maskView与CAGradientLayer那回事儿

maskView基本原理

• png图片透明像素的原理
• maskView可类比于多张png图片的叠加遮罩，原理类似
• maskView是iOS8以上才有的，如果要考虑兼容低版本，用maskLayer替代

//使用maskView的情况
@property (nonatomic, strong)UIImageView *addImageView; //声明叠加图片View


self.addImageView = [[UIImageView alloc]initWithFrame:CGRectMake(50, 50, 200, 200)];
    [self.view addSubview:self.addImageView];
self.addImageView.image = [UIImage imageNamed:@"base"];//一张底图
UIImageView *mask = [[UIImageView alloc]initWithFrame:CGRectMake(0, 0, 200, 200)];
mask.image = [UIImage imageNamed:@"mask"];//一张png图片，左边黑色，渐变到右边白色
self.addImageView.maskView = mask;//生成了叠加在一起一边半透明的图片，左边黑色部分正常显示，右边渐变白色显示模糊到无

注意：maskView并不能用addSubview来添加遮罩

maskView配合CAGradientLayer的使用

• 用CAGradientLayer直接产生带透明像素通道的layer
• 用maskView直接加载带CAGradientLayer的view
• 可以通过对CAGradientLayer进行动画的操作实现动画效果

看实例代码

    self.addImageView = [[UIImageView alloc]initWithFrame:CGRectMake(50, 450, 200, 200)];
    [self.view addSubview:self.addImageView];
    self.addImageView.image = [UIImage imageNamed:@"base"];//一张底图
    //创建出渐变的layer
    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.frame = self.addImageView.bounds;
    gradientLayer.colors = @[(__bridge id)[UIColor clearColor].CGColor,
                             (__bridge id)[UIColor blackColor].CGColor,
                             (__bridge id)[UIColor clearColor].CGColor];
    gradientLayer.locations = @[@(0.05),@(0.5),@(0.95)];
    //设置渐变方向
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint = CGPointMake(1, 0);
    //容器view ->用于加载创建出的CAGradientLayer
    UIView *containerView = [[UIView alloc]initWithFrame:self.addImageView.bounds];
    [containerView.layer addSublayer:gradientLayer];
    //设定maskView
    self.addImageView.maskView = containerView;
    CGRect frame = containerView.frame;
    frame.origin.x -=200;
    containerView.frame = frame;
    //给maskView做动画效果
    [UIView animateWithDuration:3.f animations:^{
        //改变位移
        CGRect frame = containerView.frame;
        frame.origin.x += 400;
        //重新赋值
        containerView.frame = frame;
    }];

我们不仅可以使用CAGradientLayer创建的蒙版，我们还可以对他进行动画效果的设置