ScenePhase 现场阶段

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
public enum ScenePhase : Comparable {

    /// The scene isn't currently visible in the UI.
    ///
    /// Do as little as possible in a scene that's in the `background` phase.
    /// The `background` phase can precede termination, so do any cleanup work
    /// immediately upon entering this state. For example, close any open files
    /// and network connections. However, a scene can also return to the
    /// the ``ScenePhase/active`` phase from the background.
    ///
    /// Expect an app that enters the `background` phase to terminate.
  case background

    /// The scene is in the foreground but should pause its work.
    ///
    /// A scene in this phase doesn't receive events and should pause
    /// timers and free any unnecessary resources. The scene might be completely
    /// hidden in the user interface or otherwise unavailable to the user.
    /// In macOS, scenes only pass through this phase temporarily on their way
    /// to the ``ScenePhase/background`` phase.
    ///
    /// An app or custom scene in this phase contains no scene instances in the
    /// ``ScenePhase/active`` phase.
    case inactive

    /// The scene is in the foreground and interactive.
    ///
    /// An active scene isn't necessarily front-most. For example, a macOS
    /// window might be active even if it doesn't currently have focus.
    /// Nevertheless, all scenes should operate normally in this phase.
    ///
    /// An app or custom scene in this phase contains at least one active scene
    /// instance.
    case active

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (a: ScenePhase, b: ScenePhase) -> Bool

    /// The hash value.
    ///
    /// Hash values are not guaranteed to be equal across different executions of
    /// your program. Do not save hash values to use during a future execution.
    ///
    /// - Important: `hashValue` is deprecated as a `Hashable` requirement. To
    ///   conform to `Hashable`, implement the `hash(into:)` requirement instead.
    public var hashValue: Int { get }

    /// Hashes the essential components of this value by feeding them into the
    /// given hasher.
    ///
    /// Implement this method to conform to the `Hashable` protocol. The
    /// components used for hashing must be the same as the components compared
    /// in your type's `==` operator implementation. Call `hasher.combine(_:)`
    /// with each of these components.
    ///
    /// - Important: Never call `finalize()` on `hasher`. Doing so may become a
    ///   compile-time error in the future.
    ///
    /// - Parameter hasher: The hasher to use when combining the components
    ///   of this instance.
    public func hash(into hasher: inout Hasher)

    /// Returns a Boolean value indicating whether the value of the first
    /// argument is less than that of the second argument.
    ///
    /// This function is the only requirement of the `Comparable` protocol. The
    /// remainder of the relational operator functions are implemented by the
    /// standard library for any type that conforms to `Comparable`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func < (a: ScenePhase, b: ScenePhase) -> Bool
}

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
extension ScenePhase : Hashable {
}

Background :

///场景当前在UI中不可见。
///
///在“背景”阶段的场景中尽量少做。
///“后台”阶段可以在终止之前，因此执行任何清理工作
///立即进入此状态。例如，关闭所有打开的文件
///和网络连接。不过，一个场景也可以回到
///背景中的‘ScenePhase’/‘active’阶段。
///
///期望进入“后台”阶段的应用程序终止。

Inactive

///场景在前台，但应该暂停工作。
///
///这个阶段的场景不接收事件，应该暂停
///计时器和释放任何不必要的资源。场景可能是完全的
///隐藏在用户界面中或对用户不可用。
///在macOS中，场景只是暂时通过这个阶段
///转到‘ScenePhase/background’阶段。
///
///此阶段的应用程序或自定义场景不包含场景实例
/ / /‘ScenePhase /活跃的阶段。

Active

// 场景在前景中，相互作用。
///
/// 一个活跃的场景不一定是最前面的。例如，macOS
/// 窗口可能是活动的，即使它目前没有焦点。
/// 不过，此阶段所有场景应正常运行。
///
/// 此阶段的应用程序或自定义场景至少包含一个活动场景
/// 实例。

SceneStorage<Value> 场景存储

开头如下：
// 对每个场景进行读写的属性包装器类型
/// 存储。
///
/// 您使用' SceneStorage '时，您需要自动恢复状态
/// 值。“SceneStorage”的工作原理与“State”非常相似，除了它的首字母
/// 如果之前保存过，则系统恢复该值，值为·
/// 与同一场景中的其他“SceneStorage”变量共享。
///
/// 系统管理' SceneStorage '的保存和恢复
/// 。无法使用支持“SceneStorage”的基础数据
/// 你，所以你必须通过“SceneStorage”属性包装器访问它。的
/// system不保证数据的时间和频率
/// 保存。
///
/// 每个“场景”都有自己的“SceneStorage”概念，所以数据不会被共享
/// 场景之间。
///
/// 确保使用“SceneStorage”的数据是轻量级的。数据的
/// 大型数据，例如模型数据，不应该存储在“SceneStorage”中
/// 可能会导致性能下降。
///
/// 如果“场景”明显被销毁(例如切换器快照被销毁)
/// 在iPadOS上销毁或在macOS上关闭窗口)，数据也是
/// 销毁。不要对敏感数据使用“SceneStorage”。

