*keyWindow指向的视图才可以被看到，如果指向其他的视图，那么原来的视图会被释放掉
1.获取到 Storyboard
1.1通过名字
UIStoryboard*storyboard = [UIStoryboardstoryboardWithName:@"Main"bundle:nil];
1.2通过 self.storyboard
UIStoryboard*storyboard =self.storyboard;
2.获取到 storyboard 的视图控制器
2.1通过 Storyboard ID
self.view.window.rootViewController= [storyboardinstantiateInitialViewController];
self.view.window.rootViewController= [storyboardinstantiateViewControllerWithIdentifier:@"viewController"];
3.获取到使用 xib 创建视图控制器
MyViewController*vc = [[MyViewControlleralloc]initWithNibName:@"MyViewController"bundle:nil];
4.代码设置导航栏的根视图控制器
MyNavigationController*nv = [[MyNavigationControlleralloc]initWithRootViewController:vc];
5.通过 keyWindow 来获取到视图控制器
_playingVC= [[UIApplicationsharedApplication].keyWindow.rootViewController.storyboardinstantiateViewControllerWithIdentifier:@"ViewController"];
6.keyWindow 来设置根视图控制器，可以直接跳转页面
[UIApplicationsharedApplication].keyWindow.rootViewController= [storyboardinstantiateInitialViewController];
7.自己的window
WelComeViewController*vc = [self.window.rootViewController.storyboardinstantiateViewControllerWithIdentifier:@"WelComeViewController"];
8.拿到XIB创建的UIView的引用
SendMessageView*vc = [[NSBundlemainBundle]loadNibNamed:@"SendMessageView"owner:selfoptions:nil][0];
