加载网络图片时，根据Url获取图片的尺寸进行UI布局。

   可将方法+ (CGSize)getImageSizeWithURL:(id)URL;放在UIImage的分类中。

引入系统的ImageIO.framework

/**
 *  根据图片url获取图片尺寸
 */
+ (CGSize)getImageSizeWithURL:(id)URL{
    NSURL * url = nil;
    if ([URL isKindOfClass:[NSURL class]]) {
        url = URL;
    }
    if ([URL isKindOfClass:[NSString class]]) {
        url = [NSURL URLWithString:URL];
    }
    if (!URL) {
        return CGSizeZero;
    }
    CGImageSourceRef imageSourceRef =     CGImageSourceCreateWithURL((CFURLRef)url, NULL);
    CGFloat width = 0, height = 0;
    if (imageSourceRef) {
        CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSourceRef, 0, NULL);
      if (imageProperties != NULL) {
          CFNumberRef widthNumberRef = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
          if (widthNumberRef != NULL) {
              CFNumberGetValue(widthNumberRef, kCFNumberFloat64Type, &width);
          }
          CFNumberRef heightNumberRef = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
          if (heightNumberRef != NULL) {
            CFNumberGetValue(heightNumberRef, kCFNumberFloat64Type, &height);
        }
        CFRelease(imageProperties);
    }
    CFRelease(imageSourceRef);
    }
    return CGSizeMake(width, height);
}