Question

I have to download (in asynchronous mode) and get data from a file that is generated from a server.
So I call the method that does this into the app delegate, and everything is okay.

I’d like to check the internet connection, and if there isn’t one, make the method to listen for when connection will back, and then download the file.
My question is: how can I do this?

Answer 1

You can use the apples reachability class see this

Reachability is one sample provided by Apple for this.(Description

The Reachability sample application demonstrates how to use the
SystemConfiguration framework to monitor the network state of an
iPhone or iPod touch. In particular, it demonstrates how to know when
IP can be routed and when traffic will be routed through a Wireless
Wide Area Network (WWAN) interface such as EDGE or 3G.

Reachability.h file

/*

 File: Reachability.h
 Abstract: Basic demonstration of how to use the SystemConfiguration Reachablity APIs.

 Version: 2.2

 Disclaimer: IMPORTANT:  This Apple software is supplied to you by Apple Inc.
 ("Apple") in consideration of your agreement to the following terms, and your
 use, installation, modification or redistribution of this Apple software
 constitutes acceptance of these terms.  If you do not agree with these terms,
 please do not use, install, modify or redistribute this Apple software.

 In consideration of your agreement to abide by the following terms, and subject
 to these terms, Apple grants you a personal, non-exclusive license, under
 Apple's copyrights in this original Apple software (the "Apple Software"), to
 use, reproduce, modify and redistribute the Apple Software, with or without
 modifications, in source and/or binary forms; provided that if you redistribute
 the Apple Software in its entirety and without modifications, you must retain
 this notice and the following text and disclaimers in all such redistributions
 of the Apple Software.
 Neither the name, trademarks, service marks or logos of Apple Inc. may be used
 to endorse or promote products derived from the Apple Software without specific
 prior written permission from Apple.  Except as expressly stated in this notice,
 no other rights or licenses, express or implied, are granted by Apple herein,
 including but not limited to any patent rights that may be infringed by your
 derivative works or by other works in which the Apple Software may be
 incorporated.

 The Apple Software is provided by Apple on an "AS IS" basis.  APPLE MAKES NO
 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED
 WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN
 COMBINATION WITH YOUR PRODUCTS.

 IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR
 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION AND/OR
 DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER UNDER THEORY OF
 CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF
 APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 Copyright (C) 2010 Apple Inc. All Rights Reserved.

*/


#import <Foundation/Foundation.h>
#import <SystemConfiguration/SystemConfiguration.h>

typedef enum {
    NotReachable = 0,
    ReachableViaWiFi,
    ReachableViaWWAN
} NetworkStatus;
#define kReachabilityChangedNotification @"kNetworkReachabilityChangedNotification"

@interface Reachability: NSObject
{
    BOOL localWiFiRef;
    SCNetworkReachabilityRef reachabilityRef;
}

//reachabilityWithHostName- Use to check the reachability of a particular host name. 
+ (Reachability*) reachabilityWithHostName: (NSString*) hostName;

//reachabilityWithAddress- Use to check the reachability of a particular IP address. 
+ (Reachability*) reachabilityWithAddress: (const struct sockaddr_in*) hostAddress;

//reachabilityForInternetConnection- checks whether the default route is available.  
//  Should be used by applications that do not connect to a particular host
+ (Reachability*) reachabilityForInternetConnection;

//reachabilityForLocalWiFi- checks whether a local wifi connection is available.
+ (Reachability*) reachabilityForLocalWiFi;

//Start listening for reachability notifications on the current run loop
- (BOOL) startNotifier;
- (void) stopNotifier;

- (NetworkStatus) currentReachabilityStatus;
//WWAN may be available, but not active until a connection has been established.
//WiFi may require a connection for VPN on Demand.
- (BOOL) connectionRequired;
@end

Reachability.m file

 /*
 File: Reachability.m
 Abstract: Basic demonstration of how to use the SystemConfiguration Reachablity APIs.

 Version: 2.2

 Disclaimer: IMPORTANT:  This Apple software is supplied to you by Apple Inc.
 ("Apple") in consideration of your agreement to the following terms, and your
 use, installation, modification or redistribution of this Apple software
 constitutes acceptance of these terms.  If you do not agree with these terms,
 please do not use, install, modify or redistribute this Apple software.

