How Can I Make a Function That Returns a Value From a Coroutine?

Question

I want to write a function that will always be called on the UI/main thread. Within that function, it will fetch text on a background thread (needs to access something from a file on the device) and then return that text to the main thread. Can this be done using coroutines in an idiomatic way?

Here is what I have done so far, but I fear it will run on the main thread:

fun getDisplayableName(context: Context): String = 
    if(someCondition)
        context.getString(R.string.someString)
    else runBlocking{
        var name String: String? = null
        launch(Dispatchers.IO) {
            name = // some background logic, where name may still be null
        }
        name ?: ""
    }

I want to use this function in an Android activity:

@Override
fun onCreate() {
    // Other logic
    nameTextView.text = MyHelperClass.getDisplayableName(this)
}

I'm looking for similar behavior to handling asynchronous threading with callbacks, but obviously without the callbacks part.

For the sake of discussion, assume I can't use LiveData or ViewModels.

combine it with live data so you can observe the changes without including callbacks — Gautam
– Gautam, Commented May 22, 2020 at 16:07

Francesc · Accepted Answer · 2020-05-22 16:19:02Z

1

You need a suspend function

suspend fun getDisplayableName(context: Context): String =
    if(someCondition) {
        context.getString(R.string.someString)
    } else {
        withContext(Dispatchers.IO) {
            val name = // some background logic, where name may still be null
            name.orEmpty()
        }
    }
}

You would call it like this from onCreate

lifecycleScope.launch {
    val name = getDisplayableName(this)
}

answered May 22, 2020 at 16:19

Francesc

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2 Comments

coolDude Over a year ago

Thanks much for the input. But why does it not work if I simply pass the scope as a parameter into the function? Can it not be done that way as well? Or is the suspend modifier the only way?

Francesc Over a year ago

In your code you are using runBlocking, that effectively blocks the UI thread until the operation completes. You must use a suspend function if you want to have the code run in the backgrond, using coroutines.

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