Ordering multiple Asynchronous JSON parses from website

Question

Quick brief of what I'm doing: I have two arrays, they both contain 50% of the information for a table view. Why? Because one of the arrays pulls current information from the internet, while the other array has saved user data. I had no idea as to how to get current information from the internet in a non messy way, as I'm an amateur to objective-C let alone networking in Objective-C. Anyway, so the internet Array is pulling information that corresponds to the objects in the saved array (saved in Core Data) using AFNetworking. It's therefore Asynchronous, which I want. Now here comes the problem, or at least what I can garner from the situation.

I'm doing a for loop that effectively counts through the objects in the saved array and passes the unique ID for each object so that the corresponding information from the internet can be downloaded, parsed and added to the internet array. However, since the networking is Asynchronous, the loop is effectively downloading all the information from the internet at once. Therefore the objects are being written to the internet array in order of which downloaded first. Therefore savedArray[0] does not correspond to the object in internetArray[0]. This is a pretty big flaw as you can imagine, as when I'm inserting the values into my tableView, nothing really matches up/makes sense.

I'm really looking for a way to postpone the downloading of the information, until the previous download has been completed and added to the internetArray, how on earth do I do this? Now for the code. Here is where I get the appropriate key:

for ( int i = 0; i < [mySavedObjects count]; i++) {
    MySavedObject* mySavedObject = [mySavedObjects objectAtIndex:i];
    [self retrieveMissingInformation: myObject.Id];
    [self.tableView reloadData];
}

And here is where I actually get the information (simplified for the sake of space):

- (void)retrieveMissingInformation:(NSString *) Id
{
 // create URL here.
AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request
              success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
                        NSLog(@"Success");
                        // Do some safety checks & parse the JSON, returning a searchResult.
                        [searchResults addObject:searchResult];
            } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id JSON)
                        // Do some error messages etc.
}];

[queue addOperation:operation]; //where queue is an ivar NSOperationQueue.
}

Finally, in the cellForRowAtIndexpath, I these use both:

MySavedObject *mySavedObject = [mySavedObjects objectAtIndex:indexPath.row];

&

SearchResult *searchResult = [searchResults objectAtIndex:indexPath.row];

To get the values for the cell.

Sorry for the massively large wall of text. I'm not really good enough to explain things efficiently, often getting tongue-tied on the terminology and having to resort to making up my own exmaples. Any help with ordering this mess and I'd be really grateful.

Regards,
Mike.

Zhang · Accepted Answer · 2012-11-30 02:11:51Z

Good work mate, you've stumble on a common design pattern.

As you've seen, with asynchronous request, execution happens almost concurrently so you're not guaranteed order when you think of "queues" in terms of a for loop.

The way you need to think of your download queue is in terms of recursion.

In other words, instead of:

// your current method of queue-ing the submission of data
for(int i = 0; i < arrData.count; i++)
{
    [self doAsyncDataSendForIndex:i];
}

You need to start doing something like this:

// perform the first submit for the first element in your array
[self doAsyncDataSendForIndex:dataIndex];


// a completion callback method
-(void)asyncFinishDownloading
{
    [self parseResult:serverReturnedData];
}

-(void)parseResult:(id)serverData
{
    // do all the processing of the data that you need here

    ...

    // update index for next round
    dataIndex++;

    // -----------------------------------------------------------------
    // now at the end of this method, perform a recursive call 
    // of your async download method again 
    // -----------------------------------------------------------------
    [self doAsyncDataSendForIndex:dataIndex];
}

Using the callback delegate way of queueing your download you are telling the code to only download the next data after it has finished processing the last asynchronous download.

Now you can use a number of libraries to help you with your asynchronous stuff. I myself use ASIHttpRequest and ASIFormData, you can use AFNetworking too which is newer.

-(void)asyncSubmitData:(id)someData
{
    NSURL *url = [NSURL urlWithString:@"http://www.yourserver.com/...."];

    ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:url];

    // setup your POST request parameters
    [request setPostValue:someData forKey:@"someKey"];
    ...

    // setup the completion callback delegate action
    [request setCompletionBlock:^{
        // process your server returned data here
        NSString *serverResponse = [request responseString]; // raw server response

        ...

        // increment your index and call your download method again
        dataIndex++;

        [self asyncSubmitData:[myArray objectAtIndex:dataIndex]];
    }];


    // setup fail block in case the server fails
    [request setFailedBlock:^{
        NSLog(@"Server error: %@", [[request error] localizedDescription];
    }];


    // start the asynchronous request now
    [request startAsynchronous];
}

Hope that helps.

This is absolutely fantastic. Thanks for the really detailed reply. I really like this method. I may, over the next several hours, (have to add time for making stupid mistakes you see!) implement this and the other fixes so I can learn different ways of doing it. Marked as answer due to effort put it. The other answers are still superb however!

warrenm · Accepted Answer · 2012-11-30 00:37:00Z

1

I think I understand your problem and it sounds like you're using the wrong data structures to store the data.

You have one array that is generating requests and the results to those requests are stored in a separate, unrelated array.

As you generate the request for information with an objectId, can you store the results in a NSDictionary instead of an array and use the objectId as the key to this array?

You have the NSURLRequest in the success method so you should be able to retrieve your objectId from the request through the NSURL.query if it is a parameter on the query string

edited Nov 30, 2012 at 0:37

warrenm

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answered Nov 30, 2012 at 0:16

Eamonn Moloney

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Comments

occulus · Accepted Answer · 2012-11-30 11:10:29Z

1

Postponing the downloading of info and effectively making all of your network requests serial (one at at time) would work, but isn't a very nice solution, because it's often handy to have concurrent network requests going on - for example, allowing four requests at once would probably take less time than making all requests serially.

A much better solution is to correctly handle a piece of data when it comes back. To do this you need to handle the returned data in a more structured way. Rather than just appending it to an array, do one of the following:

A. Put returned data in a dictionary. The dictionary can map from an index or appropriate key to the returned data. (Hint: if you want to put an int as a key in a dictionary, construct an NSNumber containing that int.) This option is recommended over B.

B. Insert the returned data into an array. In order for ths to make sense, before you do any fetching, you should fill up your array with placeholder objects that denote "this bit of data hasn't been fetched yet". One possible placeholder object choice would be NSNull - it's just an object to denote null/nil.

So then you can return the correct item in cellForIndexPath: by dipping into your dictionary or array.

These two data structure ideas are just the most obvious ones that come to mind, there are doubtless other ways to crack this nut.

edited Nov 30, 2012 at 11:10

answered Nov 30, 2012 at 0:21

occulus

17k6 gold badges56 silver badges77 bronze badges

3 Comments

Mackey18 Over a year ago

Perfect explanation. Really appreciate it. The Dictionary method seems like such an obvious way of getting around this issue. +1

occulus Over a year ago

Thanks. Btw, the dictionary is much preferable to the array option -- much simpler. Will modify answer to clarify that.

Mackey18 Over a year ago

Thank you, it's a really neat fix, and I actually ended up using it as it comes in handy later on.

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