Capture Wi-Fi network changing event in iOS

You can fetch details from your wifi connection:

- (NSDictionary *)getConnectionDetails
{
    NSDictionary *connectionDetails = [NSDictionary dictionary];
    CFArrayRef myArray = CNCopySupportedInterfaces();
    if (myArray) {
        CFDictionaryRef myDict = CNCopyCurrentNetworkInfo(CFArrayGetValueAtIndex(myArray, 0));
        connectionDetails = (__bridge_transfer NSDictionary*)myDict;
    }
    return connectionDetails;
}

And then if check [connectionDetails valueForKey:@"BSSID"] you will get BSSID.

Also please note that you must to import #import <SystemConfiguration/CaptiveNetwork.h>

You want SystemConfiguration, which has facilities for seeing notifications on all sorts of networking changes. In particular you'll want to use SCDynamicStoreSetNotificationKeys to listen for changes to the devices and SCNetworkConfiguration to get information about the available interfaces.

I would recommend simply using what Larme posted, and setting up an NSTimer to check every second or so, what the SSID of your current network is, if you detect a change, simply do whatever you need to do. Keep in mind, changing WiFi networks is not something that happens instantaneously, so having a 1 second resolution is not bad

In applicationDidFinishLoading

NSTimer *ssidTimer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(fetchSSIDInfo) userInfo:nil repeats:YES];

In AppDelegate

- (id)fetchSSIDInfo {
     NSArray *ifs = (__bridge_transfer id)CNCopySupportedInterfaces();
     NSLog(@"Supported interfaces: %@", ifs);
     id info = nil;
     NSString *ifnam = @"";
     for (ifnam in ifs) {
         info = (__bridge_transfer id)CNCopyCurrentNetworkInfo((__bridge CFStringRef)ifnam);
         NSLog(@"%@ => %@", ifnam, info);
         if (info && [info count]) { break; }
     }
     if ([info count] >= 1 && [ifnam caseInsensitiveCompare:prevSSID] !=  NSOrderedSame) {
          // Trigger some event
          prevSSID = ifnam;
     }

     return info;
}

Something like that. I can not check if code is typo free as I am not in front of a mac, but it should not be too different