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);
}
