[密技]：vl下的出错处理方法：

秋枫 · 发表于 2002-9-13 15:58:18

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寄件者：<a href="http://groups.google.com/groups?hl=zh-CN&lr=&ie=UTF-8&oe=UTF-8&q=author:tony.tanzillo%40worldnet.att.net+">Tony
Tanzillo</a> (<a href="mailto:tony.tanzillo@worldnet.att.net">tony.tanzillo@worldnet.att.net</a>)
主旨：Re: Globally Cath
Exeptions
新闻群组：<a href="http://groups.google.com/groups?hl=zh-CN&lr=&ie=UTF-8&oe=UTF-8&group=autodesk.autocad.customization">autodesk.autocad.customization</a>
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<td>View: <a href="http://groups.google.com/groups?hl=zh-CN&lr=&ie=UTF-8&oe=UTF-8&threadm=37F00043.775B4094%40worldnet.att.net&rnum=2&prev=/groups%3Fhl%3Dzh-CN%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D37F00043.775B4094%2540worldnet.att.net%26rnum%3D2">Complete
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日期：1999/09/27
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<pre>I'm not sure I understand what you mean by 'not clean'.
The very nature of exception handling mandates that
the application specifically prepare for and deal
with exceptions locally, so that it can resume its
execution or terminate gracefully (e.g., clean up).
You have to remember that exceptions are used both
for errors, and for signaling conditions like user-
input was not provided, an option keyword was entered,
or a collection key was not found. These types of
exceptions _must_ be handled locally and explicitly.
If you were to wrap your entire program inside one
exception handler in your main command function, it
would catch the exception, but that's essentially
like using *error*, because the execution of your
code will still be suspended at the point where the
exception occurred, and control will be transferred
to the next expression that follows the protected
expression (the code passed to vl-catch-all-apply):
(defun C:YOURCOMMAND ( / rslt)
(setq rslt
(vl-catch-all-apply
'(lambda ()
(run-your-application)
)
)
)
(if (vl-catch-all-error-p rslt)
(alert
(strcat
"This expression is evaluated immediately "
"after the one in which the exception occurred"
)
)
rslt
)
)
The above is really no different than this:
(defun C:YOURCOMMAND ( / *error*)
(defun *error* (msg)
(alert
(strcat
"This expression is evaluated immediately "
"after the one in which the exception occurred"
)
)
)
(run-your-application)
)
So, what good is the former?
There are some tricks that a very skilled programmer
can do, to wrap the body of every function in their
program in its own exception handler, but that doesn't
really solve the problem, other than perhaps serving
as a debugging tool. That's because the way you handle
an exception depends on both the nature of the error,
and how/where/when/why it occurs.
Here's a helper function that can make localized
exception handling a bit less frustrating:
;; (catch-error <Protected> <OnError>)
;;
;; (catch-error
;; '(lambda () <code to protect>...)
;; '(lambda (error) <error handling code>...)
;; )
;;
;; A wrapper for vl-catch-all-xxxxxxx
;;
;; This function provides a high-level exception-handling
;; construct that allows the programmer to factor out some
;; of the testing and flow control required when using the
;; vl-catch-all-xxxxx functions directly (which is messy).
;;
;; (catch-error) takes two arguments, both of which are
;; quoted function names or lambda lists:
;;
;; <Protected> is a function that takes no arguments
;; (usually a lambda list). It is evaluated, and if an
;; error occurs during its evaluation, then <OnError> is
;; called and passed the error string.
;;
;; <OnError> is a function that is called only if an error
;; occurs during the evaluation of <Protected>. In that case,
;; the message returned by (vl-catch-all-error-message) is
;; passed to <OnError> as its only argument.
;;
;; If no error occurs during evaluation of <Protected>, then
;; (catch-error) returns the result of <Protected>. If an error
;; occurs during evaluation of <Protected>, then (catch-error)
;; returns the result of <OnError> if supplied. If an error
;; occurs and <OnError> is not supplied (e.g., nil), then
;; (catch-error) returns the Visual LISP error object that
;; triggered the error.
;;
;; By default, errors are always handled. The only way to
;; re-raise the error is to call (exit) from within <OnError>,
;; and is generally what should be done in cases where the
;; error is unknown or the code in the <OnError> error handler
;; is not equipped to deal with a particular type of error.
;;
;; Note that calls to (catch-error) can be nested, such that
;; if a nested call does not handle an error (by calling exit
;; or vl-exit-with-error), then one or more outer calls can
;; also examine and/or handle the error, with the caveat that
;; the error message is not the same as the original one, if
;; you use (exit). If you use (vl-exit-with-error), then the
;; error message is correct, but in that case the outer most
;; call to (catch-all) must handle the error because it cannot
;; call (exit) to invoke the *error* function.
;;
;; In essence, you can think of each call to
;; catch error to be a temporary and locally-bound
;; redefinition of the *error* function (where the
;; <OnError> argument represents the body of *error*),
;; with the added benefit of being able to resume
;; execution if desired.
;;
;;
;; Example:
;;
;; The following code obtains a point or the
;; option keyword "foo", and traps the error
;; that occurs if the user responds with the
;; option keyword.
;;
;; If the error is not a result of the user
;; entering the keyword, then something else
;; has gone wrong. In that case, the error
;; handler displays the error message, and
;; aborts by calling (exit).
;;
;; If no error occurs, then the error handler
;; is not called, and the user-supplied point
;; is assigned to Result
;;
;; (defun C:CATCH-ERROR-TEST ()
;;
;; (setq eKeyWordInput
;; "Automation Error. User input is a keyword"
;; )
;;
;; (setq Util
;; (vla-get-utility
;; (vla-get-activeDocument
;; (vlax-get-acad-object)
;; )
;; )
;; )
;;
;;
;; (vla-IntializeUserInput Util 0 "Foo")
;;
;; (setq result
;; (catch-error
;; '(lambda ()
;; (vla-Getpoint
;; Util nil "Enter point/Foo: "
;; )
;; )
;; '(lambda (error)
;; (if (eq error eKeyWordInput)
;; (vla-GetInput Util)
;; (progn
;; (alert (strcat "Error: " errmsg))
;; (exit)
;; )
;; )
;; )
;; )
;; )
;;
;; (if (eq (type Result) 'Str)
;; (princ "\nUser entered 'FOO'")
;; (princ "\nUser entered a point")
;; )
;;
;; (princ)
;;
;; )
;;
(defun catch-error (catch:Protected catch:OnError / catch:err)
(setq catch:err
(vl-catch-all-apply catch:Protected)
)
(if (and (vl-catch-all-error-p catch:err)
catch:OnError
)
(Apply catch:OnError
(list (vl-catch-all-error-message catch:err))
)
catch:err
)
)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
</pre>
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        寄件者：<a href="http://groups.google.com/groups?hl=zh-CN&amp;lr=&amp;ie=UTF-8&amp;oe=UTF-8&amp;q=author:tony.tanzillo%40worldnet.att.net+">Tony 
        Tanzillo</a> (<a href="mailto:tony.tanzillo@worldnet.att.net">tony.tanzillo@worldnet.att.net</a>) 
        主旨：Re: Globally Cath 
        Exeptions 
        新闻群组：<a href="http://groups.google.com/groups?hl=zh-CN&amp;lr=&amp;ie=UTF-8&amp;oe=UTF-8&amp;group=autodesk.autocad.customization">autodesk.autocad.customization</a> 
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              <td>View: <a href="http://groups.google.com/groups?hl=zh-CN&amp;lr=&amp;ie=UTF-8&amp;oe=UTF-8&amp;threadm=37F00043.775B4094%40worldnet.att.net&amp;rnum=2&amp;prev=/groups%3Fhl%3Dzh-CN%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D37F00043.775B4094%2540worldnet.att.net%26rnum%3D2">Complete 
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        日期：1999/09/27 
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<pre>I'm not sure I understand what you mean by 'not clean'. 
 
The very nature of exception handling mandates that 
the application specifically prepare for and deal 
with exceptions locally, so that it can resume its 
execution or terminate gracefully (e.g., clean up). 
 
You have to remember that exceptions are used both 
for errors, and for signaling conditions like user- 
input was not provided, an option keyword was entered, 
or a collection key was not found. These types of 
exceptions _must_ be handled locally and explicitly. 
 
If you were to wrap your entire program inside one 
exception handler in your main command function, it 
would catch the exception, but that's essentially 
like using *error*, because the execution of your 
code will still be suspended at the point where the 
exception occurred, and control will be transferred 
to the next expression that follows the protected 
expression (the code passed to vl-catch-all-apply): 
 
(defun C:YOURCOMMAND ( / rslt) 
   (setq rslt 
      (vl-catch-all-apply 
        '(lambda () 
            (run-your-application) 
         ) 
      ) 
   ) 
   (if (vl-catch-all-error-p rslt) 
      (alert 
         (strcat 
            &quot;This expression is evaluated immediately &quot; 
            &quot;after the one in which the exception occurred&quot; 
         ) 
      ) 
      rslt 
   ) 
) 
 
The above is really no different than this: 
 
(defun C:YOURCOMMAND ( / *error*) 
   (defun *error* (msg) 
      (alert 
         (strcat 
           &quot;This expression is evaluated immediately &quot; 
           &quot;after the one in which the exception occurred&quot; 
         ) 
      ) 
   ) 
   (run-your-application) 
) 
 
So, what good is the former? 
 
There are some tricks that a very skilled programmer 
can do, to wrap the body of every function in their 
program in its own exception handler, but that doesn't 
really solve the problem, other than perhaps serving 
as a debugging tool. That's because the way you handle 
an exception depends on both the nature of the error, 
and how/where/when/why it occurs. 
 
