ios列表性能优化tips

- (UIImage *)makeCircularImageWithSize:(CGSize)size
{
  
  CGRect circleRect = (CGRect) {CGPointZero, size};

  // begin the image context since we're not in a drawRect:
  UIGraphicsBeginImageContextWithOptions(circleRect.size, NO, 0);

  // create a UIBezierPath circle
  UIBezierPath *circle = [UIBezierPath bezierPathWithRoundedRect:circleRect cornerRadius:circleRect.size.width/2];
  
  // clip to the circle
  [circle addClip];
  
  // draw the image in the circleRect *AFTER* the context is clipped
  [self drawInRect:circleRect];
  
  // create a border (for white background pictures)

  circle.lineWidth = 1;
  [[UIColor darkGrayColor] set];
  [circle stroke];
#endif
  
  // get an image from the image context
  UIImage *roundedImage = UIGraphicsGetImageFromCurrentImageContext();

  // end the image context since we're not in a drawRect:
  UIGraphicsEndImageContext();
  
  return roundedImage;
}