulting List<User>. All of that must be happening in a CoroutineScope - in my example it is created with coroutineScope, but you can use withContext or some other way.

suspend fun getUsersList(userIds: List<String>): List<User> = coroutineScope {
    buildList {
        userIds.forEach { id ->
            add(async {
                getUserFromDatabase(id)
            })
        }
    }.awaitAll()
}

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