向下放大，向上悬停效果

//
//  ViewController.m
//  detailView
//
//  Created by 殷婷婷 on 16/5/20.
//  Copyright © 2016年 yin. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()<UITableViewDelegate,UITableViewDataSource>
@property (weak, nonatomic) IBOutlet UITableView *tableView;

@property(nonatomic,weak) UIImageView *imgV;
@end
static CGFloat tableHeaderViewH = 300;
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    UIImageView *imgV = [[UIImageView alloc]initWithFrame:CGRectMake(0, -tableHeaderViewH, self.view.bounds.size.width, tableHeaderViewH)];
    imgV.image = [UIImage imageNamed:@"1.jpg"];
    imgV.contentMode = UIViewContentModeScaleAspectFill;
    //self.tableView.tableHeaderView = imgV;
    self.imgV = imgV;
    [self.tableView insertSubview:imgV atIndex:0];
    self.tableView.contentInset = UIEdgeInsetsMake(tableHeaderViewH * 0.5, 0, 0, 0);
   //此时tableview的contentoffset.y = - tableHeaderViewH * 0.5,scrollDidscroll打印。。。
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
    return 20;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    static int i = 1;
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cell"];
    if (!cell)  {
        cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"cell"];
    }
    cell.textLabel.text = [NSString stringWithFormat:@"%d",i];
    i++;
    return cell;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
    
    NSLog(@"%f",  -scrollView.contentOffset.y - tableHeaderViewH * 0.5);
    
    CGFloat delta = -scrollView.contentOffset.y - tableHeaderViewH * 0.5;
    if (delta <= 0) {
        [self.tableView bringSubviewToFront:self.imgV];
        CGRect frame = self.imgV.frame;
        frame.origin.y = -tableHeaderViewH - delta;
        self.imgV.frame = frame;
        return;
    }
     [self.tableView insertSubview:self.imgV atIndex:0];
    CGRect frame = self.imgV.frame;
    frame.size.height = tableHeaderViewH + delta;
    self.imgV.frame = frame;
    
}
@end