为什么超链接有时不会显示在NSTextField中的NSAttribute字符串中?

我在NSTextField中使用的文本是从文件加载的,如下所示。

NSString *path = [[NSBundle mainBundle] pathForResource:@"Credits"  ofType:@"rtf"];
NSAttributedString *as = [[NSAttributedString alloc] initWithPath:path documentAttributes:NULL];
[creditsLabel setAttributedStringValue:as];
[creditsLabel becomeFirstResponder];

窗口中的超链接不会以蓝色下划线呈现,除非我首先单击NSTextField上的某个位置,如两个屏幕截图所示。

以下是RTF:

{\rtf1\ansi\ansicpg1252\cocoartf1038\cocoasubrtf250
{\fonttbl\f0\fnil\fcharset0 LucidaGrande;}
{\colortbl;\red255\green255\blue255;}
\pard\tx560\tx1120\tx1680\tx2240\tx2800\tx3360\tx3920\tx4480\tx5040\tx5600\tx6160\tx6720\ql\qnatural\pardirnatural

\f0\fs24 \cf0 Copyright \'a9 2009-2010 Fully Functional Software.\
All rights reserved.\
\
BlazingStars is made possible by {\field{\*\fldinst{HYPERLINK "http://www.iterasi.net/openviewer.aspx?sqrlitid=p4mjpt7nl02tyjl08_ctaa"}}{\fldrslt DBPrefsWindowController}}, {\field{\*\fldinst{HYPERLINK "http://mattgemmell.com/source"}}{\fldrslt Image Crop}}, {\field{\*\fldinst{HYPERLINK "http://code.google.com/p/tesseract-ocr/"}}{\fldrslt Tesseract-OCR}}, {\field{\*\fldinst{HYPERLINK "http://andymatuschak.org/articles/2005/12/18/help-with-apple-help"}}{\fldrslt Andy's Help Toolkit}}, and {\field{\*\fldinst{HYPERLINK "http://wafflesoftware.net/shortcut/"}}{\fldrslt Shortcut Recorder}}.\
\
Includes icons from the BlueCons set by {\field{\*\fldinst{HYPERLINK "http://www.mouserunner.com"}}{\fldrslt Ken Saunders}}.}

_{(来源: pokercopilot.com )}

_{(来源: pokercopilot.com}