ViewController.m

//
//  ViewController.m
//  Session Demo
//
//  Created by mCURA on 27/11/17.
//  Copyright © 2017 Prince. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()<NSURLSessionDataDelegate, NSURLSessionDelegate>

@property (nonatomic, strong) NSMutableData *receivedData;

@end

@implementation ViewController

NSTimeInterval reqTimeInterval = 300.0f;

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    self.receivedData = [[NSMutableData alloc] init];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (NSURLSession *)sessionForId:(NSString *)identity {
    
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    queue.maxConcurrentOperationCount = 4;
    
    NSString *backgroundSessionIdentifier = [NSString stringWithFormat:@"backgroundPdfUploadIdentifier_%@",identity];
    
    NSURLSessionConfiguration *backgroundSession = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:backgroundSessionIdentifier];
    backgroundSession.networkServiceType = NSURLNetworkServiceTypeBackground;
    backgroundSession.requestCachePolicy = NSURLRequestReloadIgnoringCacheData;
    backgroundSession.timeoutIntervalForRequest = reqTimeInterval;
    backgroundSession.timeoutIntervalForResource = (reqTimeInterval * 2);
    
    if ([backgroundSession respondsToSelector:@selector(setWaitsForConnectivity:)]) {
        [backgroundSession setWaitsForConnectivity:true];
    }
    
    NSURLSession *session = [NSURLSession sessionWithConfiguration:backgroundSession delegate:self delegateQueue:queue];
    
    return session;
}

- (IBAction)startFetching:(id)sender {
    
    NSString *urlString = [NSString stringWithFormat:@"http://services.groupkt.com/country/get/all"];
    
    NSLog(@"url Req %@",urlString);

    // setting up the request object now
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
    [request setURL:[NSURL URLWithString:urlString]];
    [request setCachePolicy:NSURLRequestReloadIgnoringCacheData];
    [request setTimeoutInterval:reqTimeInterval];
    
    NSURLSession *session = [self sessionForId:@"mySessionId"];
    NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request];
    NSLog(@"postDataTask %@ timeout %f",postDataTask,request.timeoutInterval);
    [postDataTask resume];
}

#pragma mark- NSURLSessionDataDelegate

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *) response;
    NSLog(@"response status code: %ld", (long)[httpResponse statusCode]);
    
    self.receivedData = nil;
    self.receivedData = [[NSMutableData alloc] init];
    [self.receivedData setLength:0];
    
    completionHandler(NSURLSessionResponseAllow);
}

#pragma mark- NSURLSessionDataDelegate

-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
   didReceiveData:(NSData *)data {
    [self.receivedData appendData:data];
}

#pragma mark- NSURLSessionTaskDelegate

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    
    NSLog(@"RESPONSE --------------");
    NSLog(@"session identifier %@",session.configuration.identifier);

    if (error) {
        // Handle error
        NSLog(@"Error %@",error);
        [self responseHandler:error];
    }
    else {
        if ([session.configuration.identifier rangeOfString:@"backgroundPdfUploadIdentifier"].location != NSNotFound) {
            NSDictionary *responseData = [NSJSONSerialization JSONObjectWithData:self.receivedData options:NSJSONReadingAllowFragments error:nil];
            NSLog(@"response of background session --- %@",responseData);
        }
    }
    
    [session finishTasksAndInvalidate];
    self.receivedData = nil;
}

- (void)responseHandler:(NSError *)error {
    dispatch_async(dispatch_get_main_queue(), ^{
        if (error) {
            UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"Message" message:error.localizedDescription preferredStyle:UIAlertControllerStyleAlert];
            UIAlertAction *ok = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:nil];
            [alertController addAction:ok];
            [self presentViewController:alertController animated:YES completion:nil];
        }
    });
}

@end