计算UILabel上某段文字的frame

可扩展为:在文本的最后面添加按钮，为按钮的位置就是根据下面返回的rect变化来的。

@implementation UILabel (TextRect)
- (CGRect)boundingRectForCharacterRange:(NSRange)range
{
    NSTextStorage *textStorage = [[NSTextStorage alloc] initWithAttributedString:[self attributedText]];
    NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
    [textStorage addLayoutManager:layoutManager];
    NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:[self bounds].size];
    textContainer.lineFragmentPadding = 0;
    [layoutManager addTextContainer:textContainer];
    NSRange glyphRange;
    [layoutManager characterRangeForGlyphRange:range actualGlyphRange:&glyphRange];
    return [layoutManager boundingRectForGlyphRange:glyphRange inTextContainer:textContainer];
}

参考：多年iOS开发经验总结汇总 #46