Question

I’m running into a problem of how to display sequences of core data entities on a single graph, if there are time gaps between sequences.

I got a fixed-length bar graph (backed by UITableView) with each bar represented by a core data entity, all entities are timestamped and follow each other, so sorting such data results in a graph like below. The good thing about such graph is that NSFetchedResultsController can be used to dynamically update the table view in response to new events being added. The result is that it appears to the user that new bars are added on the left as time goes by.

My problem is that I want to expand this graph and make it 24 hour long and scrollable side to side. In this case, there will be episodes where no data will be present in the app. The end result is that if I rely on NSFetchedResultsController when there are gaps in data, and I want to display blank cells for missing data.

I’m interested in how you would approach the problem.

I see two potential solutions:

1) Drop the approach of using NSFetchedResultsController, and instead manually calculate the number of bars in a UITableView. Each bar is defined by a start date and an end date. Then I can use NSFetchRequest with predicate to query each bar’s data. If there’s an event with a timestamp that falls within the bar’s date range, then a bar is shown, otherwise a blank is shown.

The problem with approach #1 is that it is slow, and while it does allow scrolling through the entire data set, it is slow and laggy, as each fetch request is executed for each new bar. Additionally, I need to manually refresh the table in response to new data being added, which is difficult to implement properly.

2) Maybe I can pre-populate the data set with blank events, and then “fill them in” as data becomes available. This would allow me to keep the table view with fetched results controller, resulting in smoother scrolling, but at the price of significantly increasing the persistent storage used by the app (as blank entities will be created between app use).

I’m wandering if there’s an additional approach that I’m not thinking of. How would you create a side scrollable bar graph that would allow for “gaps” in the app’s persistent data storage?

Thank you for your input!

Answer 1

Your question really doesn’t have (or shouldn’t have) anything to do with Core Data or NSFetchedResultsController. How your graph view draws the data it’s given is entirely up to the graph view. There’s no reason that you couldn’t write a graph view that uses NSFetchedResultsController and deals appropriately with gaps in the data, but UITableView was never intended to deal with missing/empty cells.

The problem is not that you’re using either Core Data or NSFetchedResultsController. The problem is that you’re abusing UITableView.

Drawing a bar graph is pretty easy — it’s just a series of rectangles drawn at appropriate locations. You can give your view a fetched results controller and have it look at the fetchedObjects property to get the current list of objects. It can then render as many of those objects as it likes, and it can use the start and end date of each object to locate the drawing on the graph. Put your graph in a UIScrollView so that you don’t have to worry about scrolling. If the graph can get large, add tiling so that you’re only drawing the parts that the user is looking at, just like UITableView does.

It looks like you’re currently drawing some content above your table view, too. If you create your own graph view, you might consider drawing that additional content (the red line, the red and green bubbles, the high REM probability thing) at the same time. Doing that will make it easy to ensure that all that content is located correctly on the graph (which is probably a bit of a pain with your current scheme).