每个app有且只有一个UIApplication对象，当程序启动的时候通过调用UIApplicationMain方法得到的。可以通过sharedApplication方法得到。
UIApplication对象的主要任务是处理用户事件的处理路径，例如分发一个UIEvent到另外一个对象去处理。UIApplication对象持有众多的UIWindow对象，因此可以组织app的展示。UIApplication对象还能处理一些资源，例如通过openURL:打开邮箱客户端或者设置界面等。
获得UIApplication对象：[UIApplication sharedApplication]
获得UIApplicationDelegate对象：[[UIApplication sharedApplication] delegate]
获得UIWindow对象：
[[UIApplication sharedApplication] windows]; //UIWindow数组
[[UIApplication sharedApplication] keyWindow]; //UIWindow数组中最后调用makeKeyAndVisible方法的UIWindow对象
控制和处理UIEvent：
\- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{//分发一个event到另外一个对象去处理
[[UIApplication sharedApplication] sendAction:@selector(action: forEvent:) to:[CustomHandler sharedCustomHandler] from:self forEvent:event];
}
[[UIApplication sharedApplication] beginIgnoringInteractionEvents]; //开始忽略Event
//...中间调用动画等操作
[[UIApplication sharedApplication] endIgnoringInteractionEvents]; //结束忽略Event
[UIApplication sharedApplication].applicationSupportsShakeToEdit = YES; //晃动是否有撤销或者重做动作
打开URL资源：[[UIApplication sharedApplication] openURL:[NSURL URLWithString:UIApplicationOpenSettingsURLString]];//打开设置界面
配置通知设置：
UIUserNotificationType types = UIUserNotificationTypeBadge | UIUserNotificationTypeSound | UIUserNotificationTypeAlert;
UIUserNotificationSettings *mySettings = [UIUserNotificationSettings settingsForTypes:types categories:nil];
[[UIApplication sharedApplication] registerUserNotificationSettings:mySettings]; //注册远程推送通知
[[UIApplication sharedApplication] registerForRemoteNotifications];//注册
[[UIApplication sharedApplication] unregisterForRemoteNotifications];//注销
NSDate *date = [NSDate dateWithTimeIntervalSinceNow:10];
UILocalNotification *localNotif = [[UILocalNotification alloc] init];
localNotif.fireDate = date; //时间
localNotif.timeZone = [NSTimeZone localTimeZone]; //时区
localNotif.repeatInterval = NSCalendarUnitMinute; //间隔
localNotif.soundName = UILocalNotificationDefaultSoundName; //声音
localNotif.alertBody = @"Local Test"; //通知内容
localNotif.applicationIconBadgeNumber = 1; //数字标示
localNotif.userInfo = @{@"key":@"test"}; //info
[[UIApplication sharedApplication] scheduleLocalNotification:localNotif]; //注册通知
[[UIApplication sharedApplication] presentLocalNotificationNow:localNotif]; //立即通知
[[UIApplication sharedApplication] cancelAllLocalNotifications]; //取消所有通知
[[UIApplication sharedApplication] cancelLocalNotification:localNotif]; //取消特定的通知
NSArray *arr = [[UIApplication sharedApplication] scheduledLocalNotifications]; //所有的通知
后台运行相关：
[[UIApplication sharedApplication] applicationState]; //app状态
[[UIApplication sharedApplication] setMinimumBackgroundFetchInterval:3600]; //设置后台运行时间
NSTimeInterval remainTime = [[UIApplication sharedApplication] backgroundTimeRemaining]; //app后台运行的时间
NSLog(@"remainTIme = %f",remainTime);
int state = [[UIApplication sharedApplication] backgroundRefreshStatus]; //后台刷新的状态
NSLog(@"state = %d",state);
[[UIApplication sharedApplication] beginBackgroundTaskWithName:@"taskOne" expirationHandler:^{
}];
[[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
}];
[[UIApplication sharedApplication] endBackgroundTask:1];
远程的控制相关：
[[UIApplication sharedApplication] beginReceivingRemoteControlEvents];
[[UIApplication sharedApplication] endReceivingRemoteControlEvents];
系统的Idle Timer：
[UIApplication sharedApplication].idleTimerDisabled = YES; //不让手机休眠
APP样式：
//隐藏状态条
[[UIApplication sharedApplication] setStatusBarHidden:YES];
[[UIApplication sharedApplication] setStatusBarHidden:YES withAnimation:UIStatusBarAnimationSlide];
//设置状态条的样式
[[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleDefault];
[[UIApplication sharedApplication] statusBarStyle];
//状态条的Frame
[[UIApplication sharedApplication] statusBarFrame];
//网络是否可见
[[UIApplication sharedApplication] isNetworkActivityIndicatorVisible];
//badge数字
