基础篇-UIApplication、UIWindow以及程序的启动流程看我就够了

@property(nonatomic) NSInteger applicationIconBadgeNumber;

UIApplication *app=[UIApplication sharedApplication];
app.applicationIconBadgeNumber=123;

@property(nonatomic,getter=isNetworkActivityIndicatorVisible) BOOL networkActivityIndicatorVisible;

代码和效果: 
UIApplication *app=[UIApplication sharedApplication];
//设置指示器的联网动画
app.networkActivityIndicatorVisible=YES;

通过UIApplication管理（一个应用程序的状态栏都由它统一管理）

 如果想利用UIApplication来管理状态栏，首先得修改Info.plist的设置

 View controller-based status bar appearance ：NO
 Status bar is initially hidden  ：NO
 Status bar style ：Opaque black style

URL补充：
URL：统一资源定位符，用来唯一的表示一个资源。
URL格式:协议头：//主机地址/资源路径
网络资源：http/ ftp等   表示百度上一张图片的地址   http://www.baidu.com/images/20140603/abc.png
本地资源：file:///users/apple/desktop/abc.png(主机地址省略)

  # 当应用程序启动完毕的时候就会调用(系统自动调用)
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions

 # 即将失去活动状态的时候调用(失去焦点, 不可交互)
 - (void)applicationWillResignActive:(UIApplication *)application

 # 重新获取焦点(能够和用户交互)
 - (void)applicationDidBecomeActive:(UIApplication *)application

  # 应用程序进入后台的时候调用
  # 一般在该方法中保存应用程序的数据, 以及状态
 - (void)applicationDidEnterBackground:(UIApplication *)application

 # 应用程序即将进入前台的时候调用
 #一般在该方法中恢复应用程序的数据,以及状态
 - (void)applicationWillEnterForeground:(UIApplication *)application

  # 应用程序即将被销毁的时候会调用该方法
  #注意:如果应用程序处于挂起状态的时候无法调用该方法
 - (void)applicationWillTerminate:(UIApplication *)application

  #应用级跳转（openURL的接收）
  - (BOOL)application:(UIApplication *)application   openURL:(NSURL *)url   sourceApplication:(NSString *)sourceApplication    annotation:(id)annotation 
  // NOTE: 9.0以后使用新API接口
  - (BOOL)application:(UIApplication *)app openURL:(NSURL *)url options:(NSDictionary<NSString*, id> *)options

  #注册通知、推送等
  - (void)application:(UIApplication *)application  didRegisterForRemoteNotificationsWithDeviceToken:(NSData *)deviceToken 
  -(void)application:(UIApplication *)application didFailToRegisterForRemoteNotificationsWithError:(NSError *)error 

  # 应用程序接收到内存警告的时候就会调用
  # 一般在该方法中释放掉不需要的内存
  - (void)applicationDidReceiveMemoryWarning:(UIApplication *)application

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

  // 1.创建窗口
  self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];

  *************************** 类文件**********************************
  //创建窗口的根控制器，并且赋值
  UIViewController *rootVc = [[UIViewController alloc]init];
    self.window.rootViewController = rootVc;
  ************************  main.storyboard  ****************************
  // 2.加载main.storyboard,创建main.storyboard描述的控制器
  // UIStoryboard专门用来加载stroyboard
  // name:storyboard名称不需要后缀
  UIStoryboard *stroyboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];

  // 加载sotryboard描述的控制器
  // 加载箭头指向的控制器
    UIViewController *vc = [stroyboard instantiateInitialViewController];
  //根据绑定标识加载 
  //UIViewController *vc = [stroyboard instantiateViewControllerWithIdentifier:@"red"];

  // 设置窗口的根控制器
  self.window.rootViewController = vc;

  *****************************  xib ****************************
  // 创建窗口的根控制器
  // 通过xib创建控制器
  ViewController *vc = [[ViewController alloc] initWithNibName:@"VC" bundle:nil];
  self.window.rootViewController = vc;

  // 3.显示窗口
  [self.window makeKeyAndVisible];
  return YES;
}

   UIWindowDidBecomeVisibleNotification
   UIWindowDidBecomeHiddenNotification
   UIWindowDidBecomeKeyNotification
   UIWindowDidResignKeyNotification

UIKIT_EXTERN const UIWindowLevel UIWindowLevelNormal; //默认，值为0
UIKIT_EXTERN const UIWindowLevel UIWindowLevelAlert; //值为2000 
UIKIT_EXTERN const UIWindowLevel UIWindowLevelStatusBar ; // 值为1000

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    self.window.backgroundColor = [UIColor yellowColor];
    [self.window makeKeyAndVisible];

    UIWindow *normalWindow = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
   normalWindow.backgroundColor = [UIColor blueColor];
    normalWindow.windowLevel = UIWindowLevelNormal;
    [normalWindow makeKeyAndVisible];

    CGRect windowRect = CGRectMake(50, 50,  [[UIScreen mainScreen] bounds].size.width- 100,   [[UIScreen mainScreen] bounds].size.height - 100);
    UIWindow *alertLevelWindow = [[UIWindow alloc] initWithFrame:windowRect];
    alertLevelWindow.windowLevel = UIWindowLevelAlert;
    alertLevelWindow.backgroundColor = [UIColor redColor];
      [alertLevelWindow makeKeyAndVisible];

   UIWindow *statusLevelWindow = [[UIWindow alloc] initWithFrame:CGRectMake(0, 50, 320, 20)];
    statusLevelWindow.windowLevel = UIWindowLevelStatusBar;
    statusLevelWindow.backgroundColor = [UIColor blackColor];
    [statusLevelWindow makeKeyAndVisible];

    NSLog(@"Normal window level: %f", UIWindowLevelNormal);
    NSLog(@"Normal window level: %f", UIWindowLevelAlert);
    NSLog(@"Normal window level: %f", UIWindowLevelStatusBar);

    return  YES;
}

int main(int argc, char * argv[]) {
    @autoreleasepool {
      return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}