Core Data and NSAttributedString

I’m currently creating an app that uses some weird formatting and inline images that can be handled nicely by NSAttributedString.

The app is bundled with a pre-populated database and I was considering populating the database with the already formatted strings and images.

Core Data supports NSAttributedStrings—just set the attribute to be Transformable and you can create and store attributed strings along with most attributes.

The interesting thing is that it can also store images within the strings if you add it using NSTextAttachment:

There’s a few issues with this:

  1. Your database will get big fast. Unlike the Binary attribute, there’s no way to tell Core Data to use external storage.
  2. You’re passing in the scale of image of the device that created the image. For example, if you’re running the app on an iPhone 6, then UIImage(named:) will return an @2x image and this and only this scale is what will be stored in the database

This will be fine if it’s going to be accessed on the same device that it’s created on, but if you use any sort of syncing or you want to deliver a pre-populated database in your app bundle, then you’re going to be passing around an image that might be the wrong scale for the device.

Also, storing attributed strings this way breaks any connection with dynamic text styles, so there’s no resizing of the string after it comes out of the database if the user changes the text size in Accessibility. This makes sense as the font is hardcoded into the attributed string so there’s no way of the system knowing that it should resize it, but it does make storing attributed strings in the database less useful if you want to provide dynamic text resizing (which you should).

Because of these limitations, I think I’m going to stick with formatting the strings and adding the image on the fly for the moment, but it’s good to know what’s possible with Core Data.

iTC Communicator

I have a new script up on GitHub for updating app metadata and screenshots using the iTMSTransporter app, which I first learned about watching the iTunes Connect: Development to Distribution WWDC 2015 video.

It uses the PHP-CLI because the built in support for XML and JSON made it easier to work with the metadata file.

It’s good to be able to have this fire and forget solution, especially as I often find myself in countries with less than stellar Internet. Rather than having to sit there and drag in screenshots and wait for refreshes and reorder them manually for 5 different devices through the web interface, I can just run this instead.

Model Decisions

I’m about to get started on a new iOS app and the first major decision is what form the model is going to take.

The app will be a quiz style app where my client will be providing the questions in the form of a large JSON file.

Initially I was going to create individual model objects, have a questions manager object read in the JSON file on launch and create the various question objects, along with their parent containers. The question tree is relatively complex, with each question sitting under three ancestor nodes.

In terms of persistence, the only things that I need to keep track of is how many times a user has attempted a question and whether or not they were correct on the last attempt.

Initial Idea

My initial idea was to have a questionManager, together with various model objects. The Question objects would vend an array of Answer objects. The controller would then pass back an array of Answer objects to the Question object which would update its state and return a tuple consisting of a Bool and an NSAttributedString object indicating whether or not the answer was correct, and giving more information about the correct answer.

This would work fine and persistence would be relatively straightfoward. After the initial read of the JSON file (which will run to around 2,000 questions), I would just have all the model objects conform to NSCoding and freeze and unfreeze the questionManager between launches.

Core Data?

Trail Wallet uses Core Data to manage its data and I am very happy with it. Xcode features a decent model editor and between NSPredicate, NSFetchRequest, and NSFetchedResultsController I get a lot of power and convenience for free, not to mention that persistence is straightforward.

I haven’t yet branched into iCloud for Trail Wallet, but it is something that I am eager to implement and (if it works well—which is a big "if", given iCloud Core Data’s history) then I’ll get a lot of syncing power for very little effort (as opposed to a third party solution, or even just having to manage mapping the model to CloudKit).

Implementing Core Data in this new app which, admittedly, is overkill for something so straightforward would allow me to try out iCloud Core Data with very little risk.

All user data loss is bad, but losing the history of which questions were answered seems a lot less catastrophic than years worth of financial records. I would also get some experience in managing migration paths when iCloud is in play before I implemented it in our most important app.