为什么整个单词的 sizeWithFont 与其字母大小的总和不同？

Question

为什么整个单词的sizeWithFont 会产生与其字母大小之和不同的结果？

以下是带和不带字距调整的文本示例：

CGFloat fontSize = 36;
UIFont *font = [UIFont systemFontOfSize:fontSize];

NSArray *wordsToTest = @[@"IIIIIIIII", @"VAVAVAVAV"];

for (NSString *testWord in wordsToTest) {
    CGFloat widthOfTextWithFont = [testWord sizeWithFont:font].width;

    CGFloat widthOfTextUsingIteration = 0;
    for (int i = 0; i < [testWord length]; i++) {
        NSRange range = NSMakeRange(i, 1);
        NSString *nextLetter = [testWord substringWithRange:range];

        widthOfTextUsingIteration += [nextLetter sizeWithFont:font].width;
    }

    NSDictionary *attributesKerningDisabled = @{NSFontAttributeName : font,
                                                NSKernAttributeName : @0.0F};
    NSAttributedString *attributedStringWithDisabledKerning = [[NSAttributedString alloc] initWithString:testWord attributes:attributesKerningDisabled];


    NSDictionary *attributesKerningEnabled = @{NSFontAttributeName : font,
                                               NSKernAttributeName : @1.0F};
    NSAttributedString *attributedStringWithEnabledKerning = [[NSAttributedString alloc] initWithString:testWord attributes:attributesKerningEnabled];


    NSLog(@"Test for string: %@", testWord);
    NSLog(@"Length of the whole word: %f", widthOfTextWithFont);
    NSLog(@"Length of word, using iteration: %f", widthOfTextUsingIteration);
    NSLog(@"Length of attributed string with enabled kerning: %f", [attributedStringWithEnabledKerning size].width);
    NSLog(@"Length of attributed string with disabled kerning: %f", [attributedStringWithDisabledKerning size].width);
}

结果如下：

Test for string: IIIIIIIII
Length of the whole word: 84.000000
Length of word, using iteration: 90.000000
Length of attributed string with enabled kerning: 93.000000
Length of attributed string with disabled kerning: 84.000000

Test for string: VAVAVAVAV
Length of the whole word: 204.000000
Length of word, using iteration: 206.000000
Length of attributed string with enabled kerning: 200.000000
Length of attributed string with disabled kerning: 204.000000

这是 Apple 文档的一部分：

NSKernAttributeName 该属性的值是一个包含浮点值的 NSNumber 对象。此值指定调整紧缩对字符的点数。字距调整可防止特定字符之间出现不需要的空间，并且取决于字体。值 0 表示禁用字距调整。此属性的默认值为 0。

Answer 1

正如@AnindyaSengupta 所指出的，如果您将整个文本的所有字形推进的总和四舍五入，或者如果您将每个字形推进四舍五入，则向上取整是不同的。 说明差异的两个 CoreText 函数：

/* round up the the advances for the whole text */
static double getTextAdvances(NSString *text, CTFontRef font)
{
    unichar *characters = malloc(sizeof(unichar) * text.length);
    CGGlyph *glyphs = malloc(sizeof(CGGlyph) * text.length);
    CGSize *advances = malloc(sizeof(CGSize) * text.length);

    [text getCharacters:characters range:NSMakeRange(0, text.length)];
    CTFontGetGlyphsForCharacters(font, characters, glyphs, text.length);

    CTFontGetAdvancesForGlyphs(font, kCTFontHorizontalOrientation, glyphs, advances, text.length);

    free(characters);
    free(glyphs);

    CGFloat sum = 0.;
    for (NSUInteger i = 0; i < text.length; i++)
        sum += advances[i].width;

    free(advances);

    // round up the whole advances
    return ceilf(sum);
}

/* round up each glyphs advances */
static double getCharactersAdvancesSum(NSString *text, CTFontRef font)
{
    unichar *characters = malloc(sizeof(unichar) * text.length);
    CGGlyph *glyphs = malloc(sizeof(CGGlyph) * text.length);
    CGSize *advances = malloc(sizeof(CGSize) * text.length);

    [text getCharacters:characters range:NSMakeRange(0, text.length)];
    CTFontGetGlyphsForCharacters(font, characters, glyphs, text.length);

    CTFontGetAdvancesForGlyphs(font, kCTFontHorizontalOrientation, glyphs, advances, text.length);

    free(characters);
    free(glyphs);

    CGFloat sum = 0.;
    for (NSUInteger i = 0; i < text.length; i++)
        // round up advance width
        sum += ceilf(advances[i].width);

    free(advances);

    return sum;
}

    // test method
    CGFloat fontSize = 36;
    UIFont *font = [UIFont systemFontOfSize:fontSize];

    NSArray *wordsToTest = @[@"IIIIIIIII", @"VAVAVAVAV"];

    for (NSString *testWord in wordsToTest) {
        // … snip

        NSLog(@"Test for string: %@", testWord);
        NSLog(@"Length of the whole word: %f", widthOfTextWithFont);
        NSLog(@"Length of word, using iteration: %f", widthOfTextUsingIteration);
        // … skip kerning logs
        NSLog(@"Round up whole text advances %f", getTextAdvances(testWord, (CTFontRef)font));
        NSLog(@"Round up per Characters Advances %f", getCharactersAdvancesSum(testWord, (CTFontRef)font));
    }

测试结果：

Test for string: IIIIIIIII
Length of the whole word: 84.000000
Length of word, using iteration: 90.000000

Round up whole text advances 84.000000
Round up per Characters Advances 90.000000

Test for string: VAVAVAVAV
Length of the whole word: 204.000000
Length of word, using iteration: 206.000000

Round up whole text advances 204.000000
Round up per Characters Advances 206.000000

Answer 2

大概是因为字距调整。字距调整是改变连续字母之间的间距以使文本看起来更好读的过程。

当您获得完整字符串的大小时，将应用字距调整。对于单个字符，无需紧排，因此可以考虑 ho 间距。

(Kerning on wikipedia)

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我不相信您使用NSKernAttributeName 会关闭字距调整。我认为在这种情况下使用术语“紧缩”是不准确的，它实际上只是指字符之间的额外间距，而不是为字体定义的紧缩。