请问一下，为什么我把系统相册中的gif保存到沙盒之后就变成了静态图片？

 ALAssetRepresentation *representation = [asset defaultRepresentation];
 NSString *document = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
            NSString *filePath = [document stringByAppendingPathComponent:representation.filename];
            Byte *buffer = (Byte*)malloc(representation.size);
            NSUInteger buffered = [representation getBytes:buffer fromOffset:0.0 length:representation.size error:nil];
            NSData *data = [NSData dataWithBytesNoCopy:buffer length:buffered freeWhenDone:YES];
            [data writeToFile:filePath atomically:YES];