Dynamic NSOutlineView row heights

CalculatingCellSize.swift

class MyOutlineDelegate: NSObject, NSOutlineViewDelegate {
	//The cell we'll use for calculating sizes, should be the same cell used in the outline
	lazy var sizingCell: MyCell = {
		let cell = MyCell(frame: .zero)
		cell.translatesAutoresizingMaskIntoConstraints = false
		return cell
	}()
	
	lazy var sizingCellWidthAnchor: NSLayoutConstraint = {
		return self.sizingCell.widthAnchor.constraint(equalToConstant: 1000)
	}()

	func outlineView(_ outlineView: NSOutlineView, heightOfRowByItem item: Any) -> CGFloat {
		//Set the cell's object value to the item (or whatever property you use to set the data)
		self.sizingCell.objectValue = item
		
		//Just to make sure the width anchor is enabled, you could do this in a viewDidLoad() method rather than here
		self.sizingCellWidthAnchor.isActive = true 
		
		//You will need to calculate the width you want for the item based on indentation (for a table view this would be constant but for an outline I think you need to calculate per item)
		let width = …
		
		//We set the width (unfortunately AppKit doesn't have an equivalent to UIView.systemLayoutSizeFitting(_: CGSize)
		self.sizingCellWidthAnchor.constant = width

		//Force a layout (can't remember if this is needed or if fittingSize calls it for us, but I put it in for completeness)
		self.sizingCell.layoutIfNeeded()
		
		//Get the fitting size height for the cell height
		return self.sizingCell.fittingSize.height
	}
}