实现 iOS 头部拉伸效果

主要涉及到导航栏透明度、图片拉伸、列表头部等。

1. 导航栏透明度的实现。
2. 列表拖动距离的监听，及图片放大的实现。

---

导航透明度的设置

添加系统导航栏的Category实现

声明部分：

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

实现部分：

#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

---

监听列表拖动及实现图片放大

主要是监听滚动的距离（scrollViewDidScroll:方法）

#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

以上是对系统原生的导航栏进行透明度设置。
也可进行自定义视图设置为导航栏

效果如下：