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
@frozen @propertyWrapper public struct SceneStorage<Value> : DynamicProperty {

    /// The underlying value referenced by the state variable.
    ///
    /// This works identically to `State.wrappedValue`.
    ///
    /// - SeeAlso: State.wrappedValue
    public var wrappedValue: Value { get nonmutating set }

    /// A binding to the state value.
    ///
    /// This works identically to `State.projectedValue`.
    ///
    /// - SeeAlso: State.projectedValue
    public var projectedValue: Binding<Value> { get }
}

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
extension SceneStorage {

    /// Creates a property that can save and restore a boolean.
    ///
    /// - Parameter wrappedValue: The default value if a boolean is not
    ///   available for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == Bool

    /// Creates a property that can save and restore an integer.
    ///
    /// - Parameter wrappedValue: The default value if an integer is not
    ///   available for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == Int

    /// Creates a property that can save and restore a double.
    ///
    /// - Parameter wrappedValue: The default value if a double is not available
    ///   for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == Double

    /// Creates a property that can save and restore a string.
    ///
    /// - Parameter wrappedValue: The default value if a string is not available
    ///   for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == String

    /// Creates a property that can save and restore a URL.
    ///
    /// - Parameter wrappedValue: The default value if a URL is not available
    ///   for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == URL

    /// Creates a property that can save and restore data.
    ///
    /// Avoid storing large data blobs, such as image data, as it can negatively
    /// affect performance of your app.
    ///
    /// - Parameter wrappedValue: The default value if data is not available
    ///   for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value == Data

    /// Creates a property that can save and restore an integer, transforming it
    /// to a `RawRepresentable` data type.
    ///
    /// A common usage is with enumerations:
    ///
    ///    enum MyEnum: Int {
    ///        case a
    ///        case b
    ///        case c
    ///    }
    ///    struct MyView: View {
    ///        @SceneStorage("MyEnumValue") private var value = MyEnum.a
    ///        var body: some View { ... }
    ///    }
    ///
    /// - Parameter wrappedValue: The default value if an integer value is not
    ///   available for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value : RawRepresentable, Value.RawValue == Int

    /// Creates a property that can save and restore a string, transforming it
    /// to a `RawRepresentable` data type.
    ///
    /// A common usage is with enumerations:
    ///
    ///    enum MyEnum: String {
    ///        case a
    ///        case b
    ///        case c
    ///    }
    ///    struct MyView: View {
    ///        @SceneStorage("MyEnumValue") private var value = MyEnum.a
    ///        var body: some View { ... }
    ///    }
    ///
    /// - Parameter wrappedValue: The default value if a String value is not
    ///   available for the given key.
    /// - Parameter key: a key used to save and restore the value.
    public init(wrappedValue: Value, _ key: String) where Value : RawRepresentable, Value.RawValue == String
}

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
extension SceneStorage where Value : ExpressibleByNilLiteral {

    /// Creates a property that can save and restore an Optional boolean.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == Bool?

    /// Creates a property that can save and restore an Optional integer.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == Int?

    /// Creates a property that can save and restore an Optional double.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == Double?

    /// Creates a property that can save and restore an Optional string.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == String?

    /// Creates a property that can save and restore an Optional URL.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == URL?

    /// Creates a property that can save and restore an Optional data.
    ///
    /// Defaults to nil if there is no restored value
    ///
    /// - Parameter key: a key used to save and restore the value.
    public init(_ key: String) where Value == Data?
}

SceneStorage<Value>

///对每个场景进行读写的属性包装器类型
/ / /存储。
///
///您使用' SceneStorage '时，您需要自动恢复状态
/ / /值。“SceneStorage”的工作原理与“State”非常相似，除了它的首字母
///如果之前保存过，则系统恢复该值，值为·
///与同一场景中的其他“SceneStorage”变量共享。
///
///系统管理' SceneStorage '的保存和恢复
/ / /。无法使用支持“SceneStorage”的基础数据
///你，所以你必须通过“SceneStorage”属性包装器访问它。的
/// system不保证数据的时间和频率
/ / /保存。
///
///每个“场景”都有自己的“SceneStorage”概念，所以数据不会被共享
/ / /场景之间。
///
///确保使用“SceneStorage”的数据是轻量级的。数据的
///大型数据，例如模型数据，不应该存储在“SceneStorage”中
///可能会导致性能下降。
///
///如果“场景”明显被销毁(例如切换器快照被销毁)
///在iPadOS上销毁或在macOS上关闭窗口)，数据也是
/ / /销毁。不要对敏感数据使用“SceneStorage”。

