KVO监听导航栏

监听tableView的偏移量改变导航栏的颜色

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    self.automaticallyAdjustsScrollViewInsets = NO;
    self.tableView = [[UITableView alloc]initWithFrame:CGRectMake(0, 64, [UIScreen mainScreen].bounds.size.width,[UIScreen mainScreen].bounds.size.height-64) style:(UITableViewStylePlain)];
    self.tableView.dataSource = self;
    self.tableView.delegate = self;
    [self.view addSubview:self.tableView];
    [self.tableView addObserver:self forKeyPath:@"contentOffset" options:(NSKeyValueObservingOptionNew) context:nil];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    CGFloat offset = self.tableView.contentOffset.y;
    NSLog(@"offset=%f",offset);

    CGFloat delta = offset/64.f + 1.f;
    delta = MAX(0, delta);
    NSLog(@"delta=%f",delta);
    self.navigationController.navigationBar.alpha = delta;
    
}