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

You can do this with Apple-private methods, although that's not allowed in the App Store.

There's a private method, -setTrackedSplitView:, on NSToolbarItem. It takes an NSSplitView* as its parameter. You need to call it on the flexible-space toolbar item that you want to track a split view and pass it the split view it should track. To protect yourself against Apple removing the method, you should check if NSToolbarItem responds to the method before trying to use it.

Since the user can customize and re-order the toolbar, you generally need to enumerate the window's toolbar's items. For the first one whose identifier is NSToolbarFlexibleSpaceItemIdentifier, you set the split view it should track. For all other flexible-space items, you clear (set to nil) the split view to track. You need to do that when the window is first set up and again in the toolbar delegate's -toolbarWillAddItem: and -toolbarDidRemoveItem: methods. There's also another undocumented delegate method, -toolbarDidReorderItem:, where I've found it useful to update the toolbar.