[UIApplication sharedApplication].applicationIconBadgeNumber = 2;
//屏幕的方向
[[UIApplication sharedApplication] userInterfaceLayoutDirection];
设置状态条的方向：
[[UIApplication sharedApplication] setStatusBarOrientation:UIInterfaceOrientationLandscapeLeft animated:YES];
数据类型：
UIBackgroundTaskIdentifier : Int
typedef enum : NSUInteger {
UIRemoteNotificationTypeNone = 0,
UIRemoteNotificationTypeBadge = 1 << 0,
UIRemoteNotificationTypeSound = 1 << 1,
UIRemoteNotificationTypeAlert = 1 << 2,
UIRemoteNotificationTypeNewsstandContentAvailability = 1 << 3
} UIRemoteNotificationType;
typedef enum : NSInteger {
UIStatusBarStyleDefault,
UIStatusBarStyleLightContent,
UIStatusBarStyleBlackTranslucent,
UIStatusBarStyleBlackOpaque
} UIStatusBarStyle;
typedef enum : NSInteger {
UIStatusBarAnimationNone,
UIStatusBarAnimationFade,
UIStatusBarAnimationSlide,
} UIStatusBarAnimation;
typedef enum : NSInteger {
UIApplicationStateActive ,
UIApplicationStateInactive ,
UIApplicationStateBackground
} UIApplicationState;
const UIBackgroundTaskIdentifier UIBackgroundTaskInvalid ;
const NSTimeInterval UIMinimumKeepAliveTimeout;
typedef enum : NSUInteger {
UIBackgroundFetchResultNewData ,
UIBackgroundFetchResultNoData ,
UIBackgroundFetchResultFailed
} UIBackgroundFetchResult;
const NSTimeInterval UIApplicationBackgroundFetchIntervalMinimum ;
const NSTimeInterval UIApplicationBackgroundFetchIntervalNever;
typedef enum : NSUInteger {
UIBackgroundRefreshStatusRestricted ,
UIBackgroundRefreshStatusDenied ,
UIBackgroundRefreshStatusAvailable
} UIBackgroundRefreshStatus;
typedef enum : NSInteger {
UIInterfaceOrientationUnknown = UIDeviceOrientationUnknown ,
UIInterfaceOrientationPortrait = UIDeviceOrientationPortrait ,
UIInterfaceOrientationPortraitUpsideDown = UIDeviceOrientationPortraitUpsideDown ,
UIInterfaceOrientationLandscapeLeft = UIDeviceOrientationLandscapeRight ,
UIInterfaceOrientationLandscapeRight = UIDeviceOrientationLandscapeLeft
} UIInterfaceOrientation;
typedef enum : NSInteger {
UIUserInterfaceLayoutDirectionLeftToRight ,
UIUserInterfaceLayoutDirectionRightToLeft ,
} UIUserInterfaceLayoutDirection;
NSString *const UIApplicationOpenSettingsURLString;
NSString *const UIApplicationStatusBarOrientationUserInfoKey ;
NSString *const UIApplicationStatusBarFrameUserInfoKey;
NSString *const UIContentSizeCategoryExtraSmall ;
NSString *const UIContentSizeCategorySmall ;
NSString *const UIContentSizeCategoryMedium ;
NSString *const UIContentSizeCategoryLarge ;
NSString *const UIContentSizeCategoryExtraLarge ;
NSString *const UIContentSizeCategoryExtraExtraLarge ;
NSString *const UIContentSizeCategoryExtraExtraExtraLarge;
NSString *const UIContentSizeCategoryAccessibilityMedium ;
NSString *const UIContentSizeCategoryAccessibilityLarge ;
NSString *const UIContentSizeCategoryAccessibilityExtraLarge ;
NSString *const UIContentSizeCategoryAccessibilityExtraExtraLarge ;
NSString *const UIContentSizeCategoryAccessibilityExtraExtraExtraLarge;
NSString *const UIApplicationInvalidInterfaceOrientationException;
通知：
UIApplicationBackgroundRefreshStatusDidChangeNotification
UIApplicationDidBecomeActiveNotification
UIApplicationDidChangeStatusBarFrameNotification
UIApplicationDidChangeStatusBarOrientationNotification
UIApplicationDidEnterBackgroundNotification
UIApplicationDidFinishLaunchingNotification
UIApplicationDidReceiveMemoryWarningNotification
UIApplicationProtectedDataDidBecomeAvailable
UIApplicationProtectedDataWillBecomeUnavailable
UIApplicationSignificantTimeChangeNotification
UIApplicationUserDidTakeScreenshotNotification
UIApplicationWillChangeStatusBarOrientationNotification
UIApplicationWillChangeStatusBarFrameNotification
UIApplicationWillEnterForegroundNotification
UIApplicationWillResignActiveNotification
UIApplicationWillTerminateNotification
UIContentSizeCategoryDidChangeNotification