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Viewself.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layerCAGradientLayer *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CAGradientLayerUIView *containerView = [[UIView alloc]initWithFrame:self.addImageView.bounds];[containerView.layer addSublayer:gradientLayer];//设定maskViewself.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;}];