#restkit

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Recently, I worked on a project where the app had to download a lot of JSON data and store the results in Core Data.

I decided this was a good time to try out some frameworks, so I installed the pods for RestKit and MagicalRecord.  

With RestKit, you make a mapping, like:

@{@"job_name":@"jobName"}

Then you pass that on to the framework, along with the URL and other info, and it automatically downloads, parses, and saves that data into Core Data.

MagicalRecord also has convenience methods for importing JSON into Core Data.  Just follow the instructions in their documentation.  You go into your data model, and populate the User Info (right hand side).  Let's say you have a field called "displayName" and the JSON object has a "display_name" field. For the "displayName" field, you would add a key called "mappedKeyName" and the value would be "display_name".  

Once you've finished adding keys to the entity, to import something, you'd call:

Person *importedPerson = [Person MR_importFromObject:contactInfo];

So, just make a few edits and the framework does the rest.  Great, right?  Well, it depends.  If the JSON object is fairly simple, and you're importing 10 at a time, sure.  If the JSON object is gnarly, with multiple subarrays and dictionaries, AND you have to import 50 at a time AND establish relationships among them, not so much.  I tried both frameworks, but  I felt like I had to do too much work to shoehorn the frameworks into my code.

I ended using AFNetworking to download the JSON, then on completion, I manually looped through all the JSON dictionaries and loaded them into Core Data.  For this, I used MagicalRecord's saveWithBlock method.  While I didn't use its convenience method for importing JSON, MagicalRecord still has a lot of great utilities.

It sets up a decent NSManagedObjectContext stack for you.  The stack follows best practices, and uses a persistent store->private queue context (root)->main queue context (default).  When you save, it creates a throwaway private context, and makes it a child of the root (private) context.  Overall, MagicalRecord eliminates a lot of Core Data boilerplate.

Over the Xmas break I've had some time to work on a gem that I hope can help RubyMotion (iOS) devs more easily integrate their front-end with a RESTful Rails back-end. 

The result is Restikle. On the surface it's simple (or at least I hope it is) in that it allows you to create a CDQ schema file by parsing a Rails schema.rb along with the output of rake routes. From there, you can automatically configure RestKit at runtime to creating request and response mappings for all of the specified resources. It's designed to save a whole lot of time and boilerplate code.

I'm in the process of moving a reasonably large iOS code base (across three apps) from a set of hand-crafted POROs that are converted to and from the REST back-end by bespoke code, over to use CoreData and CDQ (in RubyMotion). For the uninitiated, RestKit has some rather complex bits and pieces of setup, that to be perfectly honest, I'm struggling to get right.

In the process of understanding what's going on, I've collected a bunch of references to blog posts and tutorials that I am ploughing through. On the off chance that some of this research might be useful to someone else, I've attached a set of links here:

Introduction to RestKit 0.20

RestKit Tutorial Code For Version 0.20

RestKit Wiki

RestKit Object Mapping

Getting Acquainted with RestKit

Developing RESTful iOS Apps with RestKit

iOS SDK: Advanced RestKit Development

I'll update this list as I find more. In the meantime, I've also posted my current problem over on the RubyMotion Google Group here.

UPDATE1: Aha! Fixed it! The problem was using addAttributeMappingsFromArray for all fields in the class, instead of making sure to use addAttributeMappingsFromArray for simple types, and addPropertyMapping for complex types. Ok, next battle ...

Just a quick note, I was getting an undefined constant error for RKLogConfigureByName, the standard way of setting up logging in RestKit. It seems that the #define statements in the _/vendor/Pods/RestKit/Code/Support/RKLog.h_ file are not getting picked up. I am using the pod and loading with RubyMotion pod support, so Iåm not sure what the issue is. I will need to investigate further but this quick workaround works, just call the lower-level method mapped via the #define. For example instead of RKLogConfigureByName use RKlcl_configure_by_name. I also list all the RKlcl_v constants to use instead of the RKLogLevel constants. I also included an example of how they are used, in this case I have put the setup in a method, and it gets called from the the standard didFinishLaunchingWithOption application block.

- (BOOL)isRestKitTimeoutError:(NSError *)error { return [error.domain isEqualToString:RKErrorDomain] && error.code == RKRequestConnectionTimeoutError; }

With only 3 days left before interview day, every minute counts.  That's why it was particularly painful to find myself stuck on the same problem for the first 4 hours of the day.  I had thought it would only take half an hour to set it up so that when users submit new questions they'd first be saved to core data and then the objects would be posted server side.  Unfortunately, this is simply the way things go sometimes. You think something will be simple and it just isn't. Or, it is, in the end, but figuring out the simple way isn't simple.

What took me so long was correctly mapping my objects with restkit when posting them to the rails backend. The difficulty is that the questions and answers are related to one another via the question's object id which is assigned to it on the backend. Thus, when the objects are initially created on the frontend, that question's object id doesn't exist yet. To fix that, restkit provides some nifty methods to post the object to the backend, then update the same object with the response that is given from the post request. Of course, it does not provide equally nice documentation for discovering this. In fact, restkit's documentation is a bit of a train wreck.  It's also only on version 0.1, which suggests the creator considers it to be in a very early stage of development and as such, developers should be a bit wary using it.  

After I finally got all of that taken care of, I spent a little time sprucing up the overall look and user interface of the app.  I think it looks ok right now which is about as good as I can hope for by Friday.  In addition to the appearance, I added a few small features, such as preventing the user from saving a form with blank fields and now the user is displayed an alert view whenever they submit a question.