IOS学习（一）视图创建

抛弃storyBoard，纯代码
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
//大小与屏幕保持一致
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
[self.window setBackgroundColor:[UIColor whiteColor]];

//让window显示在手机屏幕上
[self.window makeKeyAndVisible];
//xcode7之后，必须给windwo设置一个根控制器，否则运行会报错
[self.window setRootViewController:[[MainViewController alloc] init]];

return YES;
}