I recently started another project exploring Swift a little bit. I want to implement a Collection View using a NSFetchedResultsController to get the data out of my CoreData database. I wanted to use the example from https://github.com/AshFurrow/UICollectionView-NSFetchedResultsController and implement something similar in Swift. I don't need any of the move events so I implemented the following:
First of all I created a class to save changes made:
class ChangeItem{
var index:NSIndexPath
var type:NSFetchedResultsChangeType
init(index: NSIndexPath, type: NSFetchedResultsChangeType){
self.index = index
self.type = type
}
}
within my collection view controller I use two arrays to save the changes temporarily
var sectionChanges:[ChangeItem] = []
var objectChanges:[ChangeItem] = []
then I wait for my FetchedResultsController to change something after changes on the CoreData database were made
func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?,forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {
var item:ChangeItem?
if(type == NSFetchedResultsChangeType.Insert){
item = ChangeItem(index: newIndexPath!, type: type)
}else if(type == NSFetchedResultsChangeType.Delete){
item = ChangeItem(index: indexPath!, type: type)
}else if(type == NSFetchedResultsChangeType.Update){
item = ChangeItem(index: indexPath!, type: type)
}
if(item != nil){
self.objectChanges.append(item!)
}
}
func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {
var item:ChangeItem?
if(type == NSFetchedResultsChangeType.Insert || type == NSFetchedResultsChangeType.Delete || type == NSFetchedResultsChangeType.Update){
item = ChangeItem(index: NSIndexPath(forRow: 0, inSection: sectionIndex), type: type)
}
if(item != nil){
self.sectionChanges.append(item!)
}
}
After all changes are applied the FetchedResultsController will perform the didChangeContent method
func controllerDidChangeContent(controller: NSFetchedResultsController) {
println("something happened")
self.collectionView!.performBatchUpdates({ () -> Void in
while(self.sectionChanges.count > 0 || self.objectChanges.count > 0){
self.insertSections()
self.insertItems()
self.updateItems()
let delips = self.deleteSections()
self.deleteItems(delips)
}
}, completion: { (done) -> Void in
if(done){
println("done")
self.collectionView.reloadData()
}else{
println("not done")
self.collectionView.reloadData()
}
if(self.fetchedResultsController.sections != nil && self.fetchedResultsController.sections?.count > 0){
println("number of items in first section \(self.fetchedResultsController.sections![0].count)")
}
})
self.deselectAll()
self.collectionView.reloadData()
if(self.fetchedResultsController.sections != nil && self.fetchedResultsController.sections!.count > 0){
for i in 1 ..< self.fetchedResultsController.sections!.count{
if(self.fetchedResultsController.sections!.count > 0){
println(self.fetchedResultsController.sections![i].numberOfObjects )
}
}
}
}
which then again calls the following methods
private func deleteSections()->NSIndexSet{
var deletes = self.sectionChanges.filter({ (item) -> Bool in
if(item.type == NSFetchedResultsChangeType.Delete){
let index = (self.sectionChanges as NSArray).indexOfObject(item)
self.sectionChanges.removeAtIndex(index)
return true
}
return false
}) as [ChangeItem]
var indexSet:NSMutableIndexSet = NSMutableIndexSet()
for del in deletes{
indexSet.addIndex(del.index.section)
}
if(indexSet.count > 0 && self.collectionView.numberOfSections() > 0){
self.collectionView.deleteSections(indexSet)
}
return indexSet
}
private func insertSections(){
var inserts = self.sectionChanges.filter({ (item) -> Bool in
if(item.type == NSFetchedResultsChangeType.Insert){
let index = (self.sectionChanges as NSArray).indexOfObject(item)
self.sectionChanges.removeAtIndex(index)
return true
}
return false
}) as [ChangeItem]
var indexSet:NSMutableIndexSet = NSMutableIndexSet()
for ins in inserts{
indexSet.addIndex(ins.index.section)
}
if(indexSet.count > 0){
println("Adding \(indexSet.count) section")
println(indexSet)
self.collectionView.insertSections(indexSet)
self.collectionView.reloadSections(indexSet)
}
}
private func deleteItems(deletedSections:NSIndexSet?){
var deletes = self.objectChanges.filter({ (item) -> Bool in
if(item.type == NSFetchedResultsChangeType.Delete){
let index = (self.objectChanges as NSArray).indexOfObject(item)
self.objectChanges.removeAtIndex(index)
return true
}
return false
}) as [ChangeItem]
var indexPaths:[NSIndexPath] = []
for del in deletes{
if(del.index.section < self.fetchedResultsController.sections?.count){
if(deletedSections == nil || deletedSections!.containsIndex(del.index.section)){
indexPaths.append(del.index)
}
}
}
/*indexPaths = indexPaths.sorted({ (a, b) -> Bool in
if(a.section >= b.section && a.row >= b.row){
return true
}
return false
})
println(indexPaths)*/
//self.collectionView.numberOfItemsInSection(0)
if(indexPaths.count > 0){
println("deleting \(indexPaths.count) items")
self.collectionView.deleteItemsAtIndexPaths(indexPaths)
}
}
private func updateItems(){
var updates = self.objectChanges.filter({ (item) -> Bool in
if(item.type == NSFetchedResultsChangeType.Update){
let index = (self.objectChanges as NSArray).indexOfObject(item)
self.objectChanges.removeAtIndex(index)
return true
}
return false
}) as [ChangeItem]
var indexPaths:[NSIndexPath] = []
for update in updates{
indexPaths.append(update.index)
}
if(indexPaths.count > 0 ){
println("did update on \(indexPaths.count) items")
self.collectionView.reloadItemsAtIndexPaths(indexPaths)
}
}
private func insertItems(){
var inserts = self.objectChanges.filter({ (item) -> Bool in
if(item.type == NSFetchedResultsChangeType.Insert){
let index = (self.objectChanges as NSArray).indexOfObject(item)
self.objectChanges.removeAtIndex(index)
return true
}
return false
}) as [ChangeItem]
var indexPaths:[NSIndexPath] = []
for ins in inserts{
indexPaths.append(ins.index)
}
if(indexPaths.count > 0 && self.numberOfSectionsInCollectionView(self.collectionView) > 0){
println("Adding \(indexPaths.count) items to collection view with \(self.collectionView.numberOfItemsInSection(0))")
self.collectionView.insertItemsAtIndexPaths(indexPaths)
println("Did add items")
}
}
Deleting multiple items and sections at a time works just fine (by now), but inserting a section does not work. As you can see I implemented a output within the insertSections method. The number of new sections in my test scenario is 1 and I get the right NSIndexSet with 1 item:(0). But in the insertItems method I try to call numberOfObjects on section 0 and get the following Error
*** Assertion failure in -[UICollectionViewData numberOfItemsInSection:], /SourceCache/UIKit_Sim/UIKit3347.44/UICollectionViewData.m:5942015-06-19:00:53:01.966 Grocli[42533:1547866] CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. request for number of items in section 0 when there are only 0 sections in the collection view with userInfo (null)
Thanks in advance for reading that long article and helping me out!
