如何缩短可能包括例如 NSString表情符号为 HFS+ 文件名允许的最大长度

Question

Apple 文档说：

[...] 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.

如何限制 NSString 的长度使其真正短于 255 个字符，即使它包含可能需要多个字符才能存储的符号？

我在下面添加我当前的实现。如果我添加例如表情符号添加到字符串，而 length 回答结果字符串将远小于 255，它仍然太长而无法被 NSSavePanel 接受为文件名。

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

Answer 1

根据@JoshCaswell 的建议，我确实将 this answer 修改为类似的问题。它显然有效（我写了几个测试），但对我来说似乎很奇怪。如此明显的任务不可能如此复杂地实现？

// filename contains the NSString that should be shortened
NSMutableString *truncatedString = [NSMutableString string];
NSUInteger bytesRead = 0;
NSUInteger charIdx = 0;

while (bytesRead < 250 && charIdx < [fileName length])
{
    NSRange range = [fileName rangeOfComposedCharacterSequencesForRange:NSMakeRange(charIdx, 1)];
    NSString *character = [fileName substringWithRange:NSMakeRange(charIdx, range.length)];
    bytesRead += [character lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
    charIdx = charIdx + range.length;
    if (bytesRead <= 250)
        [truncatedString appendString:character];
}

Answer 2

rangeOfComposedCharacterSequencesForRange: 基本上与你想要的相反：你给它一个计算 255 个组成字符的范围，它给你包含这些字符的字节范围，最终可能比你要。

不幸的是，要进行相反的操作，您必须手动计算字节数。然而，对于 enumerateSubstringsInRange:options:usingBlock:，这并不难。为选项传递 NSStringEnumerationByComposedCharacterSequences 可以准确地给出它所说的：依次组成每个字符。然后您可以使用 lengthOfBytesUsingEncoding: 计算每个的大小，传递您将使用的最终编码（大概是 UTF-8）。添加字节，跟踪基于字符的索引，当你看到太多时停止。

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];

如何缩短可能包括例如 NSString表情符号为 HFS+ 文件名允许的最大长度

How to shorten an NSString that might include e.g. Emojis to the maximal length allowed for a HFS+ filename

macos

cocoa

nsstring

foundation

ios