Using 'didMoveCellFromIndexPathToIndexPathBlock' with Core Data - ios

Question

I have a custom UITableView that takes care of animations for rearranging cells and works perfectly with standard test array which doesn't use core data.
When trying with an application that uses core data with two entities 'Folder' and 'Item' with a To Many relationships I receive an error.
[(Item)] does not have a member called exchangeObjectAtIndex
for;
tableView.didMoveCellFromIndexPathToIndexPathBlock = {(fromIndexPath: NSIndexPath, toIndexPath: NSIndexPath) -> Void in
let delegate:AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
let context: NSManagedObjectContext = delegate.managedObjectContext!
self.itemsArray.exchangeObjectAtIndex(toIndexPath.row, withObjectAtIndex: fromIndexPath.row)
var error: NSError? = nil
if !context.save(&error) { abort() }
}
This is because:
The NSSet, NSMutableSet, and NSCountedSet classes declare the
programmatic interface to an unordered collection of objects.
So I tried converting the NSSet to an NSMutableArray to manage the objects order.
func itemsMutableArray() -> NSMutableArray {
return NSMutableArray(array: (item.allObjects as! [Item]).sorted{ $0.date.compare($1.date) == NSComparisonResult.OrderedAscending } )
}
But then I get the following error in my tableview; this was because the mutable array is an AnyObject so swift doesn't believe it has a title property.
cell.textLabel?.text = folderMutableArray[indexPath.row].title
So then I go back to where I started. I am just trying to create a simple list and rearrange the order of objects.
Here is my Folder Class:
class Folder: NSManagedObject {
#NSManaged var date: NSDate
#NSManaged var title: String
#NSManaged var item: NSSet
/// Method 1 - Converting to a NSMutableArray
// func itemsMutableArray() -> NSMutableArray {
// return NSMutableArray(array: (item.allObjects as! [Item]).sorted{ $0.date.compare($1.date) == NSComparisonResult.OrderedAscending } )
// }
// Method 2 - creating an array of type Item
func itemArray() -> [Item] {
let sortByDate = NSSortDescriptor(key: "Date", ascending: true)
return item.sortedArrayUsingDescriptors([sortByDate]) as! [Item]
}
}
Productivity apps do this all the time so I know it's possible but unsure how, Does anyone know where I am going wrong or have any ideas or suggestions ?

Answer 1

This is very easy to achieve. You need to add a property called index to your Item. Set it to an integer type and whenever you add a new Item set this value to the index under which you want an item to appear. This property should be added both in Core Data Model and in your Item's class as NSManaged property.
Next you need to add a transient property to your Folder class called arrayOfItems (you can rename this of course to whatever you want). In Core Data Model set it to Transient and Transformable.
In your Folder class do the following:
class Folder: NSManagedObject {
#NSManaged var date: NSDate
#NSManaged var title: String
#NSManaged var item: NSSet
#NSManaged var arrayOfItems: [Items]
override func awakeFromFetch() {
super.awakeFromFetch()
self.regenerateItems()
}
func regenerateItems() {
let desc = NSSortDescriptor(key: "index", ascending: true)
if let array = item.sortedArrayUsingDescriptors([desc]) as? [Item] {
self.arrayOfItems = array
}
}
}
Now whenever you fetch any instance of Folder you will get a correct sorted and mutable array of Items. There are only two other cases you need to consider.
And they result from the fact that awakeFromFetch is only getting called when you fetch your data from Core Data. So, you have to consider other scenarios.
Adding new Item
When you add new Item you need to either manually append the new Item to arrayOfItems or you need to call regenerateItems() once you are finished adding the new Item. For example, lets assume that somewhere in your code you create your initial data:
var folder = NSEntityDescription.insertNewObjectForEntityForName("Folder", inManagedObjectContext: self.managedObjectContext!) as! Folder
var firstItem = NSEntityDescription.insertNewObjectForEntityForName("Item", inManagedObjectContext: self.managedObjectContext!) as! Item
// assuming that you have an inverse relationship from Item to Folder
// line below will assign it and will add your firstItem to Folder's
// NSSet of items
firstItem.folder = folder
// at this point your firstItem is added to your Folder but
// arrayOfItems is empty. So, you should call
folder.arrayOfItems = [firstItem]
// or you can call folder.regenerateItems()
Code above refers to the situation when you create your initial data. If somewhere in your code you add a new Item to the folder which already has some Items you have the following:
var newItem = NSEntityDescription.insertNewObjectForEntityForName("Item", inManagedObjectContext: self.managedObjectContext!) as! Item
// assuming that you have an inverse relationship from Item to Folder
// line below will assign it and will add your newItem to Folder's
// NSSet of items
newItem.folder = someExistingFolder
// at this point your newItem is added to your Folder but
// arrayOfItems is not yep updated and does not include newItem
// so you should either call
folder.arrayOfItems.append(newItem)
// or you can call folder.regenerateItems()
Rearranging Items
Also, when you move Items in your table view you will need to change their index and order in the arrayOfItems. The easiest way to achieve this would probably be to change the order of items in the arrayOfItems and then to iterate through this array assigning correct new indexes to all items within it:
func tableView(tableView: UITableView, moveRowAtIndexPath sourceIndexPath: NSIndexPath, toIndexPath destinationIndexPath: NSIndexPath) {
let itemThatMoved = currentFolder.arrayOfItems[sourceIndexPath.row]
currentFolder.arrayOfItems.removeAtIndex(sourceIndexPath.row)
currentFolder.arrayOfItems.insert(itemThatMoved, atIndex:destinationIndexPath.row )
var currentIndex = 0
for item in currentFolder.arrayOfItems {
item.index=currentIndex
currentIndex++
}
}
Let me know if this helps or if you have any other questions.
P.S. I strongly encourage you to change the name of your relationship from item to items. Plural name items is much more logical for NSSet.