Function keep_content_in 

pub fn keep_content_in(
    keep: impl Fn(ContentParts) + 'static,
    read: impl Fn(&str) -> Option<Vec<u8>> + 'static,
)

Keep the content every change this thread commits names in keep (the app’s content store), and read the content a received change names from read, so recorded actions replay wherever their content reaches. The host sets both once per thread that runs actions.