如何缩短NSString可能包括例如。 Emojis到HFS +文件名允许的最大长度 [英] How to shorten an NSString that might include e.g. Emojis to the maximal length allowed for a HFS+ filename

问题描述

苹果文档说：

[...]当前文件系统，如HFS +（Mac OS X使用）创建具有255个字符限制的文件名[...]符号可能实际上需要相当于九个英文字符来存储[...]尝试创建更长的名称时应考虑这一点。

[...] current file systems such as HFS+ (used by Mac OS X) allow you to create filenames with a 255-character limit [...] symbols may actually take up to the equivalent of nine English characters to store [...] This should be considered when attempting to create longer names.

如何以一个真正短于255个字符的方式限制NSString的长度，即使它包含可能需要多个字符的符号store？

How do I limit the length of a NSString in a way that it is truly shorter than 255 characters, even when it includes symbols that might take more than one character to store?

我在下面添加了我的当前实现。如果我添加例如emojis到字符串，而 length 回答结果字符串将远小于255，它仍然太长，不能被 NSSavePanel 作为文件名。

I add my current implementation below. If i add e.g. emojis to the string, while length answers the resulting string would be by far smaller than 255, it is still too long to be accepted by a NSSavePanel as file name.

NSRange stringRange = {0, MIN([fileName length], 255)};
stringRange = [fileName rangeOfComposedCharacterSequencesForRange:stringRange];
fileName = [fileName substringWithRange:stringRange];

推荐答案

rangeOfComposedCharacterSequencesForRange：基本上是做与你想要的相反：你给它一个计数255组成字符的范围，它给你的字节范围，包括那些，可能会比你想要的更多。

rangeOfComposedCharacterSequencesForRange: is basically doing the opposite of what you want: you give it a range that counts 255 composed characters, and it gives you the byte range that encompasses those, which might end up being much more than you want.

不幸的是，相反，你必须手动计数字节。这不是太难，但 enumerateSubstringsInRange：options：usingBlock： 。传递 NSStringEnumerationByComposedCharacterSequences 为选项提供了它所说的：每个组成的字符轮流。然后你可以用 lengthOfBytesUsingEncoding：来计算每个的大小，传递你将使用的最终编码（大概是UTF-8）。

Unfortunately to do the reverse, you have to count the bytes manually. This isn't too hard, however, with enumerateSubstringsInRange:options:usingBlock:. Passing NSStringEnumerationByComposedCharacterSequences for the options gives you exactly what it says: each composed character in turn. You can then count the size of each with lengthOfBytesUsingEncoding:, passing the final encoding you'll be using (presumably UTF-8). Add up the bytes, keeping track of the character-based index, and stop when you've seen too many.

NSString * s = /* String containing multibyte characters */;
NSUInteger maxBytes = ...;
__block NSUInteger seenBytes = 0;
__block NSUInteger truncLength = 0;
NSRange fullLength = (NSRange){0, [s length]};

[s enumerateSubstringsInRange:fullLength
                      options:NSStringEnumerationByComposedCharacterSequences
                   usingBlock:
    ^(NSString *substring, NSRange substringRange,
      NSRange _, BOOL *stop)
    {
        seenBytes += [substring lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
        if( seenBytes > maxBytes ){
            *stop = YES;
            return;
        }
        else {
            truncLength += substringRange.length;
        }
}];

NSString * truncS = [s substringToIndex:truncLength];

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