Making multiple coroutine API calls and waiting all of them

Question

so usually when you have to make different API calls and wait, you do something like this:

viewModelScope.launch {
    withContext(dispatcherProvider.heavyTasks) {
        val apiResponse1 = api.get1() //suspend function
        val apiResponse2 = api.get2() //suspend function

        if (apiResponse1.isSuccessful() && apiResponse2.isSuccessful() { .. }
    }
}

but what happens if I've to do multiple concurrent same API Calls with different parameter:

viewModelScope.launch {
    withContext(dispatcherProvider.heavyTasks) {
        val multipleIds = listOf(1, 2, 3, 4, 5, ..)
        val content = arrayListOf<CustomObj>()

        multipleIds.forEach { id ->
             val apiResponse1 = api.get1(id) //suspend function

             if (apiResponse1.isSuccessful()) {
                 content.find { it.id == id }.enable = true
             }
        }

        liveData.postValue(content)
    }
}

Problem with second approach is that it will go through all ids of multipleIds list and make async calls, but content will be posted probably before that. How can I wait all the responses from for each loop to be finished and only then postValue of the content to view?

Maybe using async and awaiting on theme will help

Dariush Fathi
– Dariush Fathi

2020-07-14 10:27:59 +00:00
Commented Jul 14, 2020 at 10:27 — Dariush Fathi
– Dariush Fathi, Commented Jul 14, 2020 at 10:27

Adrian K · Accepted Answer · 2020-07-17 10:41:22Z

36

+100

The preferred way to ensure a couple of asynchronous tasks have completed, is using coroutineScope. It will suspend until all child jobs, e.g. all calls to launch or async, have completed.

viewModelScope.launch {
    withContext(dispatcherProvider.heavyTasks) {
        val multipleIds = listOf(1, 2, 3, 4, 5, ..)
        val content = arrayListOf<CustomObj>()
        
        coroutineScope {
            multipleIds.forEach { id ->
                launch { // this will allow us to run multiple tasks in parallel
                    val apiResponse = api.get(id)
                    if (apiResponse.isSuccessful()) {
                        content.find { it.id == id }.enable = true
                    }
                }
           }
        }  // coroutineScope block will wait here until all child tasks are completed
        
        liveData.postValue(content)
    }
}

If you do not feel comfortable with this rather implicit approach, you can also use a more functional approach, mapping your ids to a list of Deferred using async and then await them all. This will also allow you to run all tasks in parallel but end up with a list of results in the correct order.

viewModelScope.launch {
    withContext(dispatcherProvider.heavyTasks) {
        val multipleIds = listOf(1, 2, 3, 4, 5, ..)
        val content = arrayListOf<CustomObj>()

        val runningTasks = multipleIds.map { id ->
                async { // this will allow us to run multiple tasks in parallel
                    val apiResponse = api.get(id)
                    id to apiResponse // associate id and response for later
                }
        }

        val responses = runningTasks.awaitAll()

        responses.forEach { (id, response) ->
            if (response.isSuccessful()) {
                content.find { it.id == id }.enable = true
            }
        }
      
        liveData.postValue(content)
    }
}

edited Jul 17, 2020 at 10:41

answered Jul 17, 2020 at 9:10

Adrian K

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2 Comments

user3561494 Over a year ago

What do you do if each Rest call is different with a different response?

Shams Over a year ago

How about catching the errors?

Glenn Sandoval · Accepted Answer · 2020-07-14 11:19:03Z

1

In order to get a concurrent behavior you need to start a new coroutine for each id. You can move multipleIds and content outside withContext block. Also you can post the result after withContext block since withContext is a suspending function so every coroutine created inside has to finish before posting the result.

viewModelScope.launch {
    val multipleIds = listOf(1, 2, 3, 4, 5, ..)
    val content = arrayListOf<CustomObj>()

    withContext(dispatcherProvider.heavyTasks) {
        multipleIds.forEach { id ->
            launch {
                val apiResponse = api.get(id) //suspend function
                if (apiResponse.isSuccessful()) {
                    content.find { it.id == id }?.enable = true
                }
            }
        }
    }

    liveData.value = content
}

answered Jul 14, 2020 at 11:19

Glenn Sandoval

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1 Comment

MaaAn13 Over a year ago

When I put break-point at liveData.value - it always works as expected, all list items are true (api.get(id)- have been called and response got), but when I run code with out break-point, all list items are false (logs show that api call was successful), meaning, that response came later. This does not work.

Simon · Accepted Answer · 2020-07-14 09:38:24Z

0

Instead of forEach, go with map and do the same inside the { } block. Save result of map to a variable and post this variable.

answered Jul 14, 2020 at 9:38

Simon

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