By Karl Stenerud

A better reachability for a modern age.

Introduction

A long time ago in an Xcode far away, Apple provided "Reachability", an example Objective-C wrapper to demonstrate the SystemConfiguration Reachability APIs. On the whole it works well enough, but it could be so much more!

KSReachability takes reachability to the next level.

Features

• Reachability to the network in general, to a host, or to an IPV4 or IPV6 address.
• Notifications/callbacks via NSNotification, blocks, and KVO.
• Fetching status values doesn't block.
• Callbacks and KVO always occur on the main thread, so it's UI-safe.
• KSReachableOperation: A one-shot operation to perform when reachability is established.
• Supports iOS and Mac OS X.
• Can be built with or without ARC, in CLANG or GCC.

Usage

Import:

#import "KSReachability.h"

Create a KSReachability object:

self.reachability = [KSReachability reachabilityToHost:hostname];

Use blocks:

self.reachability.onReachabilityChanged = ^(KSReachability* reachability)
{
    NSLog(@"Reachability changed to %d (blocks)", reachability.reachable);
};

Or use NSNotifications:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(onReachabilityChanged:)
                                             name:kDefaultNetworkReachabilityChangedNotification
                                           object:nil];
...

self.reachability.notificationName = kDefaultNetworkReachabilityChangedNotification;
...

- (void) onReachabilityChanged:(NSNotification*) notification
{
    KSReachability* reachability = (KSReachability*)notification.object;
    NSLog(@"Reachability changed to %d (NSNotification)", reachability.reachable);
}

Or use KVO:

[self.reachability addObserver:self
                    forKeyPath:@"reachable"
                       options:NSKeyValueObservingOptionNew
                       context:NULL];
...

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    KSReachability* reachability = (KSReachability*)object;
    NSLog(@"Reachability changed to %d (KVO)", reachability.reachable);
}

Add a reachable operation:

// Create a one-shot operation that gets fired once the host is reachable.
self.reachableOperation = [KSReachableOperation operationWithHost:hostname
                                                        allowWWAN:NO
                                           onReachabilityAchieved:^
                           {
                               [self showAlertWithTitle:@"One-time message"
                                                message:@"Host is reachable!"];
                           }];

Caveats

The Meaning of Reachability

As per Apple's SCNetworkReachability documentation, a remote host is considered reachable when a data packet addressed to that host can leave the local device (i.e. the host is theoretically reachable). It does NOT guarantee that data will actually be received by the host or that the host will respond to a connection request! For example, if the host has a DNS record, but the host itself is down, it will still be considered reachable.

Delays Due to DNS Lookups

KSReachability must do a DNS lookup to determine reachability to a host by name. Since this lookup can take upwards of 10 seconds in extreme cases, it is performed in the background. As a consequence, a newly created KSReachability object will always have its state set to unreachable until this lookup completes. If you need the true reachability to a host, you must wait for the "initialized" property to change to YES (it supports KVO). As an alternative, you can set the callback "onInitializationComplete".

Full Example

I've included a full example project in this repository.

In iOSReachability, see ViewController.h and ViewController.m for details.

In MacReachability, see AppDelegate.h and AppDelegate.m for details.

License

Copyright (c) 2012 Karl Stenerud. All rights reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall remain in place in this source code.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.