Can a table view row stay put while its section is moving? - ios

Question

I am trying to create a general diffing function for doing batch updates in table views. Basically, it takes two arrays of sections, each containing an identifier for each section and an identifier for each row in each section, and calculates which sections to delete, insert or move and which individual rows to delete, insert or move. I.e., suitable input for deleteSections(_:with:), insertSections(_:with:), moveSection(_:toSection:), deleteRows(at:with:), insertRows(at:with:) and moveRow(at:to:).
I thought this could be done quite generally, but it seems I've found limits to what can be done, and I wanted to just check if I'm missing something.
So let's say I have two sections, "Fruits" and "Vegetables", containing one element each: "Banana" in Fruits, "Carrot" in Vegetables.
Let's say I want to switch so that suddenly a banana is a vegetable and a carrot is a fruit. Easy enough, I generate a moveRow(at: [0, 0], to: [1, 0] and a moveRow(at: [1, 0], to: [0, 0]) and update my data source accordingly; the rows will switch place.
Now, let's say instead I want the two sections to switch places. I will do a moveSection(0, toSection: 1) (or I can do moveSection(1, toSection: 0, or both - it doesn't matter in this case). Ok. The carrot row now moves along with the vegetables sections and the banana with the fruits.
But now... I'd like to do both of these things at the same time. The sections should switch places, but the items should stay put - or, to put it differently, they should switch which logical section they belong to but keep their physical row. This does not seem possible.
I've tried to do the moveSection and at the same time (== within the same beginUpdate/endUpdate) do moveRow(at: [0, 0], to: [0, 0]) and moveRow(at: [1, 0], to: [1, 0]), but these calls to moveRow are just the no-ops they appear to be.*
I've also tried to do deleteRows/insertRows to make the rows stay where they are; that instead gives me a crash:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Attempt to create two animations for cell'
reloadRows doesn't work either; afaik that does the same as an deleteRows/insertRows pair.
So, basically my question is:
Am I missing something? Is this possible?
(Not interested in workarounds like reconfiguring the cells or headers with the new data, I don't actually have a situation where I need to do this, I just want to know!)
Demo code if anyone wants to play around.
(* Actually, they are just no-ops when combined with both moveSection(0, toSection:1) and moveSection(1, toSection:0); if only one of these are performed, we get that internal consistancy crash!)

Answer 1

Change your switchBothNoOp() into this:
func switchBothNoOp() {
switch state {
case .first:
sections = [("Fruits", ["Banana"]),
("Vegetables", ["Carrot"])]
case .second:
sections = [("Vegetables", ["Banana"]),
("Fruits", ["Carrot"])]
}
tableView.moveSection(0, toSection: 1)
}
Then you move rows later using another begin/end updates block. For example:
func update() {
tableView.beginUpdates()
// Pick on of these:
// switchItems()
// switchSections()
switchBothNoOp()
// switchBothCrash()
tableView.endUpdates()
tableView.beginUpdates()
tableView.moveRow(at: [0, 0], to: [1, 0])
tableView.moveRow(at: [1, 0], to: [0, 0])
tableView.endUpdates()
}