IOS引导页的编写

我们在第一次打开App的时候，通常不是直接进入App主界面，而是会有一个能左右滑动、介绍App功能的界面。我是用NSUserDefaults + UIScrollview实现。
新建一个类，继承UIView，假设名为Guide。在initWithFrame方法里:

  CGFloat screenHeight = [UIScreen mainScreen].bounds.size.height;
  UIScrollView* scrollView = [[UIScrollView alloc] initWithFrame:frame];
  scrollView.backgroundColor = [UIColor whiteColor];
  scrollView.showsHorizontalScrollIndicator = NO;
  scrollView.showsVerticalScrollIndicator = NO;
  scrollView.contentSize = CGSizeMake(320*4, screenHeight);
  scrollView.pagingEnabled = YES;
  for (int i=0; i<4; i++) {
    UIImageView* imageView = [[UIImageView alloc initWithFrame:CGRectMake(i*320, 0, 320, screenHeight)];
  imageView.contentMode = UIViewContentModeScaleAspectFill;
  NSString *filePath = [[NSBundle mainBundle] pathForResource:
              [NSString stringWithFormat:@"FileName"
                        ofType:@"FileType"];
  imageView.image = [UIImage imageWithContentsOfFile:filePath];
  [scrollView addSubview:imageView];
  if (i == 3) {
    UIButton* start = [UIButton buttonWithType:UIButtonTypeCustom];
    start.frame = CGRectMake(0, 0, 100, 44);
    start.layer.cornerRadius = 5;
    start.layer.borderWidth = 0.5;
    [start setCenter:CGPointMake(1120, iPhone5?450:400)];
    [start setTitleColor:[UIColor grayColor] forState:UIControlStateNormal];
    [start addTarget:self action:@selector(closeView) forControlEvents:UIControlEventTouchUpInside];
    [start setTitle:@"Start" forState:UIControlStateNormal];
    [scrollView addSubview:start];
  }

这样，就有了一个有4张图片的引导页。
怎么去判断是不是第一次登陆呢，需要用到NSUserDefaults类。
在didFinishLaunchingWithOptions:函数中可以这样判断：

NSUserDefaults* userDefaults = [NSUserDefaults standardUserDefaults];
if([userDefaults objectForKey:@"FirstLoad"] == nil) {
    [userDefaults setBool:NO forKey:@"FirstLoad"];
    //显示引导页
}