Updating iPhone Reachability on Network Changes

Question

Edit 23.5.11

I\'m now wondering whether I\'m over engineering this. When I use an online connection, I handle it properly - either:

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Answer 1

You cannot and should not use Apple's Reachabilty sample code (or the SCNetworkReachabilityFlags) to determine if a network resource is or will be available. They give you useful tools for giving user feedback about why a connection failed and for determining if and when to retry a connection. However if you want to access a network resource you should just ask for it.

Don't present your users with "no network connection" alerts based on the reachability flags. Perform your network request and if it fails check the reachability flags and your own history of requests to see if you should tell your user that the request failed or silently retry. You shouldn't want or need to spam the user with every network failure. If a request fails consider retrying it if the host seems to be reachable and only report a failure to the user after a couple of attempts or when the reachability flags suggest that you have actually lost your network connection and not just had a single request fail.

As for reactivating the device's radios you'll need to balance your desire to try to fetch network resources with allowing the device to power down the network interfaces to save power. I don't know what your app is trying to do but I'd recommend allowing the device to power down and then trying to connect in response to a user asking the app to fetch new data. I don't see a case where you would want to reactivate a radio just to give the user a hint that some network resource may or may not currently be reachable.

See https://devforums.apple.com/message/409618

The only way to know whether a host is available is to try to connect to it. If that connection fails, you can use reachability to provide feedback to the user, and to guide your retry mechanism, but using reachability to preflight a connection is not a good idea.

The key issue here is that reachability uses local information to determine its results. This doesn't say anything about the state of the wider Internet. The origin server might be down, or any number of links between you and the origin server might be down, and reachability will happily say that it's reachable. At that point you're going to try to connect, and the connection will fail. So you're going to have to handle the error anyway, which means you might as well not do the preflight and let your standard error handling cover this case as well.

Also take a look at this question regarding checking for a resource before loading it using an external application: https://devforums.apple.com/message/411329

Making a HEAD request for the resource should give you a reasonable idea if the external app will be able to load it without requiring you to download a significant amount of data (latency will still be a concern though).