Read data from the flow.
- iOS 9.0+
- macOS 10.11+
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
Proxy Flow Error
NEAppfor a list of possible errors. If the data parameter has a length of 0 then no data can be subsequently read from the flow.