ObservableTransformer

Type Parameters:

Upstream - the upstream value type

Downstream - the downstream value type

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 ObservableTransformer<@NonNull Upstream,@NonNull Downstream>
```
Interface to compose Observables.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`@NonNull ObservableSource<Downstream>`	`apply(@NonNull Observable<@NonNull Upstream> upstream)`	Applies a function to the upstream `Observable` and returns an `ObservableSource` with optionally different element type.

- Method Detail
  - apply
```
@NonNull
@NonNull ObservableSource<Downstream> apply(@NonNull
                                            @NonNull Observable<@NonNull Upstream> upstream)
```
    Applies a function to the upstream Observable and returns an ObservableSource with optionally different element type.
    
    Parameters:
    
    upstream - the upstream Observable instance
    
    Returns:
    
    the transformed ObservableSource instance