#import "ViewController.h"
#define kScreenbounds [UIScreen mainScreen].bounds
#define kScreenWidth [UIScreen mainScreen].bounds.size.width
#define kScreenHeight [UIScreen mainScreen].bounds.size.height
// 宏定義一個高度
#define pictureHeight 200
@interface ViewController ()<UITableViewDataSource, UITableViewDelegate>
@property (nonatomic, strong) UITableView *tableView;
@property (nonatomic, strong) UIImageView *pictureImageView;
@property (nonatomic, strong) UIView *header;
@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  
  self.navigationItem.title = @"向下拉伸放大圖片";
  // 下面兩個屬性的設(shè)置是與translucent為NO,坐標變換的效果一樣
  self.edgesForExtendedLayout = UIRectEdgeNone;
  self.automaticallyAdjustsScrollViewInsets = NO;
  [self createTableView];
  
}
- (void)createTableView
{
  self.tableView = [[UITableView alloc] initWithFrame:CGRectMake(0, 0, kScreenWidth, kScreenHeight - 64) style:UITableViewStylePlain];
  _tableView.delegate = self;
  _tableView.dataSource = self;
  [_tableView registerClass:[UITableViewCell class] forCellReuseIdentifier:@"cell"];
  
  // 添加頭視圖 在頭視圖上添加ImageView
  self.header = [[UIView alloc] initWithFrame:CGRectMake(0, 0, kScreenWidth, pictureHeight)];
  _pictureImageView = [[UIImageView alloc] initWithFrame:_header.bounds];
  _pictureImageView.image = [UIImage imageNamed:@"picture"];
  /* 
   重要的屬性設(shè)置
   */
  //這個屬性的值決定了 當(dāng)視圖的幾何形狀變化時如何復(fù)用它的內(nèi)容 這里用 UIViewContentModeScaleAspectFill 意思是保持內(nèi)容高寬比 縮放內(nèi)容 超出視圖的部分內(nèi)容會被裁減 填充UIView
  _pictureImageView.contentMode = UIViewContentModeScaleAspectFill;
  // 這個屬性決定了子視圖的顯示范圍 取值為YES時，剪裁超出父視圖范圍的子視圖部分.這里就是裁剪了_pictureImageView超出_header范圍的部分.
  _pictureImageView.clipsToBounds = YES;
  [_header addSubview:_pictureImageView];
  self.tableView.tableHeaderView = _header;
  [self.view addSubview:_tableView];
  
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{

  return 20;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cell"];
  cell.textLabel.text = @"向下拉我";
  return cell;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
  
  /**
   * 這里的偏移量是縱向從contentInset算起 則一開始偏移就是0 向下為負 上為正 下拉
   */
  
  // 獲取到tableView偏移量
  CGFloat Offset_y = scrollView.contentOffset.y;
  // 下拉 縱向偏移量變小 變成負的
    if ( Offset_y < 0) {
      // 拉伸后圖片的高度
      CGFloat totalOffset = pictureHeight - Offset_y;
      // 圖片放大比例
      CGFloat scale = totalOffset / pictureHeight;
      CGFloat width = kScreenWidth;
      // 拉伸后圖片位置
      _pictureImageView.frame = CGRectMake(-(width * scale - width) / 2, Offset_y, width * scale, totalOffset);
    }

}

- (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];
  // Dispose of any resources that can be recreated.
}

@end