图片下拉放大

//  ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UITableViewController

@end

//  ViewController.m

#import "ViewController.h"

const CGFloat HMTopViewH = 350; // 设置图片高度

@interface ViewController ()

@property (nonatomic, weak) UIImageView *topView;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
//  1.  self.tableView.tableHeaderView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"biaoqingdi"]];
    // 设置内边距(cell往下移动一段距离)
    self.tableView.contentInset = UIEdgeInsetsMake(HMTopViewH * 0.5, 0, 0, 0);

    UIImageView *topView = [[UIImageView alloc] init];
    topView.image = [UIImage imageNamed:@"biaoqingdi"];
    topView.frame = CGRectMake(0, -HMTopViewH, 320, HMTopViewH);
    topView.contentMode = UIViewContentModeScaleAspectFill; // 关键代码
//    [self.tableView addSubview:topView];
    [self.tableView insertSubview:topView atIndex:0];
    self.topView = topView;

}

#pragma mark - 数据源方法
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return 20;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *ID = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ID];

    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:ID];
    }

    cell.textLabel.text = [NSString stringWithFormat:@"测试数据--%d", indexPath.row];

    return cell;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    // 向下拽了多少距离
    CGFloat down = -(HMTopViewH * 0.5) - scrollView.contentOffset.y;
    if (down < 0) {
        return;
    }

    CGRect frame = self.topView.frame;
    // 5决定图片变大的速度，值越大，速度越快
    frame.size.height = HMTopViewH + down * 5; // 关键代码
    self.topView.frame = frame;

}

@end