private boolean shouldCreateNewStorage() {
    State currentState = mCurrentState;
    return (currentState.delegate == null
        || currentState.rootDirectory == null
        || !currentState.rootDirectory.exists());
  }
Gets a concrete disk-storage instance. If nothing has changed since the last call, then the
last state is returned

@return an instance of the appropriate DiskStorage class
@throws IOException
imagepipeline-base/src/main/java/com/facebook/cache/disk/DynamicDefaultDiskStorage.java