iOS实现头部拉伸效果

@interface UINavigationBar (BackgroundColor)
- (void)lt_setBackgroundColor:(UIColor *)color;
@end

#import <objc/runtime.h>

@implementation UINavigationBar (BackgroundColor)
static char overlayKey;

- (UIView *)overlay
{
  return objc_getAssociatedObject(self, &overlayKey);
}

- (void)setOverlay:(UIView *)overlay
{
  objc_setAssociatedObject(self, &overlayKey, overlay, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)lt_setBackgroundColor:(UIColor *)color
{
  if (!self.overlay) {
    [self setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
    // insert an overlay into the view hierarchy
    self.overlay = [[UIView alloc] initWithFrame:CGRectMake(0, -20, [UIScreen mainScreen].bounds.size.width, self.bounds.size.height + 20)];
    self.overlay.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight;
    [self insertSubview:self.overlay atIndex:0];
  }
  self.overlay.backgroundColor = color;
}

@end

#import "StretchViewController.h"
#import "UINavigationBar+BackgroundColor.h"

// 背景图片的宽高比例
#define ratio 0.8

@interface StretchViewController () <UITableViewDelegate, UITableViewDataSource>

// 可放大的背景图片
@property (nonatomic, strong) UIImageView *bgView;
// 记录原始大小
@property (assign) CGRect originalFrame;

@property (nonatomic, strong) UITableView *tableView;
@end

@implementation StretchViewController

- (void)viewWillAppear:(BOOL)animated
{
  [super viewWillAppear:animated];
  //[self.navigationController setNavigationBarHidden:YES animated:animated];

  //self.navigationController.navigationBar.tintColor = [UIColor whiteColor];
  //self.navigationController.navigationBar.barTintColor = [UIColor clearColor];
  //self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];
  // 设置导航栏底部分割线为透明
  [self.navigationController.navigationBar setShadowImage:[UIImage new]];
}

- (void)viewDidLoad {
  [super viewDidLoad];
  // 设置全透明
  [self.navigationController.navigationBar lt_setBackgroundColor:[[UIColor greenColor] colorWithAlphaComponent:0]];

  // Do any additional setup after loading the view.
  self.view.backgroundColor = [UIColor lightGrayColor];

  self.bgView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.width*ratio)];
  self.bgView.image = [UIImage imageNamed:@"bg-mine"];
  self.originalFrame = self.bgView.frame;
  [self.view addSubview:self.bgView];

  self.tableView = [[UITableView alloc] initWithFrame:CGRectMake(0, 64, self.view.bounds.size.width, self.view.bounds.size.height-64) style:UITableViewStylePlain];
  self.tableView.backgroundColor = [UIColor clearColor];
  self.tableView.showsVerticalScrollIndicator = NO;
  self.tableView.delegate = self;
  self.tableView.dataSource = self;

  // 1. contentInset
  //table.contentInset = UIEdgeInsetsMake(160, 0, 0, 0);

  // 2. heatView
  UIView *headView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, 160)];
  headView.backgroundColor = [UIColor clearColor];
  self.tableView.tableHeaderView = headView;
  [self.view addSubview:self.tableView];
}

- (nonnull UITableViewCell *)tableView:(nonnull UITableView *)tableView cellForRowAtIndexPath:(nonnull NSIndexPath *)indexPath {
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cellIdentifier"];
  if (cell == nil) {
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:@"cellIdentifier"];
  }
  cell.textLabel.text = @"测试数据";
  return cell;
}

- (NSInteger)tableView:(nonnull UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
  return 10;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{

  CGFloat yOffset = scrollView.contentOffset.y; // 向上滑动，offset是增加的；向下滑动，是减少的
  if (yOffset < 160) { // 当滑动到导航栏底部时
    CGFloat colorAlpha = yOffset/160;

//    self.navigationController.navigationBar.backgroundColor = [[UIColor whiteColor] colorWithAlphaComponent:colorAlpha];
    [self.navigationController.navigationBar lt_setBackgroundColor:[[UIColor whiteColor] colorWithAlphaComponent:colorAlpha]];

  } else { // 超过导航栏底部了
    [self.navigationController.navigationBar lt_setBackgroundColor:[UIColor whiteColor]];
  }

  // 往上滑动效果、处理放大效果
  if (yOffset > 0) {
    self.bgView.frame = ({
      CGRect frame = self.bgView.frame;
      frame.origin.y = self.originalFrame.origin.y - yOffset;
      frame;
    });
  } else { // 往下移动，放大效果
    self.bgView.frame = ({
      CGRect frame = self.originalFrame;
      frame.size.height = self.originalFrame.size.height - yOffset;
      frame.size.width = frame.size.height/ratio;

      //
      frame.origin.x = self.originalFrame.origin.x - (frame.size.width - self.originalFrame.size.width)/2;
      frame;
    });
  }

}
@end