CoreText 属性字符串高度计算不准确

Question

CoreText 没有给出属性字符串的正确高度（短了一行或更多）。我在 SO 上看到了很多关于此的帖子，但无法理解或找到解决方案。有人可以解释 Core Text 高度计算是如何工作的吗？这是我编写的示例代码，显示了不准确的高度计算。

上下文

我有一个集合视图，其中单元格的高度由其中的内容决定。 我在单元格中显示文本段落。我想通过使用核心文本进行高度计算来节省一些性能。我已经看到，通过核心文本的高度计算，我可以节省大约 300 毫秒。

代码

//高度计算

+ (CGFloat)getHeight
{
    NSString *text = @"The Apple HIG recommends to use a common color for links and buttons and we did just that. By using the same color throughout the app we trained the user to always associate blue to a link.The Apple HIG recommends to use a common color for links and buttons and we did just that.By using the same color throughout the app we trained the user to always associate blue to a link.";

    NSAttributedString *attrStr = [self attributedString:text withLinespacing:3 withLineBreakMode:NSLineBreakByWordWrapping];

    CTFramesetterRef frameSetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)(attrStr));
    CGSize suggestedSize = CTFramesetterSuggestFrameSizeWithConstraints(frameSetter,
                                                                    CFRangeMake(0, attrStr.length),
                                                                    NULL,
                                                                    CGSizeMake(320, 9999),
                                                                    NULL);

    return suggestedSize.height;
}

// Cell 即将显示时加载相同的文本

- (void)loadData
{
    NSString *text = @"The Apple HIG recommends to use a common color for links and buttons and we did just that.By using the same color throughout the app we trained the user to always associate blue to a link.The Apple HIG recommends to use a common color for links and buttons and we did just that.By using the same color throughout the app we trained the user to always associate blue to a link.";

    NSAttributedString *attrStr = [[self class] attributedString:text withLinespacing:3 withLineBreakMode:NSLineBreakByWordWrapping];

    // UILabel element
    self.textLabel.attributedText = attrStr;

    self.layer.borderColor = [UIColor blueColor].CGColor;
    self.layer.borderWidth = 1.0f;
}

// 生成带前导、字体和换行符的属性字符串

+ (NSAttributedString *)attributedString:(NSString *)string
                        withLinespacing:(CGFloat)linespacing
                      withLineBreakMode:(NSLineBreakMode)lineBreakMode
{
    NSMutableAttributedString *attrStr = [[NSMutableAttributedString alloc] initWithString:string];
    NSInteger strLength = [string length];
    NSMutableParagraphStyle *style = [[NSMutableParagraphStyle alloc] init];
    style.lineSpacing = linespacing;
    style.lineBreakMode = lineBreakMode;

    [attrStr addAttributes:@{NSParagraphStyleAttributeName: style,
                         NSFontAttributeName: [UIFont fontWithName:@"HelveticaNeue" size:15]} range:NSMakeRange(0, strLength)];

    return attrStr;
}

上面的代码使用core text计算高度和UILabel来显示文本。 UILabel 对单元格 {Top:17, Leading:13px, Trailing:13px}

有 3 个约束

Answer 1

CTFramesetterSuggestFrameSizeWithConstraints 已知有问题，返回不正确的高度值。您遇到的缺失行错误非常常见，据我所知，没有好的解决方案，只有丑陋的解决方法，无法提供 100% 准确的结果。

对于 iOS7 及更高版本，我建议迁移到 TextKit。不知何故，在内部执行的计算确实可以正常工作，同时也基于 Core Text。使用NSLayoutManager 的usedRectForTextContainer: 会返回正确的结果。

您可以看到更完整的答案here。虽然不是 100% 完全在主题上，但有一些关于 Core Text 计算的错误的讨论。