Here's a helper function that can make localized 
exception handling a bit less frustrating: 
 
;; (catch-error &lt;Protected&gt; &lt;OnError&gt;) 
;; 
;; (catch-error 
;;   '(lambda () &lt;code to protect&gt;...) 
;;   '(lambda (error) &lt;error handling code&gt;...) 
;; ) 
;; 
;; A wrapper for vl-catch-all-xxxxxxx 
;; 
;; This function provides a high-level exception-handling 
;; construct that allows the programmer to factor out some 
;; of the testing and flow control required when using the 
;; vl-catch-all-xxxxx functions directly (which is messy). 
;; 
;; (catch-error) takes two arguments, both of which are 
;; quoted function names or lambda lists: 
;; 
;; &lt;Protected&gt; is a function that takes no arguments 
;; (usually a lambda list). It is evaluated, and if an 
;; error occurs during its evaluation, then &lt;OnError&gt; is 
;; called and passed the error string. 
;; 
;; &lt;OnError&gt; is a function that is called only if an error 
;; occurs during the evaluation of &lt;Protected&gt;. In that case, 
;; the message returned by (vl-catch-all-error-message) is 
;; passed to &lt;OnError&gt; as its only argument. 
;; 
;; If no error occurs during evaluation of &lt;Protected&gt;, then 
;; (catch-error) returns the result of &lt;Protected&gt;. If an error 
;; occurs during evaluation of &lt;Protected&gt;, then (catch-error) 
;; returns the result of &lt;OnError&gt; if supplied. If an error 
;; occurs and &lt;OnError&gt; is not supplied (e.g., nil), then 
;; (catch-error) returns the Visual LISP error object that 
;; triggered the error. 
;; 
;; By default, errors are always handled. The only way to 
;; re-raise the error is to call (exit) from within &lt;OnError&gt;, 
;; and is generally what should be done in cases where the 
;; error is unknown or the code in the &lt;OnError&gt; error handler 
;; is not equipped to deal with a particular type of error. 
;; 
;; Note that calls to (catch-error) can be nested, such that 
;; if a nested call does not handle an error (by calling exit 
;; or vl-exit-with-error), then one or more outer calls can 
;; also examine and/or handle the error, with the caveat that 
;; the error message is not the same as the original one, if 
;; you use (exit). If you use (vl-exit-with-error), then the 
;; error message is correct, but in that case the outer most 
;; call to (catch-all) must handle the error because it cannot 
;; call (exit) to invoke the *error* function. 
;; 
;; In essence, you can think of each call to 
;; catch error to be a temporary and locally-bound 
;; redefinition of the *error* function (where the 
;; &lt;OnError&gt; argument represents the body of *error*), 
;; with the added benefit of being able to resume 
;; execution if desired. 
;; 
;; 
;; Example: 
;; 
;;   The following code obtains a point or the 
;;   option keyword &quot;foo&quot;, and traps the error 
;;   that occurs if the user responds with the 
;;   option keyword. 
;;    
;;   If the error is not a result of the user 
;;   entering the keyword, then something else 
;;   has gone wrong. In that case, the error 
;;   handler displays the error message, and 
;;   aborts by calling (exit). 
;;    
;;   If no error occurs, then the error handler 
;;   is not called, and the user-supplied point 
;;   is assigned to Result 
;; 
;; (defun C:CATCH-ERROR-TEST () 
;;    
;;   (setq eKeyWordInput 
;;      &quot;Automation Error. User input is a keyword&quot; 
;;   ) 
;;    
;;   (setq Util 
;;      (vla-get-utility 
;;         (vla-get-activeDocument 
;;            (vlax-get-acad-object) 
;;         ) 
;;      ) 
;;   ) 
;;    
;; 
;;   (vla-IntializeUserInput Util 0 &quot;Foo&quot;) 
;;    
;;   (setq result 
;;      (catch-error 
;;           '(lambda () 
;;            (vla-Getpoint 
;;               Util nil &quot;Enter point/Foo: &quot; 
;;            ) 
;;         ) 
;;           '(lambda (error) 
;;               (if (eq error eKeyWordInput) 
;;                  (vla-GetInput Util) 
;;                  (progn 
;;                     (alert (strcat &quot;Error: &quot; errmsg)) 
;;                     (exit) 
;;                  ) 
;;               ) 
;;            ) 
;;         ) 
;;   ) 
;;            
;;   (if (eq (type Result) 'Str) 
;;      (princ &quot;\nUser entered 'FOO'&quot;) 
;;      (princ &quot;\nUser entered a point&quot;) 
;;   ) 
;;    
;;   (princ) 
;; 
;; ) 
;;               
 
(defun catch-error (catch:Protected catch:OnError / catch:err) 
    (setq catch:err 
       (vl-catch-all-apply catch:Protected) 
    ) 
    (if (and (vl-catch-all-error-p catch:err) 
             catch:OnError 
        ) 
        (Apply catch:OnError 
           (list (vl-catch-all-error-message catch:err)) 
        ) 
        catch:err 
    ) 
 ) 
 
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; 
</pre> 
 
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e2002 · 发表于 2002-9-14 22:50:14

Tony 可能是这个世界上最出色的AutoCAD开发者之一了,他的书和文章太好了!吐血推荐!!!!

adian2000 · 发表于 2002-12-6 17:28:24

可是不知道哪里有买？国内的书大部分是help的印刷版本。

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