When the original observable emits no items, this operator subscribes to the given fallback stream and emits items from that observable instead.
This can be particularly useful when consuming data from multiple sources. For example, when using the Repository Pattern. Assuming you have some data you need to load, you might want to start with the fastest access point and keep falling back to the slowest point. For example, first query an in-memory database, then a database on the file system, then a network call if the data isn't on the local machine.
This can be achieved quite simply with switchIfEmpty!
// Let's pretend we have some Data sources that complete without emitting // any items if they don't contain the data we're looking for Observable<Data> memory; Observable<Data> disk; Observable<Data> network; // Start with memory, fallback to disk, then fallback to network. // Simple as that! Observable<Data> getThatData = memory.switchIfEmpty(disk).switchIfEmpty(network);
Observable<T> switchIfEmpty(Stream<T> fallbackStream) => transform(SwitchIfEmptyStreamTransformer<T>(fallbackStream));