iOS 设置过度透明层

今天产品经理提出了一个过度透明层的需求，如下图，在此做个笔记以防忘记。

过度透明层效果图.png

具体实现

   CGRect frame = CGRectMake(20, 100, SCREEN_WIDTH-40, SCREEN_HEIGHT-200);
    /** 底层的父视图，用来设置透明色的 */
    UIView *gradientView = [[UIView alloc] initWithFrame:frame];
    gradientView.backgroundColor = [UIColor grayColor];
    [self.view addSubview:gradientView];

    /** 核心代码，设置逐渐透明色 */
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    NSArray *colors = [NSArray arrayWithObjects:
                       (id)[[UIColor colorWithWhite:0 alpha:0] CGColor],  //clearColor，透明度为0（显示为黑色，就像黑洞...）
                       (id)[[UIColor colorWithWhite:0 alpha:0.5] CGColor],//clearColor，透明度为0.5
                       (id)[[UIColor colorWithWhite:0 alpha:1] CGColor],  //clearColor，透明度为1（显示为透明）
                       nil];
    [gradientLayer setColors:colors];
    [gradientLayer setStartPoint:CGPointMake(0.0f, 0.0f)];
    [gradientLayer setEndPoint:CGPointMake(0.0f, 0.7f)];/** 设置过度透明层的高度 */
    [[gradientView layer] setMask:gradientLayer];
    [gradientLayer setFrame:gradientView.bounds];
    
    /** 把想要展示的内容加载到父视图 */
    UIScrollView *gradientScrolliew = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, frame.size.width, frame.size.height)];
    gradientScrolliew.backgroundColor = [UIColor whiteColor];
    gradientScrolliew.showsVerticalScrollIndicator = NO;
    gradientScrolliew.contentSize = CGSizeMake(SCREEN_WIDTH-40, (SCREEN_HEIGHT-200)*2);
    [gradientView addSubview:gradientScrolliew];
    
    UILabel *label = [[UILabel alloc] initWithFrame:frame];
    label.text = [self text];
    label.numberOfLines = 0;
    label.font = [UIFont systemFontOfSize:16];
    [gradientScrolliew addSubview:label];

实际上原理就是通过设置父视图的 alpha 透明度，子视图也会随着父视图的 alpha 变化而变化