 In consideration of your agreement to abide by the following terms, and subject
 to these terms, Apple grants you a personal, non-exclusive license, under
 Apple's copyrights in this original Apple software (the "Apple Software"), to
 use, reproduce, modify and redistribute the Apple Software, with or without
 modifications, in source and/or binary forms; provided that if you redistribute
 the Apple Software in its entirety and without modifications, you must retain
 this notice and the following text and disclaimers in all such redistributions
 of the Apple Software.
 Neither the name, trademarks, service marks or logos of Apple Inc. may be used
 to endorse or promote products derived from the Apple Software without specific
 prior written permission from Apple.  Except as expressly stated in this notice,
 no other rights or licenses, express or implied, are granted by Apple herein,
 including but not limited to any patent rights that may be infringed by your
 derivative works or by other works in which the Apple Software may be
 incorporated.

 The Apple Software is provided by Apple on an "AS IS" basis.  APPLE MAKES NO
 WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION THE IMPLIED
 WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN
 COMBINATION WITH YOUR PRODUCTS.

 IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL OR
 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION, MODIFICATION AND/OR
 DISTRIBUTION OF THE APPLE SOFTWARE, HOWEVER CAUSED AND WHETHER UNDER THEORY OF
 CONTRACT, TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF
 APPLE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 Copyright (C) 2010 Apple Inc. All Rights Reserved.

*/

#import <sys/socket.h>
#import <netinet/in.h>
#import <netinet6/in6.h>
#import <arpa/inet.h>
#import <ifaddrs.h>
#import <netdb.h>

#import <CoreFoundation/CoreFoundation.h>

#import "Reachability.h"

#define kShouldPrintReachabilityFlags 1

static void PrintReachabilityFlags(SCNetworkReachabilityFlags    flags, const char* comment)
{
#if kShouldPrintReachabilityFlags

    NSLog(@"Reachability Flag Status: %c%c %c%c%c%c%c%c%c %s\n",
            (flags & kSCNetworkReachabilityFlagsIsWWAN)               ? 'W' : '-',
            (flags & kSCNetworkReachabilityFlagsReachable)            ? 'R' : '-',

            (flags & kSCNetworkReachabilityFlagsTransientConnection)  ? 't' : '-',
            (flags & kSCNetworkReachabilityFlagsConnectionRequired)   ? 'c' : '-',
            (flags & kSCNetworkReachabilityFlagsConnectionOnTraffic)  ? 'C' : '-',
            (flags & kSCNetworkReachabilityFlagsInterventionRequired) ? 'i' : '-',
            (flags & kSCNetworkReachabilityFlagsConnectionOnDemand)   ? 'D' : '-',
            (flags & kSCNetworkReachabilityFlagsIsLocalAddress)       ? 'l' : '-',
            (flags & kSCNetworkReachabilityFlagsIsDirect)             ? 'd' : '-',
            comment
            );
#endif
}


@implementation Reachability
static void ReachabilityCallback(SCNetworkReachabilityRef target, SCNetworkReachabilityFlags flags, void* info)
{
    #pragma unused (target, flags)
    NSCAssert(info != NULL, @"info was NULL in ReachabilityCallback");
    NSCAssert([(NSObject*) info isKindOfClass: [Reachability class]], @"info was wrong class in ReachabilityCallback");

    //We're on the main RunLoop, so an NSAutoreleasePool is not necessary, but is added defensively
    // in case someon uses the Reachablity object in a different thread.
    NSAutoreleasePool* myPool = [[NSAutoreleasePool alloc] init];

    Reachability* noteObject = (Reachability*) info;
    // Post a notification to notify the client that the network reachability changed.
    [[NSNotificationCenter defaultCenter] postNotificationName: kReachabilityChangedNotification object: noteObject];

    [myPool release];
}

- (BOOL) startNotifier
{
    BOOL retVal = NO;
    SCNetworkReachabilityContext    context = {0, self, NULL, NULL, NULL};
    if(SCNetworkReachabilitySetCallback(reachabilityRef, ReachabilityCallback, &context))
    {
        if(SCNetworkReachabilityScheduleWithRunLoop(reachabilityRef, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode))
        {
            retVal = YES;
        }
    }
    return retVal;
}

- (void) stopNotifier
{
    if(reachabilityRef!= NULL)
    {
        SCNetworkReachabilityUnscheduleFromRunLoop(reachabilityRef, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
    }
}

- (void) dealloc
{
    [self stopNotifier];
    if(reachabilityRef!= NULL)
    {
        CFRelease(reachabilityRef);
    }
    [super dealloc];
}

+ (Reachability*) reachabilityWithHostName: (NSString*) hostName;
{
    Reachability* retVal = NULL;
    SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(NULL, [hostName UTF8String]);
    if(reachability!= NULL)
    {
        retVal= [[[self alloc] init] autorelease];
        if(retVal!= NULL)
        {
            retVal->reachabilityRef = reachability;
            retVal->localWiFiRef = NO;
        }
    }
    return retVal;
}

