导航栏控制器的渐变

效果图

实现步骤
1、设置导航栏视图控制器中的navigationBar背景图标

[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];

2、设置导航栏视图控制器中的navigationBar子视图的背景颜色

barView = self.navigationController.navigationBar.subviews.firstObject;
barView.backgroundColor = [UIColor redColor];

3、实现代理方法- (void)scrollViewDidScroll:(UIScrollView *)scrollView并在该方法中根据滑动偏移量计算背景色的透明度

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

    // 1
    if (offY >= 300.0) {
        barAlpha = 1.0;
    } else if (offY >= 0) {
        barAlpha = offY / 300.0;
    } else {
        barAlpha = 0.0;
    }
    barView.backgroundColor = [[UIColor redColor] colorWithAlphaComponent:barAlpha];
}

代码示例

@interface NavViewController () <UITableViewDelegate, UITableViewDataSource>
{
    UITableView *tableview;

    CGFloat barAlpha;
    UIView *barView;
}

@end

@implementation NavViewController

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

    self.navigationItem.title = @"导航栏渐变";

    self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"bottom" style:UIBarButtonItemStyleDone target:self action:@selector(bottomClick)];

    // 隐藏分割线
    self.navigationController.navigationBar.shadowImage = [UIImage new];
    // 设置导航栏背景图标
    [self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
    // 设置导航栏背景透明度
    barAlpha = 0.0;
    barView = self.navigationController.navigationBar.subviews.firstObject;
    barView.backgroundColor = [UIColor redColor];

    //
    tableview = [[UITableView alloc] initWithFrame:self.view.bounds style:UITableViewStylePlain];
    [self.view addSubview:tableview];
    tableview.autoresizingMask = UIViewAutoresizingFlexibleHeight;
    tableview.backgroundView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"tableBground"]];
    tableview.delegate = self;
    tableview.dataSource = self;
}

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

- (void)dealloc
{
    NSLog(@"dealloc: %@", self);
}

- (void)bottomClick
{
    [tableview setContentOffset:CGPointMake(0.0, (tableview.contentSize.height - tableview.bounds.size.height)) animated:YES];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return 100;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"UITableViewCell"];
    if (cell == nil)
    {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"UITableViewCell"];
        cell.backgroundColor = [UIColor colorWithWhite:0.5 alpha:0.5];
    }
    cell.textLabel.text = [NSString stringWithFormat:@"row: %@", @(indexPath.row + 1)];
    return cell;
}

- (BOOL)tableView:(UITableView *)tableView shouldHighlightRowAtIndexPath:(NSIndexPath *)indexPath
{
    return NO;
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    CGFloat offY = scrollView.contentOffset.y;
    if (offY >= 300.0) {
        barAlpha = 1.0;
    } else if (offY >= 0) {
        barAlpha = offY / 300.0;
    } else {
        barAlpha = 0.0;
    }
    barView.backgroundColor = [[UIColor redColor] colorWithAlphaComponent:barAlpha];
}

@end