storyboard兼容版本处理

//storyboard版本兼容

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  // 1
  UIStoryboard *mainStoryboard = nil;
  if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"6.0")) {
    mainStoryboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
  } else {
    mainStoryboard = [UIStoryboard storyboardWithName:@"MainStoryboard-legacy" bundle:nil];
  }
 
  // 2
  self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
  self.window.rootViewController = [mainStoryboard instantiateInitialViewController];
  [self.window makeKeyAndVisible];
 
  return YES;
}