iOS tableView实现头部拉伸并改变导航条渐变色

#import "TableViewController.h"

static NSString *ident = @"cell";

#define RGBA(r,g,b,a) [UIColor colorWithRed:r/255.0f green:g/255.0f blue:b/255.0f alpha:a]
#define RGB(r,g,b) RGBA(r,g,b,1.0f)
#define ZhuTiColor RGB(76,16,198)
#define ZhuTiColorAlpha(alpha) RGBA(76, 16, 198, alpha)

// 判断是否是iPhone X
#define iPhoneX ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? CGSizeEqualToSize(CGSizeMake(1125, 2436), [[UIScreen mainScreen] currentMode].size) : NO)
// 状态栏高度
#define STATUS_BAR_HEIGHT (iPhoneX ? 44.f : 20.f)
// 导航栏高度
#define NAVIGATION_BAR_HEIGHT (iPhoneX ? 88.f : 64.f)
// tabBar高度
#define TAB_BAR_HEIGHT (iPhoneX ? (49.f + 34.f) : 49.f)
// home indicator
#define HOME_INDICATOR_HEIGHT (iPhoneX ? 34.f : 0.f)

#define ScreenWidth ([UIScreen mainScreen].bounds.size.width)
#define ScreenHeight ([UIScreen mainScreen].bounds.size.height)

#define imageHight 200


@interface TableViewController ()

@property (nonatomic,strong) UIImageView *headImage;
@property (nonatomic, strong) UIView *headerBackView;
@property (nonatomic, strong) UIView *mengView;

@end

@implementation TableViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  [self.tableView registerClass:[UITableViewCell class] forCellReuseIdentifier:ident];
  self.view.backgroundColor = [UIColor redColor];
  self.tableView.tableHeaderView = self.headerBackView;
  [self.headerBackView addSubview:self.headImage];
  [self.headImage addSubview:self.mengView];
  [self navCleanFromAlpha:0];
  
}

-(void)navCleanFromAlpha:(CGFloat)alpha
{
  [self.navigationController.navigationBar setBackgroundImage:[self createImageWithColor:ZhuTiColorAlpha(alpha)] forBarMetrics:UIBarMetricsDefault];
  self.navigationController.navigationBar.shadowImage = [UIImage new];
}

-(UIImage*) createImageWithColor:(UIColor*) color
{
  CGRect rect=CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);
  UIGraphicsBeginImageContext(rect.size);
  CGContextRef context = UIGraphicsGetCurrentContext();
  CGContextSetFillColorWithColor(context, [color CGColor]);
  CGContextFillRect(context, rect);
  UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return theImage;
}

-(UIImageView *)headImage
{
  if(!_headImage)
  {
    _headImage= [[UIImageView alloc]initWithFrame: self.headerBackView.bounds];
    _headImage.image = [UIImage imageNamed:@"1024"];
  }
  return _headImage;
}

-(UIView *)mengView
{
  if (!_mengView)
  {
    _mengView = [[UIView alloc]initWithFrame:self.headerBackView.bounds];
    _mengView.backgroundColor = RGBA(1, 1, 1, 0.1);
  }
  return _mengView;
}

-(UIView *)headerBackView
{
  if (!_headerBackView)
  {
    _headerBackView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, ScreenWidth, imageHight)];
    [_headerBackView setBackgroundColor:[UIColor lightGrayColor]];
  }
  return _headerBackView;
}

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

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
  //---------------------- 图片拉升 -------------------------
  //图片高度
  CGFloat imageHeight = self.headerBackView.frame.size.height;
  //图片宽度
  CGFloat imageWidth = ScreenWidth;
  //图片上下偏移量
  CGFloat imageOffsetY = scrollView.contentOffset.y;
  
//  NSLog(@"图片上下偏移量 imageOffsetY:%f ->",imageOffsetY);
  
  //上移
  if (imageOffsetY < 0)
  {
    CGFloat totalOffset = imageHeight + ABS(imageOffsetY);
    CGFloat f = totalOffset / imageHeight;
    
    self.headImage.frame = CGRectMake(-(imageWidth * f - imageWidth) * 0.5, imageOffsetY, imageWidth * f, totalOffset);
    self.mengView.frame = self.headImage.bounds;
  }
  
  //------------------- 导航条颜色渐变 ----------------------------
  
  CGFloat tableViewOffsetY = [self.tableView rectForSection:0].origin.y - NAVIGATION_BAR_HEIGHT;
  CGFloat contentOffsetY = scrollView.contentOffset.y;

  if (contentOffsetY >= tableViewOffsetY)
  {
//    scrollView.contentOffset = CGPointMake(0, tableViewOffsetY); //定位
    [self navCleanFromAlpha:1];

  }
  else
  {
    CGFloat alpha = scrollView.contentOffset.y/imageHight;
    
    if (alpha >= 1) { alpha = 1; }
    if (alpha <= 0) { alpha = 0; }
    NSLog(@"%.2f",alpha);
    [self navCleanFromAlpha:alpha];
  }
  
  
}

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


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ident forIndexPath:indexPath];
  
  // Configure the cell...
  
  cell.textLabel.text = [NSString stringWithFormat:@"asdada = %zd",indexPath.row];
  
  return cell;
}