如何将NSValue转换为NSData并返回? [英] How to convert NSValue to NSData and back?

问题描述

A有一些NSValue(通过KVC valueForKey获取)，我需要将其附加到NSData对象，以便使用Game Center通过网络发送它.显然，我还需要将NSValue从NSData转换回更多的KVC(setValue:forKey:).

A have a number of NSValue (obtained via KVC valueForKey) that I need to append to an NSData object in order to send it over the network using Game Center. Obviously I will also need to convert the NSValue back from NSData for more KVC (setValue:forKey:).

我不知道每个NSValue的确切类型，所以我仅限于使用NSValue和NSData提供的接口.我应该提到NSValue绝不是指针类型.

I don't know the exact type of each NSValue, so I'm limited to using the interfaces provided by NSValue and NSData. I should mention that the NSValue are never pointer types.

我很惊讶没有[NSData dataWithValue:value]，也没有类似[value dataWithEncoding:]之类的东西.但是也许我是盲人，没有看到明显的选择.我曾考虑过使用getValue:将值复制到void*缓冲区中，但是除了使用objCType并将其与所有可能的类型进行比较之外，我应该如何确定缓冲区的长度?

I'm surprised that there's no [NSData dataWithValue:value] and neither something like [value dataWithEncoding:] or similar. But maybe I'm blind and not seeing the obvious choice. I thought about using getValue: to copy the value into a void* buffer, but how am I supposed to determine the buffer length other than by using objCType and comparing that with all possible types?

我应该怎么做?

注意: NSKeyedArchiver毫无疑问，因为它效率极低.将4字节的浮点型归档为142字节的NSData，将8字节的CGPoint归档为NSData时使用260字节.保持发送的数据量最少对我正在做的事情至关重要.

NOTE: NSKeyedArchiver is out of the question because it is terribly inefficient. A 4 Byte float is archived to a 142 Bytes NSData, a 8 Byte CGPoint uses 260 Bytes when archived to NSData. Keeping the amount of data sent to a minimum is crucial to what I'm doing.

推荐答案

Martin Gordon的答案越来越近了，但是幸运的是，您不必手动解析objCType字符串.有一个功能可以做到:这个问题引导我解决问题.

Martin Gordon's answer is getting close, but fortunately you don't have to manually parse the objCType string. There's a function that does that: NSGetSizeAndAlignment. From that you get the size/length of the value obtained from getValue:. This question lead me to the solution.

我最终为NSData创建了一个类别，该类别使我可以从NSNumber或NSValue对象创建NSData:

I ended up creating a category for NSData that allows me to create NSData from NSNumber or NSValue objects:

@interface NSData (GameKitCategory)
+(NSData*) dataWithValue:(NSValue*)value;
+(NSData*) dataWithNumber:(NSNumber*)number;
@end

@implementation NSData (GameKitCategory)
+(NSData*) dataWithValue:(NSValue*)value
{
    NSUInteger size;
    const char* encoding = [value objCType];
    NSGetSizeAndAlignment(encoding, &size, NULL);

    void* ptr = malloc(size);
    [value getValue:ptr];
    NSData* data = [NSData dataWithBytes:ptr length:size];
    free(ptr);

    return data;
}

+(NSData*) dataWithNumber:(NSNumber*)number
{
    return [NSData dataWithValue:(NSValue*)number];
}
@end

我还在此NSValue/NSNumber数据之前添加了一个小标头，该标头使我可以解码该数据所针对的属性以及数据部分中的字节数.这样，我可以将值恢复到remote属性.

I also add a small header before this NSValue/NSNumber data that allows me to decode which property the data is for and how many bytes are in the data section. With that I can restore the value to the remote property.

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