//后台通知:屏幕操作通知等等

UIKIT_EXTERN NSString *const UIApplicationDidEnterBackgroundNotification       NS_AVAILABLE_IOS(4_0);

UIKIT_EXTERN NSString *const UIApplicationWillEnterForegroundNotification      NS_AVAILABLE_IOS(4_0);

UIKIT_EXTERN NSString *const UIApplicationDidFinishLaunchingNotification;

UIKIT_EXTERN NSString *const UIApplicationDidBecomeActiveNotification;

UIKIT_EXTERN NSString *const UIApplicationWillResignActiveNotification;

UIKIT_EXTERN NSString *const UIApplicationDidReceiveMemoryWarningNotification;

UIKIT_EXTERN NSString *const UIApplicationWillTerminateNotification;

UIKIT_EXTERN NSString *const UIApplicationSignificantTimeChangeNotification;

UIKIT_EXTERN NSString *const UIApplicationWillChangeStatusBarOrientationNotification __TVOS_PROHIBITED; // userInfo contains NSNumber with new orientation

UIKIT_EXTERN NSString *const UIApplicationDidChangeStatusBarOrientationNotification __TVOS_PROHIBITED;  // userInfo contains NSNumber with old orientation

UIKIT_EXTERN NSString *const UIApplicationStatusBarOrientationUserInfoKey __TVOS_PROHIBITED;            // userInfo dictionary key for status bar orientation

UIKIT_EXTERN NSString *const UIApplicationWillChangeStatusBarFrameNotification __TVOS_PROHIBITED;       // userInfo contains NSValue with new frame

UIKIT_EXTERN NSString *const UIApplicationDidChangeStatusBarFrameNotification __TVOS_PROHIBITED;        // userInfo contains NSValue with old frame

UIKIT_EXTERN NSString *const UIApplicationStatusBarFrameUserInfoKey __TVOS_PROHIBITED;                  // userInfo dictionary key for status bar frame

UIKIT_EXTERN NSString *const UIApplicationBackgroundRefreshStatusDidChangeNotification NS_AVAILABLE_IOS(7_0) __TVOS_PROHIBITED;

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsURLKey                   NS_AVAILABLE_IOS(3_0); // userInfo contains NSURL with launch URL

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsSourceApplicationKey     NS_AVAILABLE_IOS(3_0); // userInfo contains NSString with launch app bundle ID

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsRemoteNotificationKey    NS_AVAILABLE_IOS(3_0) __TVOS_PROHIBITED; // userInfo contains NSDictionary with payload

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsLocalNotificationKey     NS_AVAILABLE_IOS(4_0) __TVOS_PROHIBITED; // userInfo contains a UILocalNotification

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsAnnotationKey            NS_AVAILABLE_IOS(3_2); // userInfo contains object with annotation property list

UIKIT_EXTERN NSString *const UIApplicationProtectedDataWillBecomeUnavailable    NS_AVAILABLE_IOS(4_0);

UIKIT_EXTERN NSString *const UIApplicationProtectedDataDidBecomeAvailable       NS_AVAILABLE_IOS(4_0);

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsLocationKey              NS_AVAILABLE_IOS(4_0); // app was launched in response to a CoreLocation event.

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsNewsstandDownloadsKey    NS_AVAILABLE_IOS(5_0) __TVOS_PROHIBITED; // userInfo contains an NSArray of NKAssetDownload identifiers

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsBluetoothCentralsKey     NS_AVAILABLE_IOS(7_0); // userInfo contains an NSArray of CBCentralManager restore identifiers

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsBluetoothPeripheralsKey  NS_AVAILABLE_IOS(7_0); // userInfo contains an NSArray of CBPeripheralManager restore identifiers

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsShortcutItemKey          NS_AVAILABLE_IOS(9_0) __TVOS_PROHIBITED; // userInfo contains the UIApplicationShortcutItem used to launch the app.

 // Key in options dict passed to application[will | did]FinishLaunchingWithOptions and info for UIApplicationDidFinishLaunchingNotification  

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsUserActivityDictionaryKey    NS_AVAILABLE_IOS(8_0); // Sub-Dictionary present in launch options when user activity is present

UIKIT_EXTERN NSString *const UIApplicationLaunchOptionsUserActivityTypeKey          NS_AVAILABLE_IOS(8_0); // Key in user activity dictionary for the activity type

UIKIT_EXTERN NSString *const UIApplicationOpenSettingsURLString NS_AVAILABLE_IOS(8_0);//打开系统定位

// Keys for application:openURL:options:  

UIKIT_EXTERN NSString *const UIApplicationOpenURLOptionsSourceApplicationKey NS_AVAILABLE_IOS(9_0);   // value is an NSString containing the bundle ID of the originating application

UIKIT_EXTERN NSString *const UIApplicationOpenURLOptionsAnnotationKey NS_AVAILABLE_IOS(9_0);   // value is a property-list typed object corresponding to what the originating application passed in UIDocumentInteractionController‘s annotation property

UIKIT_EXTERN NSString *const UIApplicationOpenURLOptionsOpenInPlaceKey NS_AVAILABLE_IOS(9_0);   // value is a bool NSNumber, set to YES if the file needs to be copied before use

// Content size category constants   //内容大小类别常数

UIKIT_EXTERN NSString *const UIContentSizeCategoryExtraSmall NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategorySmall NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryMedium NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryExtraLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryExtraExtraLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryExtraExtraExtraLarge NS_AVAILABLE_IOS(7_0);

// Accessibility sizes  //可访问行

UIKIT_EXTERN NSString *const UIContentSizeCategoryAccessibilityMedium NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryAccessibilityLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryAccessibilityExtraLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryAccessibilityExtraExtraLarge NS_AVAILABLE_IOS(7_0);

UIKIT_EXTERN NSString *const UIContentSizeCategoryAccessibilityExtraExtraExtraLarge NS_AVAILABLE_IOS(7_0);

// Notification is emitted when the user has changed the preferredContentSizeCategory for the system //通知发出时，改变了用户对系统的preferredcontentsizecategory

UIKIT_EXTERN NSString *const UIContentSizeCategoryDidChangeNotification NS_AVAILABLE_IOS(7_0); // userInfo dictionary will contain new value for UIContentSizeCategoryNewValueKey

UIKIT_EXTERN NSString *const UIContentSizeCategoryNewValueKey NS_AVAILABLE_IOS(7_0); // NSString instance with new content size category in userInfo

// This notification is posted after the user takes a screenshot (for example by pressing both the home and lock screen buttons)//此通知发布后，用户需要一个截图(例如，通过按下home键和锁屏按钮)

UIKIT_EXTERN NSString *const UIApplicationUserDidTakeScreenshotNotification NS_AVAILABLE_IOS(7_0);

// Extension point identifier constants   //扩展点标识符常量

UIKIT_EXTERN NSString *const UIApplicationKeyboardExtensionPointIdentifier NS_AVAILABLE_IOS(8_0);

NS_ASSUME_NONNULL_END

iOS UIApplication 里面各const实际用意