+ (Reachability*) reachabilityWithAddress: (const struct sockaddr_in*) hostAddress;
{
    SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithAddress(kCFAllocatorDefault, (const struct sockaddr*)hostAddress);
    Reachability* retVal = NULL;
    if(reachability!= NULL)
    {
        retVal= [[[self alloc] init] autorelease];
        if(retVal!= NULL)
        {
            retVal->reachabilityRef = reachability;
            retVal->localWiFiRef = NO;
        }
    }
    return retVal;
}

+ (Reachability*) reachabilityForInternetConnection;
{
    struct sockaddr_in zeroAddress;
    bzero(&zeroAddress, sizeof(zeroAddress));
    zeroAddress.sin_len = sizeof(zeroAddress);
    zeroAddress.sin_family = AF_INET;
    return [self reachabilityWithAddress: &zeroAddress];
}

+ (Reachability*) reachabilityForLocalWiFi;
{
    struct sockaddr_in localWifiAddress;
    bzero(&localWifiAddress, sizeof(localWifiAddress));
    localWifiAddress.sin_len = sizeof(localWifiAddress);
    localWifiAddress.sin_family = AF_INET;
    // IN_LINKLOCALNETNUM is defined in <netinet/in.h> as 169.254.0.0
    localWifiAddress.sin_addr.s_addr = htonl(IN_LINKLOCALNETNUM);
    Reachability* retVal = [self reachabilityWithAddress: &localWifiAddress];
    if(retVal!= NULL)
    {
        retVal->localWiFiRef = YES;
    }
    return retVal;
}

#pragma mark Network Flag Handling

- (NetworkStatus) localWiFiStatusForFlags: (SCNetworkReachabilityFlags) flags
{
    PrintReachabilityFlags(flags, "localWiFiStatusForFlags");

    BOOL retVal = NotReachable;
    if((flags & kSCNetworkReachabilityFlagsReachable) && (flags & kSCNetworkReachabilityFlagsIsDirect))
    {
        retVal = ReachableViaWiFi;  
    }
    return retVal;
}

- (NetworkStatus) networkStatusForFlags: (SCNetworkReachabilityFlags) flags
{
    PrintReachabilityFlags(flags, "networkStatusForFlags");
    if ((flags & kSCNetworkReachabilityFlagsReachable) == 0)
    {
        // if target host is not reachable
        return NotReachable;
    }

    BOOL retVal = NotReachable;

    if ((flags & kSCNetworkReachabilityFlagsConnectionRequired) == 0)
    {
        // if target host is reachable and no connection is required
        //  then we'll assume (for now) that your on Wi-Fi
        retVal = ReachableViaWiFi;
    }


    if ((((flags & kSCNetworkReachabilityFlagsConnectionOnDemand ) != 0) ||
        (flags & kSCNetworkReachabilityFlagsConnectionOnTraffic) != 0))
    {
            // ... and the connection is on-demand (or on-traffic) if the
            //     calling application is using the CFSocketStream or higher APIs

            if ((flags & kSCNetworkReachabilityFlagsInterventionRequired) == 0)
            {
                // ... and no [user] intervention is needed
                retVal = ReachableViaWiFi;
            }
        }

    if ((flags & kSCNetworkReachabilityFlagsIsWWAN) == kSCNetworkReachabilityFlagsIsWWAN)
    {
        // ... but WWAN connections are OK if the calling application
        //     is using the CFNetwork (CFSocketStream?) APIs.
        retVal = ReachableViaWWAN;
    }
    return retVal;
}

- (BOOL) connectionRequired;
{
    NSAssert(reachabilityRef != NULL, @"connectionRequired called with NULL reachabilityRef");
    SCNetworkReachabilityFlags flags;
    if (SCNetworkReachabilityGetFlags(reachabilityRef, &flags))
    {
        return (flags & kSCNetworkReachabilityFlagsConnectionRequired);
    }
    return NO;
}

- (NetworkStatus) currentReachabilityStatus
{
    NSAssert(reachabilityRef != NULL, @"currentNetworkStatus called with NULL reachabilityRef");
    NetworkStatus retVal = NotReachable;
    SCNetworkReachabilityFlags flags;
    if (SCNetworkReachabilityGetFlags(reachabilityRef, &flags))
    {
        if(localWiFiRef)
        {
            retVal = [self localWiFiStatusForFlags: flags];
        }
        else
        {
            retVal = [self networkStatusForFlags: flags];
        }
    }
    return retVal;
}
@end

For more information of how to use it. Just refer the above link.