objective-c - CTLineGetGlyphRuns NSAttributedString 的奇怪行为

Question

我有一个字符串，假设"Hello"我需要在前面和末尾追加，","所以字符串看起来像这样：",Hello,"现在我希望文本是白色的，而“，”是不同的颜色。所以我做这样的事情：

[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange(range.location+1, range.length-1)];
[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor yellowColor] range:NSMakeRange(range.location, 1)];
[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor yellowColor] range:NSMakeRange(range.length-1, 1)];

后来我用CTFrameSetterRed,CGContextRef

CTLineRef line = CFArrayGetValueAtIndex((CFArrayRef)lines, index);
CFArrayRef glyphRuns = CTLineGetGlyphRuns(line);
CFIndex glyphCount = CFArrayGetCount(glyphRuns);

在这个",Hello,"例子中，glyphCount 返回了3HOWEVER！当我将代码更改为：

[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange(range.location+1, range.length-1)];
[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange(range.location, 1)];
[attributedString addAttribute:(__bridge NSString *)kCTForegroundColorAttributeName value:[UIColor whiteColor] range:NSMakeRange(range.length-1, 1)];

（现在整个消息“，你好”是白色的，这是唯一的变化） glyphCount 返回1 为什么会这样以及如何避免它？这看起来相当奇怪。