How to run a NSURLConnection in background in iOS

Question

I am working with an app which is todo list organizer, where user adds notes. I am using coredata DB to store the notes. As I am providing sync feature, I am parsing JSON data to server, and also getting JSON data from server.
I am using NSURLConnection API and its delegate functions

- (void)pushData
{
      loop through the notes array and send notes 1 by one

       [[request setValue:@"application/json;charset=utf-8" forHTTPHeaderField:@"Content-Type"];

            [request setHTTPMethod:@"POST"];
            [request setHTTPBody:jsonData];

            m_dataPush = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];

            [m_dataPush start];
}


- (void)connectionDidFinishLoading:(NSURLConnection *)connection 
{
      Process response from server, save to core DB
      and again pushData if any modified and again process the response
}

I call this API, on appEnterBackground and appBecomeActive, because, I want the data to updated on multiple devices.

The problems, which I am facing is that

1) When the notes are more, app is getting stuck, when we exit and open the app and start adding notes.

2) I tried using GCD, but then my NSURLConnection doesnot send me any response

Regards Ranjit

Answer 1

Ranjit: Based on your comments in the different responses, I suspect you are sending the 1st request from the main thread. When you receive the 1st response, you process it in the background, and then send the 2nd request also from the background. The subsequent requests should be sent from the main thread

[self performSelectorOnMainThread:@selector(myMethodToOpenConnection:)
                       withObject:myObject
                    waitUntilDone:NO];

otherwise the thread exits before the delegate is called

Answer 2

You can use NSOperation Queue with NSURLConnection like this

//allocate a new operation queue
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
//Loads the data for a URL request and executes a handler block on an
//operation queue when the request completes or fails.
[NSURLConnection
 sendAsynchronousRequest:urlRequest
 queue:queue
 completionHandler:^(NSURLResponse *response,
                     NSData *data,
                     NSError *error) {
     if ([data length] >0 && error == nil){
         //process the JSON response
         //use the main queue so that we can interact with the screen
         NSString *myData = [[NSString alloc] initWithData:data
                                                  encoding:NSUTF8StringEncoding];
         NSLog(@"JSON data = %@", myData);
         NSDictionary *myDict = [myData JSONValue];
         }
 }];

it will do all the processing in the background.

Answer 3

NSURLConnection provides a convenience method called sendAsynchronousRequest: completionHandler: that does the GCD work for you. You can tell it to run the completion handler on the main thread.

Using it, your code would get simpler as follows:

// place a declaration in your .h to make it public
- (void)pushDataWithCompletion:(void (^)(BOOL, NSError*))completion;

- (void)pushDataWithCompletion:(void (^)(BOOL, NSError*))completion
{
    // setup your connection request...
    [[request setValue:@"application/json;charset=utf-8" forHTTPHeaderField:@"Content-Type"];
    [request setHTTPMethod:@"POST"];
    [request setHTTPBody:jsonData];

    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {

        // whatever you do on the connectionDidFinishLoading
        // delegate can be moved here
        if (!error) {
            // did finish logic here, then tell the caller you are done with success
            completion(YES, nil);
        } else {
            // otherwise, you are done with an error
            completion(NO, error);
        }
    }];
}

Exactly what you pass back in the block depends on what the callers care about. It's common to make some aspect of the data you collected one of the block params.

EDIT - I left out the pointer notation (*) after NSError above.

Also, say you have an array of objects that needs to be processed by the server. This method is good for one call. To handle several, lets give it a parameter. Say that each note is an NSString *;

- (void)pushNote:(NSString *)note withCompletion:(void (^)(BOOL, NSError*))completion {
    // Code is the same except it forms the request body using the note parameter.
}

If the real task is to do work for several notes, you need a method that calls this one repeatedly, then tells its caller that its done.

- (void)pushNotes:(NSArray *)notes withCompletion:(void (^)(BOOL, NSError*))completion {

    // if there are no more notes, we are done
    if (!notes.count) return completion(YES, nil);

    NSString *nextNote = notes[0];
    NSArray *remainingNotes = [notes subarrayWithRange:NSMakeRange(1, notes.count-1)];

    [self pushNote:nextNote withCompletion:^(BOOL success, NSError*error) {
        // if success, do the rest, or else stop and tell the caller
        if (success) {
            [self pushNotes:remainingNotes withCompletion:completion];
        } else {
            completion(NO, error);
        }
    }]; 
}