CompletableOnSubscribe

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
```
@FunctionalInterface
public interface CompletableOnSubscribe
```
A functional interface that has a subscribe() method that receives a CompletableEmitter instance that allows pushing an event in a cancellation-safe manner.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`void`	`subscribe(@NonNull CompletableEmitter emitter)`	Called for each `CompletableObserver` that subscribes.

- Method Detail
  - subscribe
```
void subscribe(@NonNull
               @NonNull CompletableEmitter emitter)
        throws java.lang.Throwable
```
    Called for each CompletableObserver that subscribes.
    
    Parameters:
    
    emitter - the safe emitter instance, never null
    
    Throws:
    
    java.lang.Throwable - on error