Smooth animation in UICollectionView when changing data source - ios

Question

I have a UISegmentControl that I use to switch the datasource for a UICollectionView. The datasources are different types of objects.
For example the objects might look like this
struct Student {
let name: String
let year: String
...
}
struct Teacher {
let name: String
let department: String
...
}
And in the view that contains the CollectionView, there would be code like this:
var students = [Student]()
var teachers = [Teachers]()
... // populate these with data via an API
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
if(segmentControl.titleForSegment(at: segmentControl.selectedSegmentIndex) == "Students") {
return students?.count ?? 0
} else {
return teachers?.count ?? 0
}
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "personCell", for: indexPath) as! PersonCell
if(segmentControl.titleForSegment(at: segmentControl.selectedSegmentIndex)! == "Students") {
cell.title = students[indexPath.row].name
cell.subtitle = students[indexPath.row].year
} else {
cell.title = teachers[indexPath.row].name
cell.subtitle = teachers[indexPath.row].subject
}
return cell
}
#IBAction func segmentChanged(_ sender: AnyObject) {
collectionView.reloadData()
}
This correctly switches between the two datasources, however it does not animate the change. I tried this:
self.collectionView.performBatchUpdates({
let indexSet = IndexSet(integersIn: 0...0)
self.collectionView.reloadSections(indexSet)
}, completion: nil)
But this just crashes (I think this is because performBatchUpdates gets confused about what to remove and what to add).
Is there any easy way to make this work, without having a separate array storing the current items in the collectionView, or is that the only way to make this work smoothly?
Many thanks in advance!

Answer 1

If your Cell's UI just look the same from different datasource, you can abstract a ViewModel upon your datasource, like this:
struct CellViewModel {
let title: String
let subTitle: String
...
}
Then every time you got data from an API, generate ViewModel dynamically
var students = [Student]()
var teachers = [Teachers]()
... // populate these with data via an API
var viewModel = [CellViewModel]()
... // populate it from data above by checking currently selected segmentBarItem
if(segmentControl.titleForSegment(at: segmentControl.selectedSegmentIndex)! == "Students") {
viewModel = generateViewModelFrom(students)
} else {
viewModel = generateViewModelFrom(teachers)
}
So you always keep one datasource array with your UICollectionView.
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return viewModel?.count ?? 0
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "personCell", for: indexPath) as! PersonCell
cell.title = viewModel[indexPath.row].title
cell.subtitle = viewModel[indexPath.row].subTitle
return cell
}
#IBAction func segmentChanged(_ sender: AnyObject) {
collectionView.reloadData()
}
Then try your performBatchUpdates:
self.collectionView.performBatchUpdates({
let indexSet = IndexSet(integersIn: 0...0)
self.collectionView.reloadSections(indexSet)
}, completion: nil)