/// Called when the view is about to made visible, before it is added to the hierarchy.
/// Because the view is not yet in the hierarchy at the time this method is called, it
/// is too early in the appearance transition for many usages. Prefer -viewIsAppearing:
/// instead of this method when possible. ...
- (void)viewWillAppear:(BOOL)animated;
/// Called after the view has fully transitioned to visible, when any transition animations have completed.
- (void)viewDidAppear:(BOOL)animated;
/// Called when the view is about to be dismissed, covered, or otherwise hidden.
- (void)viewWillDisappear:(BOOL)animated;
/// Called after the view has fully been dismissed, covered, or otherwise hidden, when any transition animations have completed.
- (void)viewDidDisappear:(BOOL)animated;
// If a custom container controller manually forwards its appearance callbacks, then rather than calling
// viewWillAppear:, viewDidAppear: viewWillDisappear: or viewDidDisappear: on the children these methods
// should be used instead. This will ensure that descendent child controllers appearance methods will be
// invoked. It also enables more complex custom transitions to be implemented since the appearance callbacks are
// now tied to the final matching invocation of endAppearanceTransition.
- (void)beginAppearanceTransition:(BOOL)isAppearing animated:(BOOL)animated __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_5_0);
- (void)endAppearanceTransition __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_5_0);
@property(null_resettable, nonatomic,strong) UIView *view; // The getter first invokes [self loadView] if the view hasn't been set yet. Subclasses must call super if they override the setter or getter.
UIViewController.h:117-118：
- (void)loadView; // This is where subclasses should create their custom view hierarchy if they aren't using a nib. Should never be called directly.
- (void)loadViewIfNeeded API_AVAILABLE(ios(9.0)); // Loads the view controller's view if it has not already been set.
UIViewController.h:119：
@property(nullable, nonatomic, readonly, strong) UIView *viewIfLoaded API_AVAILABLE(ios(9.0)); // Returns the view controller's view if loaded, nil if not.
- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated; // Uses a horizontal slide transition. Has no effect if the view controller is already in the stack.
- (nullable UIViewController *)popViewControllerAnimated:(BOOL)animated; // Returns the popped controller.
- (nullable NSArray<__kindof UIViewController *> *)popToViewController:(UIViewController *)viewController animated:(BOOL)animated; // Pops view controllers until the one specified is on top. Returns the popped controllers.
- (nullable NSArray<__kindof UIViewController *> *)popToRootViewControllerAnimated:(BOOL)animated; // Pops until there's only a single view controller left on the stack. Returns the popped controllers.
UINavigationController.h:67-70：
@property(nullable, nonatomic,readonly,strong) UIViewController *topViewController; // The top view controller on the stack.
@property(nullable, nonatomic,readonly,strong) UIViewController *visibleViewController; // Return modal view controller if it exists. Otherwise the top view controller.
@property(nonatomic,copy) NSArray<__kindof UIViewController *> *viewControllers; // The current view controller stack.
UINavigationController.h:72：
- (void)setViewControllers:(NSArray<UIViewController *> *)viewControllers animated:(BOOL)animated API_AVAILABLE(ios(3.0)); // If animated is YES, then simulate a push or pop depending on whether the new top view controller was previously in the stack.
// An array of children view controllers. This array does not include any presented view controllers.
@property(nonatomic,readonly) NSArray<__kindof UIViewController *> *childViewControllers API_AVAILABLE(ios(5.0));
...
- (void)addChildViewController:(UIViewController *)childController API_AVAILABLE(ios(5.0));
// Called when the navigation controller shows a new top view controller via a push, pop or setting of the view controller stack.
- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated;
- (void)navigationController:(UINavigationController *)navigationController didShowViewController:(UIViewController *)viewController animated:(BOOL)animated;
UIViewController.h:185-186——viewWillDisappear: 注释 "Called when the view is about to be dismissed, covered, or otherwise hidden"：被覆盖（covered）本身就是 disappearance 的一种，表明 UIKit 把"被盖住"与"消失"同等对待，而非保留在层级里仅被遮挡。
// ... This method will add the toViewController's view to the superview of
// the fromViewController's view and the fromViewController's view will be removed from its superview after the
// transition completes. ...
// ... it is important to ensure that the toViewController's view is added to the visible view hierarchy
// while the fromViewController's view is removed.
（UIViewController.h:447-449 为 "will add the toViewController's view ... and the fromViewController's view will be removed from its superview after the transition completes"，UIViewController.h:454-455 为 "ensure that the toViewController's view is added to the visible view hierarchy while the fromViewController's view is removed"。）UINavigationController 的 push/pop 正是这种容器 transition：转场结束后旧 VC 的 view 被 removeFromSuperview。
反证（VC 仍持有 view）：UIViewController.h:119viewIfLoaded "Returns the view controller's view if loaded, nil if not"——view 的"loaded"状态独立于其是否在 window 层级中；第 1 节已证 VC 在栈中被强持有，而 view 属性是 strong（UIViewController.h:116）。
官方文档佐证（URL，本环境不可达，未能在线验证）：
Apple《View Controller Programming Guide for iOS》（已归档）：
[镜像来源 URL] https://developer.apple.com/library/content/featuredarticles/ViewControllerPGforiPhoneOS/index.html （该 URL 取自 GitHub 镜像仓库 Yannmm/View-Controller-Programming-Guide-for-iOS-Chinese-Translation 中 Apple 原文链接，镜像内容经 api.github.com 获取）该指南（及 Apple 文档站现行 UINavigationController、UIViewController 页面）对导航栈的描述为：导航控制器在转场时把新栈顶的 view 装入内容区，旧栈顶 view 移出层级、VC 留栈。[推断——指南镜像章节（Overview/Presentations 等）中未逐字出现该句，此段综合 Apple 文档惯常表述与本节头文件注释；请以上方头文件证据为准]
- (void)viewWillUnload API_DEPRECATED("", ios(5.0, 6.0)) API_UNAVAILABLE(tvos) API_UNAVAILABLE(visionos, watchos);
- (void)viewDidUnload API_DEPRECATED("", ios(3.0, 6.0)) API_UNAVAILABLE(tvos) API_UNAVAILABLE(visionos, watchos); // Called after the view controller's view is released and set to nil. For example, a memory warning which causes the view to be purged. Not invoked as a result of -dealloc.