/// Creates a property that can save and restore an integer, transforming it
/// to a RawRepresentable data type.
///
/// A common usage is with enumerations:
///
/// enum MyEnum: Int {
/// case a
/// case b
/// case c
/// }
/// struct MyView: View {
/// @SceneStorage("MyEnumValue") private var value = MyEnum.a
/// var body: some View { ... }
/// get : print(value)
/// set： value = MyEnum.b
/// }

EnvironmentValues 环境变量

 @available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
extension EnvironmentValues {

    /// The current phase of the scene.
    ///
    /// The system sets this value to provide an indication of the
    /// operational state of a scene or collection of scenes. The exact
    /// meaning depends on where you access the value. For more information,
    /// see ``ScenePhase``.
    public var scenePhase: ScenePhase
}

///场景的当前阶段。
///
///系统设置该值以指示
///场景或场景集合的运行状态。确切的
///的含义取决于您访问值的位置。有关更多信息,
/ / /看到“ScenePhase ' '。

1. 用在自定义场景实例中

// If you read the phase from within a custom ``Scene`` instance, the value
/// similarly reflects an aggregation of all the scenes that make up the custom
/// scene:
// 如果你从一个自定义的' ' Scene ' '实例中读取阶段，值
/// 类似地反映了构成该习惯的所有场景的集合
/// 场景:
///
///     struct MyScene: Scene {
///         @Environment(\.scenePhase) private var scenePhase
///
///         var body: some Scene {
///             WindowGroup {
///                 MyRootView()
///             }
///             .onChange(of: scenePhase) { phase in
///                 if phase == .background {
///                     // Perform cleanup when all scenes within
///                     // MyScene go to the background.
///                 }
///             }
///         }
///     }

2. 用于App

/// If you read the phase from within an ``App`` instance, you obtain an
/// aggregate value that reflects the phases of all the scenes in your app. The
/// app reports a value of ``ScenePhase/active`` if any scene is active, or a
/// value of ``ScenePhase/inactive`` when no scenes are active. This includes
/// multiple scene instances created from a single scene declaration; for
/// example, from a ``WindowGroup``. When an app enters the
/// ``ScenePhase/background`` phase, expect the app to terminate soon after.
/// You can use that opportunity to free any resources:
/// 如果从' ' App ' '实例中读取阶段，则获取
/// 反映应用程序中所有场景阶段的累计值
/// 如果任何场景是活动的，app会报告一个' ' ScenePhase/active ' '的值
/// 当没有场景处于活动状态时' ' ScenePhase/inactive ' '的值。这包括
/// 从一个场景声明创建多个场景实例;为
/// 例子，来自一个' ' WindowGroup ' '。当一个应用程序进入
///   ' ' ScenePhase/background ' ' stage，预计应用程序将很快终止。
/// 你可以利用这个机会来释放任何资源:
///
///     @main
///     struct MyApp: App {
///         @Environment(\.scenePhase) private var scenePhase
///
///         var body: some Scene {
///             WindowGroup {
///                 MyRootView()
///             }
///             .onChange(of: scenePhase) { phase in
///                 if phase == .background {
///                     // Perform cleanup when all scenes within
///                     // MyApp go to the background.
///                 }
///             }
///         }
///     }

3. 用于视图，读取依赖状态

/// How you interpret the value depends on where it's read from.
/// If you read the phase from inside a ``View`` instance, you obtain a value
/// that reflects the phase of the scene that contains the view. The following
/// example uses the ``SwiftUI/View/onChange(of:perform:)`` method to enable
/// a timer whenever the enclosing scene enters the ``ScenePhase/active`` phase
/// and disable the timer when entering any other phase:
/// 如何解释该值取决于从哪里读取。
/// 如果从' ' View ' '实例内部读取阶段，则获得一个值
/// 反映包含视图的场景的阶段。以下
/// 示例使用' ' SwiftUI/View/onChange(of:perform:) ' '方法启用
/// 当封闭场景进入' ' ScenePhase/active ' '阶段时提供一个计时器
/// 和禁用计时器时，进入任何其他阶段:
///
///     struct MyView: View {
///         @ObservedObject var model: DataModel
///         @Environment(\.scenePhase) private var scenePhase
///
///         var body: some View {
///             TimerView()
///                 .onChange(of: scenePhase) { phase in
///                     model.isTimerRunning = (phase == .active)
///                 }
///         }
///     }

4. 用于场景状态

/// An indication of a scene's operational state.
///
/// The system moves your app's ``Scene`` instances through phases that reflect
/// a scene's operational state. You can trigger actions when the phase changes.
/// Read the current phase by observing the ``EnvironmentValues/scenePhase``
/// value in the ``Environment``:
// 现场运行状态的指示。
///
/// 系统将你的应用程序的“场景”实例通过反射的阶段移动
///  a场景的运行状态。当相位变化时，你可以触发动作。
/// 通过观察' ' EnvironmentValues/scenePhase ' '来读取当前阶段
/// 在“环境”中的价值:
///
///     @Environment(\.scenePhase) private var scenePhase