ios - NSString 绘制矩形 :withAttributes: not correctly centered when using NSKernAttributeName

Question

当我使用 drawInRect:withAttributes: 并传入带有 NSTextAlignmentCenter 的段落样式和 NSKernAttributeName 的非零值时，字符串没有正确居中。我做错了什么还是这是预期的行为？有解决方法吗？

截图:

您可以清楚地看到顶部文本没有正确居中。

我的演示代码:

- (void)drawRect:(CGRect)rect
{
    // Drawing code
    UIFont *font = [UIFont systemFontOfSize:15.0];
    [self drawString:@"88" inRect:rect font:font textColor:[UIColor blackColor]];

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);
    CGContextAddRect(context, rect);
    CGContextStrokePath(context);
    CGContextRestoreGState(context);
}

- (void)drawString:(NSString *)text
            inRect:(CGRect)contextRect
              font:(UIFont *)font
         textColor:(UIColor *)textColor
{
    CGFloat fontHeight = font.lineHeight;
    CGFloat yOffset = floorf((contextRect.size.height - fontHeight) / 2.0) + contextRect.origin.y;

    CGRect textRect = CGRectMake(contextRect.origin.x, yOffset, contextRect.size.width, fontHeight);

    NSMutableParagraphStyle *paragraphStyle = [[NSParagraphStyle defaultParagraphStyle] mutableCopy];
    paragraphStyle.lineBreakMode = NSLineBreakByClipping;
    paragraphStyle.alignment = NSTextAlignmentCenter;

    [text drawInRect:textRect withAttributes:@{NSKernAttributeName: @(self.kerning),
                                            NSForegroundColorAttributeName: textColor,
                                            NSFontAttributeName: font,
                                            NSParagraphStyleAttributeName: paragraphStyle}];
}

谢谢!

更新，感谢this comment ，我将 NSBackgroundColorAttributeName: [UIColor greenColor] 添加到属性中并得到以下结果:

Answer 1

紧缩只能应用于每个紧缩对的第一个 字符。如果要在所有 n 字符之间显示带字距调整的字符串，则字距调整必须为前 n-1 个字符设置属性。

代替:

[text drawInRect:textRect withAttributes:@{NSKernAttributeName: @(self.kerning),
                                        NSForegroundColorAttributeName: textColor,
                                        NSFontAttributeName: font,
                                        NSParagraphStyleAttributeName: paragraphStyle}];

您必须创建一个属性字符串，以便您可以设置字距调整属性对于特定范围而不是整个字符串:

NSMutableAttributedString *as = [[NSMutableAttributedString alloc]
                                 initWithString:text
                                 attributes:@{
                                              NSForegroundColorAttributeName: textColor,
                                              NSFontAttributeName: font,
                                              NSParagraphStyleAttributeName: paragraphStyle}];
[as addAttribute:NSKernAttributeName
           value:@(self.kerning)
           range:NSMakeRange(0, [text length] - 1)];

[as drawInRect:textRect];

这里是字符串“1234”和字距调整 -4.0 的结果: