用kvo监听TableViewCell图片大小变化

使用三方SDWebImage, 使用sd_setImageWithUrl，设置占位图片，当TableViewCell上的图片没有刷新出来的时候，保证其位置不会被占用。
eg. [cell.titltImageView sd_setImageWithURL:[NSURL URLWithString:car.newsImage] placeholderImage:[UIImage imageNamed:@”zhanwei2_1”]];
//用kvo监听
方法:重点内容
static void *imageViewFrameKey = &imageViewFrameKey;

• (void)observeValueForKeyPath:(NSString )keyPath ofObject:(id)object change:(NSDictionary )change context:(void *)context;
• eg:

• (void)observeValueForKeyPath:(NSString )keyPath ofObject:(id)object change:(NSDictionary )change context:(void *)context
  {
  if (context == imageViewFrameKey) {

  //此时监听到imageView的frame发生变化，改成自定义的frame值
  UIImageView *imageView = (UIImageView *)object;
  CGRect newRect = CGRectMake(6, 4, 82, 51);
  if (!CGRectEqualToRect(imageView.frame, newRect)) {
      imageView.frame = newRect;
  }
  

  } else {
  [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
  }
  }重点内容