Tries to get item from CurrentState. If item wasn't found, current state is reloaded and then it tries again.

Gets current state together with time of last change.

Gets data from cache (if item in cache with key made from changedSince exists). Otherwise it gets items from fetchChangedDataFunc. This function returns only data changed after changedSince. Null is returned if nothing was found.

Invalidates current state. Before next fetch of current state data (property CurrentState, method UpdateAndTryGetItem(), etc.) data will be reloaded from data source.

Invalidates cache and then tries to get item.