/**
  * Sets the display mode.
  *
  * @param displayMode the mode to set
  */
 public synchronized void setDisplayMode(BookShelfFragment.DisplayMode displayMode) {
   sp.edit().putString(PREF_KEY_DISPLAY_MODE, displayMode.name()).apply();
 }
 /** @return the display mode that has been set or the default. */
 public synchronized BookShelfFragment.DisplayMode getDisplayMode() {
   return BookShelfFragment.DisplayMode.valueOf(
       sp.getString(PREF_KEY_DISPLAY_MODE, BookShelfFragment.DisplayMode.GRID.name()));
 }