Align NSToolbarItems with NSSplitView columns

Question

Finder and Notes have a peculiar behaviour that I am seeking to reproduce. The ‘flexible space’ in the NSToolbar seems to take the dimensions of the split view into account.

Answer 1

When using macOS 11 or newer, you can insert NSTrackingSeparatorToolbarItem items to the toolbar, which will split up your toolbar in sections, aligned with the dividers of a NSSplitView object.

This example adds the new separator items to a toolbar that already contains the rest of the buttons, configured in Interface Builder or in code. The target splitview concerns a standard configuration of 3 splitviews, including a sidebar panel.

class WindowController: NSWindowController, NSToolbarDelegate {

    let mainPanelSeparatorIdentifier = NSToolbarItem.Identifier(rawValue: "MainPanel")

    override func windowDidLoad() {
        super.windowDidLoad()

        self.window?.toolbar?.delegate = self
    
        // Calling the inserts async gives more time to bind with the split viewer, and prevents crashes
        DispatchQueue.main.async {

            // The .sidebarTrackingSeparator is a built-in tracking separator which always aligns with the sidebar splitview
            self.window?.toolbar?.insertItem(withItemIdentifier: .sidebarTrackingSeparator, at: 0)
        
            // Example of a custom mainPanelSeparatorIdentifier
            // Index at '3' means that there are 3 toolbar items at the left side
            // of this separator, including the first tracking separator
            self.window?.toolbar?.insertItem(withItemIdentifier: mainPanelSeparatorIdentifier at: 3)
        }
    }

    func toolbar(_ toolbar: NSToolbar, itemForItemIdentifier itemIdentifier: NSToolbarItem.Identifier, willBeInsertedIntoToolbar flag: Bool) -> NSToolbarItem? {
    
        if let splitView = (self.contentViewController as? NSSplitViewController)?.splitView {
        
            // You must implement this for custom separator identifiers, to connect the separator with a split view divider
            if itemIdentifier == mainPanelSeparatorIdentifier {
                return NSTrackingSeparatorToolbarItem(identifier: itemIdentifier, splitView: splitView, dividerIndex: 1)
            }
        }
        return nil
    }
}