Android Kotlin Coroutine used with function parameters acts odd

Question

I'm using a coroutine to format a list.

This is how it looks

val job : Job? = null

private fun formatList(originalList:MutableList<MY_OBJECT>, callback : (MutableList<MY_OBJECT>) -> Unit){

    if(job != null && !job?.isCompleted!!){

        job?.cancel()
    }

    job = viewModelScope.launch(Dispatchers.IO) {

          originalList.foreach { item ->
               //do something with the item.
          }
    }
}

This method can be called several times during runtime, and to avoid it from doing the samething, I added a cancel call if the job isn't done yet.

The problem is, in runtime, the stuffs inside the foreach block randomly produces some index related crashes.

Why is this happening? Is this something about things going behind coroutine execution? Or are there something I don't know about the foreach loop?

Pavan Varma · Accepted Answer · 2020-06-01 05:55:28Z

1

Making your code cancellable

For job.cancel() to work properly, you need to make your code inside coroutine cancellable.

job = viewModelScope.launch(Dispatchers.IO) {
      originalList.foreach { item ->
           if(!isActive) return@launch
           //do something with the item.
      }
      if(!isActive) return@launch
      // do something
}

Here the line if(!isActive) return@launch is checking if the coroutine is still active or not. Checking for isActive before and after computationally intensive code is good practice.

Note: if you are not doing any network or database calls, always try to use Dispatchers.Default which is optimized for computation instead of Dispatchers.IO.

answered Jun 1, 2020 at 5:55

Pavan Varma

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3 Comments

March3April4 Over a year ago

One more question. Does this mean that the foreach block will not stop even if the job itself has been cancelled?

Pavan Varma Over a year ago

yes, it will not stop automatically. we have to manually close it depending upon isActive.

March3April4 Over a year ago

Thanks. Manually stopping the loop did the trick. By the way, the logic inside the foreach block consists of heavy actions such as comparing every item's Date value several times. Will Dispatchers.Default block the UI?

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