layer.mask 的使用

a.layer.mask = b.layer;
b 的不透明的部分，会变成a的内容的显示部分。

聊天气泡.png

代码：

     UIImageView *girlsImage = [[UIImageView alloc]initWithFrame:CGRectMake(30, 100, 192, 108)];
    girlsImage.image = [UIImage imageNamed:@"zisehua.jpg"];
    [self.view addSubview:girlsImage];
    
    
    UIImage *bumble = [UIImage imageNamed:@"bunble.png"];
    UIImageView *bumbleImage = [[UIImageView alloc]initWithFrame:girlsImage.frame];
    bumbleImage.image = [bumble stretchableImageWithLeftCapWidth:30 topCapHeight:30];
    
    CALayer *layer = bumbleImage.layer;
    layer.frame = girlsImage.bounds;
    girlsImage.layer.mask = layer;