Enable/disable a toolbar item in a Catalyst app.

ToolbarController.swift

final class ToolbarController, NSObject, NSToolbarDelegate {
    var isBackButtonEnabled = false {
        didSet {
            if isBackButtonEnabled != oldValue {
                reloadBackItem()
            }
        }
    }
    
    func toolbar(_ toolbar: NSToolbar, itemForItemIdentifier itemIdentifier: NSToolbarItem.Identifier, willBeInsertedIntoToolbar flag: Bool) -> NSToolbarItem? {
        switch itemIdentifier {
        case .back:
            return makeBackToolbarItem()
        default:
            return nil
        }
    }
}

private extension ToolbarController {
    private func makeBackToolbarItem() -> NSToolbarItem {
        let imageConfiguration = UIImage.SymbolConfiguration(scale: .large)
        let image = UIImage(systemName: "chevron.backward", withConfiguration: imageConfiguration)
        let barButtonItem = UIBarButtonItem(image: image, style: .plain, target: nil, action: nil)
        let item = NSToolbarItem(itemIdentifier: .back, barButtonItem: barButtonItem)
        item.isNavigational = true
        item.toolTip = L10n.Toolbar.Tooltip.back
        if isBackButtonEnabled {
            item.target = self
            item.action = #selector(back)
        } else {
            item.target = nil
            item.action = nil
        }
        return item
    }
  
    private func reloadBackItem() {
        // Remove and re-insert the back item to update it's enabled/disabled state.
        // Just setting the target/action won't immediately reload the item.
        // It will only happen when the user performs an action like scrolling.
        if let idx = toolbar.items.firstIndex(where: { $0.itemIdentifier == .back }) {
            toolbar.removeItem(at: idx)
            toolbar.insertItem(withItemIdentifier: .back, at: 0)
        }
    }
}