@interface RootViewController ()@property (nonatomic, copy) UIScrollView *scrollView;@property (nonatomic, copy) UIView *headView;@property (nonatomic, copy) UIView *headView2;@end@implementation RootViewController- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view.    // 创建一个UIScrollView    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(10, 20, 300, 440)];    [self.view addSubview:_scrollView];    _scrollView.backgroundColor = [UIColor orangeColor];    _scrollView.contentSize = CGSizeMake(0, 2000);    _scrollView.delegate = self;    // 创建两个UIView并添加到scrollView上    _headView = [[UIView alloc] initWithFrame:CGRectMake(0, 200, 300, 50)];    [_scrollView addSubview:_headView];    _headView.backgroundColor = [UIColor grayColor];        _headView2 = [[UIView alloc] initWithFrame:CGRectMake(0, 800, 300, 40)];    [_scrollView addSubview:_headView2];    _headView2.backgroundColor = [UIColor colorWithRed:0.255 green:0.718 blue:1.000 alpha:1.000];    //为scrollView添加观察者    [_scrollView addObserver:self forKeyPath:@"contentOffset" options:(NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew) context:@"headView"];    }// 被观察者的属性发生变化就调用此方法- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{    if (context == @"headView") {        if ((_scrollView.contentOffset.y > 200 && _scrollView.contentOffset.y <= 760)) {            _headView.frame = CGRectMake(0, _scrollView.contentOffset.y, 300, 50);        }        if (_scrollView.contentOffset.y >= 760 && _scrollView.contentOffset.y <= 800.0) {            float y = 760;                        _headView.frame = CGRectMake(0, y, 300, 50);        }        if (_scrollView.contentOffset.y >= 800) {            _headView2.frame = CGRectMake(0, _scrollView.contentOffset.y, 300, 40);        }    }    else {        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];    }}@end