EDIT
The Jobs based answer below can work.
But for anyone who has use case I had (one expensive level generator algorithm to run in background rather than many parallel ones), just using Async/Await Tasks requires much less refactoring. This video was helpful:
https://www.youtube.com/watch?v=oWFJl56IL4Y
Original answer
This seems to work. I didn't realise that it will launch scheduled jobs even if you don't call Complete();.
In fact the job will execute even if you don't call JobHandle.ScheduleBatchedJobs(); but the docs seem to say you should call that, so I'm assuming in more complex code there can be a condition when the job won't pick up. Perhaps I'm wrong about this.
public class JobTest : MonoBehaviour
{
JobHandle myJob;
bool started = false;
bool done = false;
void Update()
{
if (!started)
{
if (Time.realtimeSinceStartup > 5)
{
print("scheduling");
myJob = scheduleLongJob();
JobHandle.ScheduleBatchedJobs();
started = true;
}
}
if (myJob.IsCompleted && started)
{
myJob.Complete(); // do I need this? What does it do in this case?
print("job completed");
done = true;
}
}
JobHandle scheduleLongJob()
{
LongJob myJob = new LongJob();
return myJob.Schedule();
}
}
public struct LongJob : IJob
{
public void Execute()
{
float u = 0;
for (int i = 0; i < 50000000; i++)
{
u += math.exp10(math.sqrt(10f));
}
Debug.Log(u);
}
}