Perl/TkFAQ-10.18.如何限制输入框中内容的宽度
原文：
10.18. How do I limit an Entry's insertion width?
[OBSOLETE; explain use of new -validatecommand with 8.0] Nick Ing-Simmons recommends writing a new Entry widget with the insert method appropriately overridden by one that does limit the width. His code is avaialable as a separate package from: http://www.perltk.org/contrib/etc/LEntry-0_00.tar.gz
Now Brent Powers points out a possible problem with that approach and recommends an insert() method as follows: Date: Thu, 22 Aug 1996 10:32:44 -0400 From: "Brent B. Powers" <powers@ml.com> Subject: Re: How to set max characters for Entry widget In-reply-to: <199608211445.PAA09248@pluto> Ummm, before we set this into the distribution or FAQ, maybe we should make it work properly. An example: Imagine maxwidth configured to 8, the user fills in ABCDEFGH, moves the cursor back 4 places, and types I. The SUPER::insert call sets the string to ABCDIEFGH, which this code then modifies to ABCDIEFG. Hmmm, how about sub insert { my($w, @args) = @_; my($max) = $w->cget(-maxwidth); my($sval) = $w->get; if (length($sval) >= $max) { $w->SUPER::insert(@args); if (length($w->get) > length($sval) { ## Reject it; my($idx) = $w->index('insert'); # get current cursor position $w->delete(0, 'end'); $w->insert(0, $sval); $w->icursor($idx); $w->bell; } else { $w->SUPER::insert(@args); } } Of course, that still doesn't deal with the selection, but ...
To which Nick Ing-Simmons responded (Thu Aug 22 1996): 'paste' and <ButtonRelease-2> call insert method, what other selection issues are there?
译文：
10.18. 如何限制输入框中内容的宽度？
【Perl/Tk的作者Nick Ing-Simmons曾经建议重写一个新的Entry组件——适当的重载它的insert方法以便可以限制宽度。】
下面是Brent Powers为实现此功能提供的一个insert()方法，并且指出了其可能存在的问题：
日期: Thu, 22 Aug 1996 10:32:44 -0400
发自: "Brent B. Powers" <powers@ml.com>
主题: Re: 如何设置Entry中的最大字符数目？
In-reply-to: <199608211445.PAA09248@pluto>
嗯，在我们把下面的代码加入到“常见问题”中去前，也许我们应该先把它修正的更好一些。
一个例子：假设maxwidth选项已经被设置为8，用户先键入了ABCDEFGH，然后移动光标到, moves
第四个字符后面，键入了I。SUPER::insert的调用将使此字符串变成了ABCDIEFGH，但是下面
的代码将会把他更正为ABCDIEFG.
嗯，怎么样？
sub insert {
my($w, @args) = @_;
my($max) = $w->cget(-maxwidth);
my($sval) = $w->get;
if (length($sval) >= $max) {
$w->SUPER::insert(@args);
if (length($w->get) > length($sval) {
## Reject it;
my($idx) = $w->index('insert'); # get current cursor position
$w->delete(0, 'end');
$w->insert(0, $sval);
$w->icursor($idx);
$w->bell;
} else {
$w->SUPER::insert(@args);
}
}
当然，这里还没有考虑字符选择的问题，但是……
对此，Nick Ing-Simmons作出了回答(Thu Aug 22 1996)：
'粘贴'和<ButtonRelease-2>会调用insert方法，此外还有什么其它“选择”的问题呢？
（译者注：在Tk8.0以后，似乎可以通过设置-validateCommand选项来实现此功能吧。有兴趣的朋友可以自己参考一下Entry的手册：perldoc Tk::Entry）