注释原文即历史行为的自述："Called after the view controller's view is released and set to nil. For example, a memory warning which causes the view to be purged."
UIViewController.h:207（现行行为的最权威一行注释）：
- (void)didReceiveMemoryWarning; // Called when the parent application receives a memory warning. On iOS 6.0 it will no longer clear the view by default.
- (void)didReceiveMemoryWarning; // Called when the parent application receives a memory warning. On iOS 6.0 it will no longer clear the view by default.
- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application; // try to clean up as much memory as possible. next step is to terminate app
- (void)applicationWillTerminate:(UIApplication *)application;
注释直白："try to clean up as much memory as possible. next step is to terminate app"——警告之后就是杀进程（jetsam）。
/// Tells the delegate that the application has become active
/// - Note: This method is not called if `UIScene` lifecycle has been adopted.
- (void)applicationDidBecomeActive:(UIApplication *)application API_DEPRECATED("Use UIScene lifecycle and sceneDidBecomeActive(_:) from UISceneDelegate or the UIApplication.didBecomeActiveNotification instead.", ios(2.0, 26.0), tvos(9.0, 26.0), visionos(1.0, 26.0)) API_UNAVAILABLE(watchos);
UIApplication.h:373：
/// Tells the delegate that the application is now in the background
/// - Note: This method is not called if `UIScene` lifecycle has been adopted.
- (void)applicationDidEnterBackground:(UIApplication *)application API_AVAILABLE(ios(4.0)) API_DEPRECATED("Use UIScene lifecycle and sceneDidEnterBackground(_:) from UISceneDelegate or the UIApplication.didEnterBackgroundNotification instead.", ios(4.0, 26.0), visionos(1.0, 26.0)) API_UNAVAILABLE(watchos);
同组：applicationWillResignActive:（UIApplication.h:369）、applicationWillEnterForeground:（UIApplication.h:377）。全部带 - Note: This method is not called if 'UIScene' lifecycle has been adopted. 与指向 scene 对应方法的弃用信息。
// Conditions that help the system shell determine whether the scene should be destroyed for certain actions
@property (nonatomic, copy) NSSet<UISceneDestructionCondition *> *destructionConditions API_AVAILABLE(ios(26.0), tvos(26.0), visionos(26.0)) API_UNAVAILABLE(watchos) NS_REFINED_FOR_SWIFT;
/// Called when the view is becoming visible at the beginning of the appearance transition,
/// after it has been added to the hierarchy and been laid out by its superview. This method
/// is very similar to -viewWillAppear: and is always called shortly afterwards (so changes
/// made in either callback will be visible to the user at the same time), but unlike
/// -viewWillAppear:, at the time when -viewIsAppearing: is called all of the following are
/// valid for the view controller and its own view:
/// - View controller and view's trait collection
/// - View's superview chain and window
/// - View's geometry (e.g. frame/bounds, safe area insets, layout margins)
/// Choose this method instead of -viewWillAppear: by default, as it is a direct replacement
/// that provides equivalent or superior behavior in nearly all cases.
- (void)viewIsAppearing:(BOOL)animated API_AVAILABLE(ios(13.0), tvos(13.0)) API_UNAVAILABLE(watchos);
注意两点：(a) 它解决的是 viewWillAppear: 时机过早（"before it is added to the hierarchy"，UIViewController.h:162-163）的问题；(b) 本 SDK 将其标注为 API_AVAILABLE(ios(13.0))——该方法于 2023 年（iOS 17 SDK / WWDC23）首次引入，Apple 随后把可用性回溯下调至 iOS 13（back-deployment），当前头文件以 ios(13.0) 标注[推断——ios(13.0) 标注为头文件可证；"iOS 17 SDK 首次引入后回溯"为知识背景，本环境无法在线验证 Apple 文档变更史]。
/// Call to manually request a properties update for this view controller. // 190-191
/// Multiple requests may be coalesced into a single update alongside the next layout pass.
- (void)setNeedsUpdateProperties API_AVAILABLE(ios(26.0), ...) // 192
/// Override point for subclasses to update properties of this view controller or its view. // 193-194
- (void)updateProperties NS_REQUIRES_SUPER API_AVAILABLE(ios(26.0), ...) // 195
- (void)updatePropertiesIfNeeded API_AVAILABLE(ios(26.0), ...) // 198
以及 childViewControllerForInterfaceOrientationLock（UIViewController.h:725）、prefersInterfaceOrientationLocked（740）、setNeedsUpdateOfPrefersInterfaceOrientationLocked（743）——界面方向锁定；UIScene.h:59destructionConditions（scene 销毁条件）；UINavigationController.h:89-91interactiveContentPopGestureRecognizer（API_AVAILABLE(ios(26.0)...)，全屏内容区 pop 手势）。