A block that will be executed by the system on an internal system thread when some data is read from the flow. The block is passed either the data that was read or a non-nil error if an error occurred. See NEAppProxyFlowError in NEAppProxyFlow for a list of possible errors. If the data parameter has a length of 0 then no data can be subsequently read from the flow.
This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.