Q:

Coroutines the proper way to add a job as child of another?

Given we have job1 : Job and job2 : Job and we want to make job2 a child of job1 (they where created separately have no relation).
What is the correct way to declare that relationship? so that when job1 is cancelled job2 is cancelled as well...
I tried job1.attachChild(e1.job2 as ChildJob) but this is internal api.
I do not want to do some hack when i launch job2 from job1 coroutine.

A:

You can use a Job(parent: Job?) factory function which receives parent job as a parameter. It has the following definition:
public fun Job(parent: Job? = null): Job

that means parameter parent is an optional.
So you can create your jobs like this:
var parentJob: Job = Job()
var childJob: Job = Job(parentJob)

Also take a look at SupervisorJob, which can be used to customize the default behavior of the Job. SupervisorJob factory function has similar definition:
fun SupervisorJob(parent: Job? = null): Job

