If you use URLSession to start a task with iOS 10 and the network is not available the connection fails at once with an error. There was a small change in iOS 11 that means you can now tell your URLSession to wait until network connectivity is available before trying the connection.
Configuring The Session
To configure a session to wait for connectivity set the waitsForConnectivityboolean to true when creating the session configuration. For example, starting with a default session configuration:
let configuration = URLSessionConfiguration.default
configuration.timeoutIntervalForResource =300
if #available(iOS11, *) {
configuration.waitsForConnectivity = true
}
let session =URLSession(configuration: configuration, delegate:self, delegateQueue:nil)
Notes:
The default resource timeout interval is 7 days which might be longer than you want to wait. I changed it to five minutes here.
As with all session configuration settings you must set the waitsForConnectivity flag before creating the session. The session takes a copy of the configuration so changing it afterwards has no effect on the created session.
This API is new in iOS 11 so wrap waitsForConnectivity in an #available if supporting iOS 10 or earlier.
The default for waitsForConnectivity is false but note that background sessions always wait for connectivity.
Set the delegate when creating the session if you want to know when the session waits for connectivity.
Also remember that URLSession keeps a strong reference to its delegate. If the session owner is also the delegate you can end up with a retain cycle unless you call invalidateAndCancel on the session at some point to release the delegate and break the cycle.
session.invalidateAndCancel()// Don't use session after this
Waiting For Connectivity
Using iOS 10 or earlier if you start a data task and there is no connectivity it fails at once. You get back an error either via the completion handler or the delegate method urlSession(_:task:didCompleteWithError:) depending on which you are using.
In iOS 11 if you set waitsForConnectivity to true the session waits (it will eventually timeout) and only starts the task if the network connection becomes available. You also get a callback to the URLSessionTaskDelegate method urlSession(_:taskIsWaitingForConnectivity:) so you can take some action:
func urlSession(_ session: URLSession, taskIsWaitingForConnectivity task: URLSessionTask){
// waiting for connectivity, update UI, etc.
}
Notes:
The session only waits when trying the initial connection. If the network drops after making the connection you get back an error via the completion handler or session delegate as with iOS 10.
How long the session waits for connectivity depends on the session resource timeout timeoutIntervalForResource. This defaults to 7 days so you may want to change it to some shorter value when creating the session configuration.
Read More
See this WWDC talk for the changes in URLSession in iOS 11:
WWDC 2017 Session 709 Advances in Networking, Part 2
I skipped a few details on how to use URLsession so if you need a recap see this earlier WWDC talk:
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