Subj : Re: problem on setter of object;'s property
To   : lyg <liyg@tech.coretek.com.cn>
From : Brendan Eich <brendan@meer.net>
Date : Mon Feb 09 2004 08:32 am

lyg wrote:

>when i set the value property of a option object , why the GetOptionProperty
>function is not called ?
>
>what is wrong ? Thanks!
>
>/*declare option object's property GETTER*/
>static JSBool GetOptionProperty (JSContext *cx, JSObject *obj, jsval id,
>jsval *vp);
>
>static JSClass OptionClass = {
>          "Option",0,
>           JS_PropertyStub,JS_PropertyStub, JS_PropertyStub,
>JS_PropertyStub,JS_EnumerateStub, JS_ResolveStub, JS_ConvertStub,
>JS_FinalizeStub,JSCLASS_NO_OPTIONAL_MEMBERS
>};
>
>enum  OPTION_PROP_ID {
>
>  OPTION_VALUE = 0,
>  OPTION_TEXT,
>};
>
>static JSPropertySpec OptionProperties[] =
>{
>   {"value",        OPTION_VALUE,
>JSPROP_ENUMERATE|JSPROP_READONLY, GetOptionProperty},
>   {"text",             OPTION_TEXT,
>JSPROP_ENUMERATE|JSPROP_READONLY, GetOptionProperty},
>   {0}
>};
>
>static JSBool
>Option(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
>{
>  if (0 < argc && 5 > argc)
>  
>

This might be more readable of it used number-line order (0 < argc && 
argc < 5).  Why 5?

>  {
>    char *temp;
>    temp = JS_GetStringBytes( JSVAL_TO_STRING(argv[0]) );
>
>    JS_SetProperty(cx, obj, "text",  &argv[0]);
>    if (argc > 1)
>      JS_SetProperty(cx, obj, "value", &argv[1]);
>  
>

How is this supposed to work, given that value is readonly?

>    return JS_TRUE;
>  }
>  else return JS_FALSE;
>  
>

1.  else after return is a non-sequitur.
2.  Never return false without first calling JS_ReportError or 
JS_SetPendingException, or calling something that does that before it 
returns false or null.

>}
>
>JSObject *
>js_InitOptionClass(JSContext *cx, JSObject *obj)
>{
>    JSObject *proto;
>
>    proto = JS_InitClass(cx, obj, NULL, &OptionClass, Option, 4,
>            NULL, NULL, OptionProperties, NULL);
>    if (!proto)
>       return NULL;
>    OBJ_SET_SLOT(cx, proto, JSSLOT_PRIVATE, JSVAL_FALSE);
>  
>

Do not use OBJ_SET_SLOT or include jsobj.h!  Use only jsapi.h.

Why would you set the private slot on an object whose class lacks the 
JSCLASS_HAS_PRIVATE flag?  That's an error, and if you had used the 
proper API (JS_SetPrivate) and tested a DEBUG-compiled version of this 
code, you would have hit an assertion telling you so.

>    return proto;
>}
>
>static JSBool GetOptionProperty (JSContext *cx, JSObject *obj, jsval id,
>jsval *vp)
>{
>  int i = 9;
>  return JS_TRUE;
>}
>  
>

Despite the above small and large problems, you have not shown enough to 
say why the value getter is not being called.  But fix the above first, 
retest, and debug a little -- try breakpointing in js_GetProperty and 
stepping into functions called from it.

/be

.