rx.Single is a beta feature in RxJava that represents a single value, rather than a stream of values that an rx.Observable represents. This is useful in simplifying the mental model required for consumers of your API (e.g. a Retrofit service).
rx.Observable
- Respond with an error.
- Never respond.
- Respond successfully with no data and end.
- Respond successfully with a single value and end.
- Respond successfully with multiple values and end.
- Respond successfully with one or more values and don’t end.
rx.Single
- Respond with an error.
- Never respond.
- Respond successfully with a single value.
Example
Consider a public API that retrieves a user’s profile from a network API.
Observable<Profile> getProfile();The behavior of getProfile() is not immediately obvious - can getProfile() emit multiple values? when does it end? Maybe it emits a new Profile when it gets updated?
What if we use rx.Single instead?
Single<Profile> getProfile();Now the behavior of getProfile() explicit - the consumers only need to consider a single value, and that behavior is enforced by the Java type system.
TL;DR
Consider using rx.Single when an API/Service layer should only emit one item.