// 畫弧線
- (void)drawHalfCircle {
  loadingLayer = [self drawCircle];
   
  // 這個是用于指定畫筆的開始與結束點
  loadingLayer.strokeStart = 0.0;
  loadingLayer.strokeEnd = 0.75;
}
 
- (CAShapeLayer *)drawCircle {
  CGRect frame = CGRectMake(0, 0, self.frame.size.height, self.frame.size.height);
  CAShapeLayer *circleLayer = [CAShapeLayer layer];
  // 指定frame，只是為了設置寬度和高度
  circleLayer.frame = frame;
  // 設置居中顯示
  circleLayer.position = CGPointMake(self.frame.size.height/2, self.frame.size.height/2);
  // 設置填充顏色
  circleLayer.fillColor = [UIColor clearColor].CGColor;
  // 設置線寬
  circleLayer.lineWidth = 1;
  // 設置線的顏色
  circleLayer.strokeColor = kSelfborderColor.CGColor;
  // 使用UIBezierPath創(chuàng)建路徑
  UIBezierPath *circlePath = [UIBezierPath bezierPathWithOvalInRect:frame];
  // 設置CAShapeLayer與UIBezierPath關聯(lián)
  circleLayer.path = circlePath.CGPath;
  // 將CAShaperLayer放到某個層上顯示
  [self.layer addSublayer:circleLayer];
  return circleLayer;
}