use NSFetchedResultsController to arrange CoreData model into sections by date

Question

I currently am using CoreData for local storage of my model. I have successfully implemented an NSfetchedResultsController to manage the retrieval and presentation of the data in a tableView.

I wish the data to be organized in sections by local day. So each section will be a unique local day. Similar to most chat apps.

I know that I can save a date property in my managedObject in addition to the timestamp and use that as the sectionNameKeyPath, however that will not work when the user changes the timezone on their device. In this case I will have to go back and change every entry in coreData and change the local day entry.

Is there a simpler way to achieve organizing by day taking into account the possibility of the time zone changing?

let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Blog")
let blogSort = NSSortDescriptor(key: "dateCreated", ascending: true)
request.sortDescriptors = [blogSort]
fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: "localDate", cacheName: nil)

In this example, dateCreated is a timestamp for the date the blog was created and localDate is the actual calendar date.

use Calendar.current to convert timestamp to a date string and save it as a string — alionthego, May 15 '18 at 04:32
I think from what I have read the answer is in using a transient property in coredata. just not sure how to implement that using swift 4 — alionthego, May 15 '18 at 04:41
Have a look at https://stackoverflow.com/a/30544650/1187415, it demonstrates how to define a transient property `sectionIdentifier` which can be use as `sectionNameKeyPath` for the fetched results controller. — Martin R, May 15 '18 at 05:29

Mannopson · Answer 1 · 2018-05-15T05:33:31.800

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This is not an answer for your question. But I think this is helpful for you.

let date = Date.init()
let dateFormatter = DateFormatter.init()
dateFormatter.dateStyle = .short
let stringDate = dateFormatter(stringFrom: date)

And then you can save this string as a localDate to the managedObject.

managedObject.localDate = stringDate

edited May 15 '18 at 05:33

answered May 15 '18 at 05:06

Mannopson

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I know how to do that. That will always group sections by local date. But if the user then goes to Europe and the timezone on the phone changes, your date string will be the local date of wherever they left from and the sections will not look right. The answer has to be in transient properties on coreData and many examples for objective C and older swift but no current examples. – alionthego May 15 '18 at 05:18
I have not tested yet! But I think it’s possible. So, save the `localDate` as a `Date.init()` to the model. And convert this value to the `Calender.current` while retrieving from the model. – Mannopson May 15 '18 at 05:46

Joakim Danielson · Answer 2 · 2018-05-15T06:11:48.067

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I would use a calculated property in the Blog class for this. This is how I would approach this

Save the creation date with the same time zone (GMT is easiest), using ISO8601DateFormatter
Add a transient attribute to the Blog entity, say localCreateDate: Date and calculate it by parsing your stored creation date into a Date instance using the local time zone
Use the new localCreateDate in the fetch controller as the sectionNameKeyPath, see this answer or this by Martin R

edited May 15 '18 at 06:11

answered May 15 '18 at 05:38

Joakim Danielson

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You cannot use a computed property in a Core Data sort descriptor. – Martin R May 15 '18 at 05:52
@MartinR, you're right. I was thinking about a transient attribute. – Joakim Danielson May 15 '18 at 05:59
You cannot use transient attributes in a Core Data sort descriptor either. However, you *can* use them as sectionNameKeyPath, that is what I tried to demonstrate in https://stackoverflow.com/a/30544650/1187415. – Martin R May 15 '18 at 06:05
Argh, I am seriously careless today. I was following [my own answer](https://stackoverflow.com/a/49466329/9223839) and still I get it so wrong. – Joakim Danielson May 15 '18 at 06:08
@MartinR, thanks for the input. – Joakim Danielson May 15 '18 at 06:14
The creation date should be stored as a Date, not as a (formatted) String. – Martin R May 15 '18 at 06:47
@MartinR I used String because that is what OP used. – Joakim Danielson May 15 '18 at 08:38

use NSFetchedResultsController to arrange CoreData model into sections by date

2 Answers2