iOS实现scrollView或者scrollView的子类下拉图片放大的效果

代码是通过Tableview来说明的，用在其他情况下同样适用

- (void)viewDidLoad {
    [super viewDidLoad];
    _imageview = [[UIImageView alloc]init];
    _imageview.image = [UIImage imageNamed:@"F2.jpg"];
    self.imageview.frame =CGRectMake(0, -150, self.tableView.frame.size.width, 150);
    //此处不能通过添加headerView的方式，因为添加headerview只能设置view的高
    [self.tableView addSubview:_imageview];
    self.tableView.contentInset = UIEdgeInsetsMake(150, 0, 0, 0);
    
}


- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return 100;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell * cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:nil];
    cell.textLabel.text = @"我是cell";
    return cell;
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}


-(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    //通过滑动的便宜距离重新给图片设置大小
    CGFloat yOffset = scrollView.contentOffset.y;
    if(yOffset<-150)
    {
        CGRect f= self.imageview.frame;
        f.origin.y= yOffset;
        f.size.height = -yOffset;
        self.imageview.frame = f;
    }
}