func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).

        // Create the SwiftUI view that provides the window contents.
        let contentView = BaseTabbarView()

        // Use a UIHostingController as window root view controller.
        if let windowScene = scene as? UIWindowScene {
            
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = UIHostingController(rootView: contentView)
            self.window = window
            window.makeKeyAndVisible()
        }
    }

import SwiftUI

@main
struct MultiPlatformApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

struct BaseTabbarView_Previews: PreviewProvider {
    /// 预览视图，你试着在这里多添加两个看看效果呀
    static var previews: some View {
        
        BaseTabbarView()
    }
}

NavigationView{
            
      NavigationLink.init(
          destination: Text("Destination"),
          label: {
              Text("Navigate")
          })
          .navigationTitle(title)
          /// 留意下这个显示模式 displayMode 分三种，具体的可以点进去看看
          /// inline 就是我们常见的模式
          /// .navigationBarTitle(title,displayMode: .inline)
}

/// A view that switches between multiple child views using interactive user
/// interface elements.
///
/// To create a user interface with tabs, place views in a `TabView` and apply
/// the ``View/tabItem(_:)`` modifier to the contents of each tab. The following
/// creates a tab view with three tabs:
///
///     TabView {
///         Text("The First Tab")
///             .tabItem {
///                 Image(systemName: "1.square.fill")
///                 Text("First")
///             }
///         Text("Another Tab")
///             .tabItem {
///                 Image(systemName: "2.square.fill")
///                 Text("Second")
///             }
///         Text("The Last Tab")
///             .tabItem {
///                 Image(systemName: "3.square.fill")
///                 Text("Third")
///             }
///     }
///     .font(.headline)
///
/// Tab views only support tab items of type ``Text``, ``Image``, or an image
/// followed by text. Passing any other type of view results in a visible but
/// empty tab item.
@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 7.0, *)
public struct TabView<SelectionValue, Content> : View where SelectionValue : Hashable, Content : View {

    /// Creates an instance that selects from content associated with
    /// `Selection` values.
    public init(selection: Binding<SelectionValue>?, @ViewBuilder content: () -> Content)

    /// The content and behavior of the view.
    public var body: some View { get }

    /// The type of view representing the body of this view.
    ///
    /// When you create a custom view, Swift infers this type from your
    /// implementation of the required `body` property.
    public typealias Body = some View
}

// Create the SwiftUI view that provides the window contents.
let contentView = BaseTabbarView()

// Use a UIHostingController as window root view controller.
if let windowScene = scene as? UIWindowScene {
            
      let window = UIWindow(windowScene: windowScene)
      window.rootViewController = UIHostingController(rootView: contentView)
      self.window = window
      window.makeKeyAndVisible()
}

import SwiftUI

struct BaseTabbarView: View {
    
    /// 理解State这个属性 //www.cnblogs.com/xiaoniuzai/p/11417123.html
    /*
     通过使用 @State 修饰器我们可以关联出 View 的状态. SwiftUI 将会把使用过 @State 修饰器的属性存储到一个特殊的内存区域，并且这个区域和 View struct 是隔离的. 当 @State 装饰过的属性发生了变化，SwiftUI 会根据新的属性值重新创建视图
     */
    @State private var selectedTab = 0
    
    var body: some View {
        
        /// public init(selection: Binding<SelectionValue>?, @ViewBuilder content: () -> Content)
        TabView(selection: $selectedTab){
            
            HomeView(title: "首页")
                .tabItem {
                    Image(selectedTab == 0 ? "icon_home_selected" : "icon_home")
                    Text("首页")
                }
                .onTapGesture {
                    selectedTab = 0
                }
                .tag(0)
            
            ServiceView(title: "周边")
                .tabItem {
                    Image(selectedTab == 1 ? "icon_around_selected" : "icon_around")
                    Text("周边")
                }
                .onTapGesture {
                    selectedTab = 1
                }
                .tag(1)
            
            AroundView(title: "服务")
                .tabItem {
                    Image(selectedTab == 2 ? "icon_service_selected" : "icon_service")
                    Text("服务")
                }
                .onTapGesture {
                    selectedTab = 2
                }
                .tag(2)
            
            MineView(title: "我的").environmentObject(NavigationAction())
                .tabItem {
                    Image(selectedTab == 3 ? "icon_mine_selected" : "icon_mine")
                    Text("我的")
                }
                .onTapGesture {
                    selectedTab = 3
                }
                .tag(3)
        /// 这个着重颜色设置可以设置tabbaritem字体的颜色
        }.accentColor(.blue)
    }
}

struct BaseTabbarView_Previews: PreviewProvider {
    /// 预览视图，你试着在这里多添加两个看看效果呀
    static var previews: some View {
        
        BaseTabbarView()
    }
}