All RX android related

Observables

from
http://blog.danlew.net/2015/06/22/loading-data-from-multiple-sources-with-rxjava/
https://github.com/dlew/rxjava-multiple-sources-sample/tree/master/src/main/java/net/danlew/sample

// Our sources (left as an exercise for the reader)
Observable<Data> memory = ...;  
Observable<Data> disk = ...;  
Observable<Data> network = ...;

// Retrieve the first source with data
Observable<Data> source = Observable  
  .concat(memory, disk, network)
  .first();
  
  The difference between the two calls is that first() will throw a NoSuchElementException 
  if none of the sources emits valid data, whereas takeFirst() will simply complete without
  exception.
  
// save/cache data
Observable<Data> networkWithSave = network.doOnNext(data -> {  
  saveToDisk(data);
  cacheInMemory(data);
});

Observable<Data> diskWithCache = disk.doOnNext(data -> {  
  cacheInMemory(data);
});



// kick off stale data
Observable<Data> source = Observable  
  .concat(memory, diskWithCache, networkWithSave)
  .first(data -> data.isUpToDate());