code: private Func1<ArrayList<SoundCloudTrack>, ArrayList<SoundCloudTrack>> cacheTracks() {
          return new Func1<ArrayList<SoundCloudTrack>, ArrayList<SoundCloudTrack>>() {
              @Override
              public ArrayList<SoundCloudTrack> call(ArrayList<SoundCloudTrack> soundCloudTracks) {
                  if (soundCloudTracks.size() > 0) {
                      mCacheRam.tracks = soundCloudTracks;
                  }
                  return soundCloudTracks;
              }
          };
      }
docs: "Cache" the tracks list of the supported artist retrieved from network in RAM
  to avoid requesting SoundCloud API for next call.
  
  @return {@link rx.functions.Func1} used to save the retrieved tracks list