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
}