其思想是找到NSSplitViewController（内部仍用作引擎）并订阅以观察其第一个拆分项（即侧栏）isCollapsed状态。
使用Xcode 13.3/macOS 12.2.1进行测试

这里是主要部分：

// find split view through hierarchy
  while sview != nil, !sview!.isKind(of: NSSplitView.self) { 
    sview = sview?.superview
  }
  guard let sview = sview as? NSSplitView else { return }
  
  controller = sview.delegate as? NSSplitViewController   // delegate is our controller
  if let sideBar = controller?.splitViewItems.first {     // now observe for state
    observer = sideBar.observe(\.isCollapsed, options: [.new]) { [weak self] _, change in
      if let value = change.newValue {
        self?.sideCollapsed?.wrappedValue = value    // << here !!
      }
    }
  }

和用法

SideViewInsideNavigationView()
  .background(SplitViewAccessor(sideCollapsed: $collapsed))   // << inject finder !!

完成findings and code is here

在iOS 16+中使用新的NavigationSplitView，这变得容易得多。例如：

@State private var splitViewVisibility = NavigationSplitViewVisibility.all

var body: some View {
    NavigationSplitView(columnVisibility: $splitViewVisibility) {
        SidebarView()
    } detail: {
        DetailVew()
    }
    .onChange(of: splitViewVisibility) { newValue in
        print(newValue)
    }
}