通过服务器获取

后台开发人员在返回图片地址的同时，返回图片尺寸比例数据。

利用ImageIO创建CGImageSourceRef对象

此方法会下载图片，且同步执行。

CGImageSourceRef imageSource = CGImageSourceCreateWithURL((CFURLRef)[NSURL URLWithString:fileurl], NULL);
    CGFloat width = 0, height = 0;
    
    if (imageSource)
    {
        CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
        
        if (imageProperties != NULL)
        {
            CFNumberRef widthNum  = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
            if (widthNum != NULL) {
                CFNumberGetValue(widthNum, kCFNumberFloat64Type, &width);
            }
            
            CFNumberRef heightNum = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
            if (heightNum != NULL) {
                CFNumberGetValue(heightNum, kCFNumberFloat64Type, &height);
            }
            CFRelease(imageProperties);
        }
        CFRelease(imageSource);
    }