working on app like dropbox and making a feature like dropbox offline download, so when a file is added from a system it will download in other system but as a placeholder file once user double click or open that file or after copy pasting to another location on demand need to download the file and after that do the action open or paste so here I need file open event and paste event to be block untill it downloads. how to achieve this uing obj c, c , c++ or swift