UITableView头部ImageView下拉放大效果，导航栏透明渐变

#import "ViewController.h"
#import "UINavigationBar+Awesome.h"

#define ImageWidth [[UIScreen mainScreen] bounds].size.width
static CGFloat imageH = 200;
static CGFloat navH = 64;

@interface ViewController ()<UITableViewDelegate,UITableViewDataSource>

@property (strong, nonatomic) UITableView *tableView;
@property (nonatomic, strong) UIImageView *headerView;
@property (nonatomic, strong) UIImage *shadowImage;

@end

@implementation ViewController

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.

self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc]initWithTitle:@"返回" style:UIBarButtonItemStyleDone target:self action:nil];
self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc]initWithTitle:@"确定" style:UIBarButtonItemStyleDone target:self action:nil];

self.automaticallyAdjustsScrollViewInsets = NO;

self.tableView = [[UITableView alloc]initWithFrame:self.view.bounds style:UITableViewStylePlain];
self.tableView.delegate = self;
self.tableView.dataSource = self;
self.tableView.contentInset = UIEdgeInsetsMake(imageH, 0, 0, 0);
[self.view addSubview:self.tableView];

self.headerView = [[UIImageView alloc]init];
self.headerView.frame = CGRectMake(0, -imageH, ImageWidth, imageH);
self.headerView.image = [UIImage imageNamed:@"IMG_0106.JPG"];
self.headerView.contentMode = UIViewContentModeScaleAspectFill;
[self.tableView addSubview:self.headerView];
[self.tableView insertSubview:self.headerView atIndex:0];
}

- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];

self.shadowImage = self.navigationController.navigationBar.shadowImage;
[self.navigationController.navigationBar setShadowImage:[UIImage new]];

CGFloat offsetY = self.tableView.contentOffset.y;
[self changeNavAlphaWithConnentOffset:offsetY];
}

- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:animated];

[self.navigationController.navigationBar lt_reset];
self.navigationController.navigationBar.shadowImage = self.shadowImage;
}

-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return 50;
}

-(CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
return 50;
}

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *ID = @"XXXX";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ID];
if (!cell) {
cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:ID];
}
cell.textLabel.text = [NSString stringWithFormat:@"%ld",indexPath.row];
return cell;
}

-(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
UIViewController *vc = [[UIViewController alloc]init];
vc.view.backgroundColor = [UIColor whiteColor];
[self.navigationController pushViewController:vc animated:YES];
}

-(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGFloat offsetY = scrollView.contentOffset.y;
NSLog(@"%f",offsetY);

if (offsetY < -imageH) {
NSLog(@"开始改变");
CGRect f = self.headerView.frame;
f.origin.y = offsetY;
f.size.height =  -offsetY;
self.headerView.frame = f;
}

[self changeNavAlphaWithConnentOffset:offsetY];
}

-(void)changeNavAlphaWithConnentOffset:(CGFloat)offsetY
{
UIColor *color = [UIColor colorWithRed:246/255.0 green:246/255.0 blue:246/255.0 alpha:1];
if (offsetY > -navH * 2 ) {
NSLog(@"渐渐不透明");
CGFloat alpha = MIN(1, 1 - ((-navH * 2 + navH - offsetY) / navH));
[self.navigationController.navigationBar lt_setBackgroundColor:[color colorWithAlphaComponent:alpha]];
self.title = @"个人主页";
}
else {
NSLog(@"渐渐透明");
[self.navigationController.navigationBar lt_setBackgroundColor:[color colorWithAlphaComponent:0]];
self.title = @"";
}
}