导航栏透明下拉半透明

需求：

1. 导航栏透明
2. 下拉变色

实现：
第一步：创建基类
BaseScrollViewController 继承自UIViewController

@interface BaseScrollViewController () 

@property (nonatomic, strong) UIScrollView *scrollView;

@end

loadView：This is where subclasses should create their custom view hierarchy if they aren't using a nib. Should never be called directly.这是当他们没有正在使用nib视图页面，子类将会创建自己的自定义视图层。绝不能直接调用。
从nib/storyBoard载入视图 。除非你没有使用xib文件创建视图。

- (void)loadView{
    // 根视图更为滚动视图
    self.scrollView = [[UIScrollView alloc]initWithFrame:[UIScreen mainScreen].bounds];
    self.view = self.scrollView;
    self.scrollView.delegate = self;
}

viewWillAppear： Called when the view is about to made visible. Default does nothing视图即将可见时调用。默认情况下不执行任何操作。由于产品要求下一页的NavBar的和上页的并不同，所以在此方法里写，这样切换界面都可以调用到。

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    self.automaticallyAdjustsScrollViewInsets = NO;
    [self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
    // 设置透明
    [[[self.navigationController.navigationBar subviews]objectAtIndex:0] setAlpha:0];
    self.view.backgroundColor = [AXUtils colorWithString:@"ffffff"];
    self.navigationController.delegate = self;
    [self scrollViewDidScroll:self.scrollView];
    //    [self.navigationController.navigationBar setShadowImage:[UIImage new]];
}

viewWillDisappear ：Called when the view is dismissed, covered or otherwise hidden. Default does nothing视图被驳回时调用，覆盖或以其他方式隐藏。默认情况下不执行任何操作。
若是自定义的NavBar，在此reset销毁

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
//    [self.navigationController.navigationBar reset];
}

下拉变色的实现方法

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    CGFloat offsetY = scrollView.contentOffset.y;
    // 滑动超过设定值即半透明
    if (offsetY > ScaleHeight(50)) {
        CGFloat alpha = MIN(0.8, 1 - ((ScaleHeight(50) + 64 - offsetY) / 64));
        [self.navigationController.navigationBar setBackgroundImage:[AXUtils imageWithString:@"000000"] forBarMetrics:UIBarMetricsDefault];
        [[[self.navigationController.navigationBar subviews]objectAtIndex:0] setAlpha:alpha];
    } else {
        [[[self.navigationController.navigationBar subviews]objectAtIndex:0] setAlpha:0];
    }
}

总结：UI切图就好了，嘿嘿嘿~