ios 为什么不能在viewdidlaod里面使用贝塞尔曲线

oid)viewDidLoad {
[super viewDidLoad];

//创建出CAShapeLayer
self.shapeLayer = [CAShapeLayer layer];
self.shapeLayer.frame = CGRectMake(0, 0, 200, 200);//设置shapeLayer的尺寸和位置
self.shapeLayer.position = self.view.center;
self.shapeLayer.fillColor = [UIColor clearColor].CGColor;//填充颜色为ClearColor

//设置线条的宽度和颜色
self.shapeLayer.lineWidth = 1.0f;
self.shapeLayer.strokeColor = [UIColor redColor].CGColor;

//创建出圆形贝塞尔曲线
UIBezierPath *circlePath = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, 200, 200)];

//让贝塞尔曲线与CAShapeLayer产生联系
self.shapeLayer.path = circlePath.CGPath;

//添加并显示