How to use iOS Reachability

Reachability is a network helper utility class, its used to get various informations about the connection status

What is the main purposes of Reachability?

Reachability is used to query the network status, and to register your listeners to get informed when connectivity changes.

Is this the main purpose of Reachability, to show the user a message?

No, its main usage is to test if there is Internet connectivity, or to get notified if the connectivity changes

For example if NSURLConnection request has failed do I need to use Reachability somehow to resend the request?

Yes, you could use it. For example, what I normally do in my project is to save all the requests that have been made to a remote server.

Let's say I want to download 10 files. When any file fails the download process due to no Internet connection, I save them to an array of failed downloads.

Then, when Reachability informs me that the Internet connection has been restored, I iterate through this array and start the download process again.

What is the proper use of Reachability?

It depends, on your patterns and needs.

Is it common to use only one instance when app launch, and then listen to network changes?

Yes, that is what I do. In my projects, I only have 1 instance of a download manager class, and this class has the only alive instance of Reachability.

Or should I check myself the reachability status before every network request?

You can do that without having multiple instance of Reachability classes. What I normally do is to have a method inside my download manager that tells me using Reachability if there is connection or not.

Is it enough to use reachabilityWithHostName or do I need also reachabilityForLocalWiFi and reachabilityForInternetConnection?

I'm not sure about this one, but what I normally do is to test connectivity on all the means. I don't differentiate between 3G or WiFi, but there are some implementations where this info (WiFi or 3G) could be useful.

Reachability is an example project that Apple has made. People use this as an API over the SystemConfiguration framework. As you already have seen there are methods to check if a host is reachable and so on.

The way I use the Reachabilty project is that I have made a class with a class method that returns a boolean if the host I am requesting data from is available. If it is then the method returns YES and if it is not, it returns NO (obviously).

Now, in the application where I am making use of connections you need/should check if it is possible to start a connection as the Apple documentation states. I have a simple if and then show an appropriate alert message that the request could not be completed at the moment.

There are no requirements that you should automatically try again if the request could not be carried out the first time. The main purpose of this is to keep your application away from crashing, and at the same time give the user an message that it could not be done.

I do not use the notifications myself, but that is because I am not interested in monitoring whether or not connection can be carried out. This is something that you will have to decide by your application demands.

You are not forced to use all the methods in the Reachablitiy class, it is sufficient to use one of them. It is documented what the different methods offer and when they are ment to be used in the header file.

Remember to include the SystemConfiguration framework.

Reachability makes a best guess whether the internet is reachable or not. It tells you whether you have access through WiFi, or whether you have access only through Mobile Data. There is no guarantee that it is correct. If it says you have a WiFi connection, that connection can be lost one second later. The only way to find whether accessing a URL will work or not is to make that access and see what happens. There are situations where access will work when Reachability says it doesn't and vice versa.

Here's what I use Reachibility for: After downloads failed because there was no internet connection, I'll start retrying when Reachability detects a change (actually a few seconds later; URL access often fails immediately after Reachability says WiFi comes back). After downloads failed where Mobile Data was disallowed, check if Mobile Data is available then ask the user to allow use of Mobile Data.