如何判断NavigationSplitView的可见列数及相关视图状态？

1. 判断当前可见列数 #

使用NavigationSplitView的columnVisibility绑定属性，它的类型是NavigationSplitViewVisibility枚举，包含三种核心状态：

• .singleColumn：仅显示1列（如iPhone设备、iPad分屏窄模式）
• .doubleColumn：显示2列（如iPad分屏中宽模式、macOS部分布局）
• .all：显示全部3列（如iPad全屏、macOS全屏）

修改示例代码添加列数判断逻辑：

struct ContentView: View {
    @State private var departments = ["D1"]
    @State private var employees = ["E1", "E2", "E3"]
    @State private var selectedDepartment: String?
    @State private var selectedEmployee: String?
    // 新增列可见性绑定
    @State private var columnVisibility = NavigationSplitViewVisibility.automatic

    var body: some View {
        NavigationSplitView(columnVisibility: $columnVisibility) {
            List(departments, id: \.self, selection: $selectedDepartment) { department in
                Text(department)
            }
        } content: {
            List(employees, id: \.self, selection: $selectedEmployee) { employee in
                Text(employee)
            }
            // 监听列可见性变化，判断当前列数
            .onChange(of: columnVisibility) { newState in
                switch newState {
                case .singleColumn:
                    print("当前可见1列")
                case .doubleColumn:
                    print("当前可见2列")
                case .all:
                    print("当前可见3列")
                @unknown default:
                    break
                }
            }
        } detail: {
            if let selectedEmployee {
                Text(selectedEmployee)
            } else {
                Text("No employee selected")
            }
        }
    }
}

2. 判断视图是否被推入导航栈 #

利用iOS 16+提供的@Environment(\.navigationStackDepth)环境变量，它返回当前导航栈的层级数：

• 层级数为0：当前视图是导航栈的根视图
• 层级数大于0：有视图被推入到当前栈中

示例用法：

struct ContentList: View {
    @Environment(\.navigationStackDepth) private var stackDepth
    let employees: [String]
    @Binding var selectedEmployee: String?

    var body: some View {
        List(employees, id: \.self, selection: $selectedEmployee) { employee in
            NavigationLink(value: employee) {
                Text(employee)
            }
        }
        .onAppear {
            if stackDepth > 0 {
                print("当前视图被推入导航栈")
            } else {
                print("当前视图是导航栈根视图")
            }
        }
    }
}

3. 判断视图是否为iPad侧滑覆盖视图 #

通过组合设备类型与水平尺寸类判断：当设备是iPad，且水平尺寸类为.compact时，通常处于侧滑覆盖（Slide Over）模式。

示例代码：

struct ContentView: View {
    @Environment(\.horizontalSizeClass) private var horizontalSizeClass
    // ... 其他状态变量

    var body: some View {
        NavigationSplitView {
            // sidebar内容
            List(departments, id: \.self, selection: $selectedDepartment) { department in
                Text(department)
            }
        } content: {
            // content内容
            List(employees, id: \.self, selection: $selectedEmployee) { employee in
                Text(employee)
            }
        } detail: {
            // detail内容
            if let selectedEmployee {
                Text(selectedEmployee)
            } else {
                Text("No employee selected")
            }
        }
        .onAppear {
            let isSlideOver = UIDevice.current.userInterfaceIdiom == .pad && horizontalSizeClass == .compact
            if isSlideOver {
                print("当前处于iPad侧滑覆盖视图")
            }
        }
    }
}

注：若需要更精准判断，可结合UIWindowScene的effectiveGeometry属性检查场景宽度（Slide Over标准宽度约320pt），上述方法已覆盖绝大多数场景。

内容的提问来源于stack exchange，提问作者user1046037