在iOS11之前，view展示圆角的处理过程为：
//Objective-C
UIView *view0 = [[UIView alloc]initWithFrame:CGRectMake(30, 100, 200, 40)];
view0.backgroundColor = [UIColor grayColor];
view0.layer.cornerRadius = 15;
//view0.clipsToBounds = YES;
view0.layer.masksToBounds = YES;
[self.view addSubview:view0];
//swift
let view = UIView()
view.clipsToBounds = true
view.layer.cornerRadius = 10
view.frame = .init(x: 30, y: 100, width: 200, height: 40)
view.backgroundColor = UIColor.lightGray
self.view.addSubview(view)
如此，便可以显示一个圆角的view：
image.png
iOS11对圆角功能进行了改善，layer新增了maskedCorners属性：
@property CACornerMask maskedCorners
CA_AVAILABLE_STARTING (10.13, 11.0, 11.0, 4.0);
CACornerMask是一个结构体类型：
typedef NS_OPTIONS (NSUInteger, CACornerMask)
{
kCALayerMinXMinYCorner = 1U << 0,
kCALayerMaxXMinYCorner = 1U << 1,
kCALayerMinXMaxYCorner = 1U << 2,
kCALayerMaxXMaxYCorner = 1U << 3,
};
该属性用于设置view哪个角显示圆角：
//Objective-C
if (@available(iOS 11.0, *)) {
view2.layer.maskedCorners = kCALayerMaxXMaxYCorner | kCALayerMaxXMinYCorner;
} else {
// Fallback on earlier versions
}
//swift
view2.layer.maskedCorners = [.layerMaxXMaxYCorner,.layerMinXMaxYCorner]
设置该属性便可以指定view圆角显示：
image.png
此外，iOS11中，view圆角的圆角也支持动画：
//Objective-C
[UIView animateWithDuration:2 delay:1.0 options:UIViewAnimationOptionCurveLinear animations:^{
view1.layer.cornerRadius = 0;
} completion:nil];
//swift
UIViewPropertyAnimator.init(duration: 1.0, curve: .linear) {
view3.layer.cornerRadius = 0
}.startAnimation()
311-3.gif
在iOS11之前，圆角是不支持UIView动画的。
