AFHTTPRequestOperation doesn't work after stand by mode - iphone

Question

I am developing a app which upload multiple files.
For uploading, I use AFHTTPRequestOperation. It successfully works, But If I lock and after it unlock the screen,then It stop uploading files.
My code for upload file is here
NSUserDefaults *defaultUser = [NSUserDefaults standardUserDefaults];
NSString *userId = [defaultUser stringForKey:#"UserId"];
AFHTTPClient *client= [AFHTTPClient clientWithBaseURL:[NSURL URLWithString:[NSString stringWithFormat:#"%#%#",UploadURL,userId]]];
NSDictionary *params = [NSDictionary dictionaryWithObjectsAndKeys:nil];
NSMutableURLRequest *request = [client multipartFormRequestWithMethod:#"POST" path:nil parameters:params constructingBodyWithBlock: ^(id <AFMultipartFormData>formData) {
[formData appendPartWithFileData: data name:#"f" fileName:[NSString stringWithFormat:#"%d_image.jpeg",rand()] mimeType:#"image/jpeg"];
}];
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setUploadProgressBlock:^(NSUInteger bytesWritten, long long totalBytesWritten, long long totalBytesExpectedToWrite) {}];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"error: %#", [operation error]);
if(error.code == -1001){
UIAlertView *myAlert = [[UIAlertView alloc] initWithTitle:#"Error!"
message:#"The request timed out." delegate:self cancelButtonTitle:#"Ok" otherButtonTitles:nil];
[myAlert show];
}
}];
[operation start];
Can anyone give me suggestion for handle this situation.
Thanks.

Answer 1

(1) Add this code:
[operation setShouldExecuteAsBackgroundTaskWithExpirationHandler:^{
// Handle iOS shutting you down (possibly make a note of where you
// stopped so you can resume later)
}];
(2) Change this line:
[operation start];
to
[client enqueueHTTPRequestOperation:operation];
(3) Keep a strong reference to your AFHTTPClient (this is typically done for AFHTTPClient with the singleton pattern) so that your operations don't get deallocated