How to edit metadata of image in iOS8?

ALAssetsLibrary *lib = [[ALAssetsLibrary alloc] init];
    [lib assetForURL:nil resultBlock:^(ALAsset *asset) {
        NSDictionary *metadata = rep.metadata;
        if (metadata) {
            NSDictionary *GPSDict=metadata[@"{GPS}"];
            NSDictionary *TIFFDict=metadata[@"{TIFF}"];
            if (GPSDict){
                double longitude = [[GPSDict objectForKey:@"Longitude"] doubleValue];
                double latitude = [[GPSDict objectForKey:@"Latitude"] doubleValue];
                if ([[GPSDict objectForKey:@"LatitudeRef"] isEqualToString:@"S"]) {
                    latitude = -latitude;
                }
                if ([[GPSDict objectForKey:@"LongitudeRef"] isEqualToString:@"W"]) {
                    longitude = -longitude;
                }
                if (TIFFDict){
                    NSUserDefaults *pref = [NSUserDefaults standardUserDefaults];
                    [pref setObject:[TIFFDict objectForKey:@"DateTime"] forKey:@"PHOTODATE"];
                    [pref synchronize];
                }
                coordinate2D = CLLocationCoordinate2DMake(latitude, longitude);
            }else {

                latitude = locationManager.location.coordinate.latitude;
                 longitude = locationManager.location.coordinate.longitude;

                 [GPSDictionary setObject:[NSNumber numberWithFloat:fabs(latitude)]
                 forKey:(NSString*)kCGImagePropertyGPSLatitude];
                 [GPSDictionary setObject:(latitude > 0 ? @"N": @"S") forKey:(NSString*)kCGImagePropertyGPSLatitudeRef];
                 [GPSDictionary setObject:[NSNumber numberWithFloat:fabs(longitude)]
                 forKey:(NSString*)kCGImagePropertyGPSLongitude];
                 [GPSDictionary setObject:(longitude > 0 ? @"E": @"W") forKey:(NSString*)kCGImagePropertyGPSLongitudeRef];        //

                 if (metadata&& GPSDictionary) {
                 [metadata setValue:GPSDictionary forKey:(NSString*)kCGImagePropertyGPSDictionary];
                 }
                 coordinate2D = CLLocationCoordinate2DMake(latitude, longitude);
            }
        }
        else
        {

        }
    } failureBlock:^(NSError *error) {
        //User denied access
        NSLog(@"Unable to access image: %@", error);
    }];

I am using above code to get metadata of image.But now i want to edit this metadata.I want to add custom location in image if the location information is not present in the {GPS} dictionary.

From the apple’s documentation : Applications are only allowed to edit assets that they originally wrote. So if your application is writing image in Photos Library then only you will be able to edit its metadata.

You can check whether metadata is editable or not using ALAsset’s editable property.

I was able to update metadata using setImageData:metadata:completionBlock: method.

Please refer following code : I am passing same image data with updated metadata. Also I have tested changing orientation not the GPS data but this code helps you to start :

ALAsset *asset = // your asset
    if(asset.editable) {
        NSDictionary *metadata = asset.defaultRepresentation.metadata;

        NSDictionary *gpsInfo=metadata[@"{GPS}"];

        if (gpsInfo) {
            NSMutableDictionary *mutableGPSInfo = [gpsInfo mutableCopy];
            [mutableGPSInfo setObject:@"yourNewLatitude" forKey:@"Latitude"];
            [mutableGPSInfo setObject:@"yourNewLongitude" forKey:@"Longitude"];

            NSMutableDictionary *mutableMetadata = [metadata mutableCopy];
            [mutableMetadata setObject:[mutableGPSInfo copy] forKey:@"{GPS}"];

            ALAssetRepresentation *rep = [asset defaultRepresentation];
            Byte *buffer = (Byte*)malloc(rep.size);
            NSUInteger buffered = [rep getBytes:buffer fromOffset:0.0 length:rep.size error:nil];
            NSData *data = [NSData dataWithBytesNoCopy:buffer length:buffered freeWhenDone:YES];

            [asset setImageData:data metadata:[mutableMetadata copy] completionBlock:^(NSURL *assetURL, NSError *error) {
                if (error) {
                    NSLog(@"Error : %@",error);
                } else {
                    NSLog(@"Asset metadata is successfully edited");
                }
            }];
        }
    } else {
        NSLog(@"oops..! Asset can not be edited");
    }