Yes, there was an option I missed. NSTreeController seems to handle this really well, with a few small quirks. I had to add a 'rules' property to my rule. Simple extension: extension Rule { @objc public var rules: NSSet? { return self.passToLayer?.rules } } Setting up the binding was a little weird, as I've never done that in code before. Wound up with roughly this: @IBOutlet var ruleViewContainer:NSView! var ruleOutlineView:NSOutlineView = NSOutlineView() private var layerToShow:Layer private var treeController:NSTreeController = NSTreeController() ... override func viewWillAppear() { ... let outlineContainer = NSScrollView(frame:ruleViewContainer.bounds) outlineContainer.autoresizingMask = [.width, .height] outlineContainer.documentView = ruleOutlineView outlineContainer.hasVerticalScroller = true outlineContainer.hasHorizontalScroller = true ruleViewContainer.addSubview(outlineContainer) ruleOutlineView.usesAlternatingRowBackgroundColors = true ruleOutlineView.usesAutomaticRowHeights = true ruleOutlineView.delegate = self ruleOutlineView.dataSource = self ruleOutlineView.doubleAction = #selector(self.doubleClick) ruleOutlineView.target = self treeController.childrenKeyPath = "rules" treeController.bind(NSBindingName.content, to: layerToShow, withKeyPath: "rules", options: nil) ruleOutlineView.bind(NSBindingName.content, to: treeController, withKeyPath: "arrangedObjects", options: nil) } Now all the items in the NSOutlineView are of type NSTreeNode. This broke my existing drag-and-drop and outlineView(_ outlineView:, viewFor tableColumn:, item:). Easy enough to fix. Just wrap all the references to item like this: let representedItem = (item as! NSTreeNode).representedObject Then use representedItem just like you used item before. I handle the updates in my API interaction object, which then updates my Core Data store if the server confirms the attempted change was successful. The UI then updates. I still have a few weird glitches. Moved objects are sometimes out of order. That should be much easier to figure out, though.

Figured it out. I used a modified version of this NSImage+TintColor extension to create a swatch dictionary: let namedColors:[String] = [ "aquamarine", "black", "blue", ... } let myColorSwatches:[String:NSImage] = { (colorNames:[String]) -> [String:NSImage] in var toReturn:[String:NSImage] = [:] for colorName in colorNames { let image = NSImage(systemSymbolName: "rectangle.fill", accessibilityDescription: nil)! image.isTemplate = false image.lockFocus() NSColor(named: colorName)!.set() let imageRect = NSRect(origin: .zero, size: image.size) imageRect.fill(using: .sourceIn) image.unlockFocus() toReturn[colorName] = image } return toReturn }(namedColors) ... which I can then reference in my SwiftUI Picker: Picker("Color:", selection: pickerColor) { ForEach(namedColors, id: \.self) { colorName in HStack { Image(nsImage: myColorSwatches[colorName]!) Text(colorName) } } } And just as a reminder in case anybody else wants to do this, I have a color set defined in my Assets.xcassets for each named color.

Figured it out. My supercategories were defined as a struct with two properties: name and frc. While reading over some forum hits for the umpteenth time, this one - https://developer.apple.com/forums/thread/68477 finally clicked. Switched them over to use a class instead, and now everything is happy. From: Swift struct sidebarCategoryItem { let name:String let frc:NSFetchedResultsControllerCPMObject_CD } to this instead: Swift class sidebarCategoryItem { let name:String let frc:NSFetchedResultsControllerCPMObject_CD init(name initName:String, frc initFRC:NSFetchedResultsControllerCPMObject_CD) { self.name = initName self.frc = initFRC } } +5 lines of code, and -a million lines added for troubleshooting.

Okay, this is definitely something weird about the parent object. I tried this: Swift let parent = myOutlineView.child(topLevelSectionIndex!, ofItem: nil)! print("parent: \(String(describing: parent))") print("parent row: \(myOutlineView.row(forItem: parent))") let row2Parent = myOutlineView.item(atRow: 2)! print("row2Parent: \(String(describing:row2Parent))") print("row2Parent row: \(myOutlineView.row(forItem: row2Parent))") and I get this in my log: text parent: supercategory parent row: -1 row2Parent: supercategory row2Parent row: 2 The String(describing:) of both objects is identical, though the objects are clearly different internally. And if I tell insertItems to insert something into row2Parent, it works! The problem is I only know the row indexes when my application has no data downloaded. As I fetch stuff and as the user expands and collapses sections, the rows for the supercategories will change. I clearly can't get it just by the row number, and it isn't telling me the row number for the object I got from its child(_:ofItem:) method. I've tried getting the object directly from the NSOutlineViewDataSource method outlineView(_:child:ofItem:). No change. Debug descriptions of the objects are identical, but it still shows row -1. Tried getting the object directly from the array I use for the supercategories. No change. Does anybody know what sources insertItems(_:at:inParent:) accepts for the object in the inParent?

Realized a bit ago I forgot to include the code which sets topLevelSectionIndex. It was in a block of debug prints which I trimmed from the code above: Swift let topLevelSectionIndex:Int? = { for (index, section) in objectSections.enumerated() { if controller == section.frc { return index } } return nil }() let myOutlineView:NSOutlineView = myScrollView.documentView! as! NSOutlineView switch type { ... I tried adding this code to my case .insert: just above the insertItems: Swift print("Parent is expandable? \(myOutlineView.isExpandable(parent))") print("Parent is expanded? \(myOutlineView.isItemExpanded(parent))") myOutlineView.expandItem(parent) print("Parent is expanded? \(myOutlineView.isItemExpanded(parent))") and it gave me this in my logs: text Parent is expandable? true Parent is expanded? false Parent is expanded? false I get the same results whether the supercategory is expanded or collapsed before I download my data and my code is called. This makes me think the parent object I'm getting isn't the parent object the NSOutlineView wants me to use.