Hi guys and a big thanks in advance: I want to solve the following problem and I honestly don't know where to start googling it 😕 Lets say we have a database class and it has two methods:

class Database {
    fun prefillDatabase() {
      // launch a non blocking job here that puts some data in the database
      // image this is done on application start
    }

    suspend fun getEntities() {
      // launch a blocking coroutine to get all entities of the database but only AFTER the job in prefillDatabase() completed
      // imagine this is called later in the application on button click.
    }
}

Is this a case for .join()? The job started in prefillDatabase() could already be well completed before getEntities() is called

I assume the first one (prefill) is called in the app start up and ‘getEntities’ is called somewhere else completely different, but you want to make sure that ‘prefill’ has done its job before ‘getEntities’ start doing its work. Correct?

Fulley correct, yes! And ahhh! The execution in getEntities would suspend on .join() until its execution is complete and would do nothing if its already completed, right?

That’s the idea. The

join

function is a suspend function.

Hmm, after looking at the documentation:

* If the [Job] of the invoking coroutine is cancelled or completed when this
     * suspending function is invoked or while it is suspended, this function
     * throws [CancellationException].

But thats ok now I know what I have to try and where to look, a big thanks!

I think that documentation doesn't mean what you think it means -- it means that if the

getEntities

coroutine is cancelled/completed,

join

throws

CancellationException

-- not if

prefill

is completed.

Join is way to do that, you should just save this job to some filed, you even can init db instance inside and cache using async{} which caches result and you can receive it using suspend function .await() It also allows to be sure that db will be prefilled only once and even make it lazy, never call prefillDatabase explicitly, but let first getEntities do this