@GuardedBy("mLock")
  private boolean maybeUpdateFileCacheSize() {
    long now = mClock.now();
    if ((!mCacheStats.isInitialized())
        || mCacheSizeLastUpdateTime == UNINITIALIZED
        || (now - mCacheSizeLastUpdateTime) > FILECACHE_SIZE_UPDATE_PERIOD_MS) {
      return maybeUpdateFileCacheSizeAndIndex();
    }
    return false;
  }
If file cache size is not calculated or if it was calculated a long time ago
(FILECACHE_SIZE_UPDATE_PERIOD_MS) recalculated from file listing.

@return true if it was recalculated, false otherwise.
imagepipeline-base/src/main/java/com/facebook/cache/disk/DiskStorageCache.java