CAShapeLayer的strokeColor不工作

#import <UIKit/UIKit.h>

@interface UIView (Shape)

- (void)setShape:(CGPathRef)shape;

@end

#import "UIView+Shape.h"

@implementation UIView (Shape)
- (void)setShape:(CGPathRef)shape
{
    if (shape == nil) {
        self.layer.mask = nil;
    }

    CAShapeLayer* shapeLayer = [CAShapeLayer layer];
    shapeLayer.path = shape;
    shapeLayer.fillColor = [[UIColor whiteColor] CGColor];
    shapeLayer.strokeColor = [[UIColor redColor] CGColor];
    shapeLayer.lineWidth = 0.5;
    self.layer.mask = shapeLayer;

}
@end

这是我的密码。我想将形状的边框颜色设置为红色,但边框始终是透明的颜色。我必须添加一个子层,代码如下所示。它起作用了,为什么?

 - (void)setShape:(CGPathRef)shape strokeColor:(UIColor *)color
 {
    if (shape == nil) {
        self.layer.mask = nil;
    }

    CAShapeLayer* shapeLayer = [CAShapeLayer layer];
    shapeLayer.path = shape;
//    shapeLayer.fillColor = [[UIColor whiteColor] CGColor];
//    shapeLayer.strokeColor = [[UIColor redColor] CGColor];
//    shapeLayer.lineWidth = 0.5;
    self.layer.mask = shapeLayer;

    CAShapeLayer *borderLayer = [CAShapeLayer layer];
    borderLayer.path = shape;
    borderLayer.lineWidth = 2;
    borderLayer.fillColor = [[UIColor clearColor] CGColor];
    borderLayer.strokeColor = [color CGColor];
    [self.layer addSublayer:borderLayer];
}

对不起,我的英语不好。