Question

I’m rather confused by this stack trace (only the confusing part is shown):

-[NSXMLDocument length]: unrecognized selector sent to instance 0x10187e010
An uncaught exception was raised
-[NSXMLDocument length]: unrecognized selector sent to instance 0x10187e010
(
   0   CoreFoundation                      0x00007fff8f5d6286 __exceptionPreprocess + 198
   1   libobjc.A.dylib                     0x00007fff9213bd5e objc_exception_throw + 43
   2   CoreFoundation                      0x00007fff8f6624ce -[NSObject doesNotRecognizeSelector:] + 190
   3   CoreFoundation                      0x00007fff8f5c3133 ___forwarding___ + 371
   4   CoreFoundation                      0x00007fff8f5c2f48 _CF_forwarding_prep_0 + 232
   5   CoreFoundation                      0x00007fff8f548c66 CFDataGetLength + 118
   6   CoreFoundation                      0x00007fff8f5791df CFStringCreateFromExternalRepresentation + 31
   7   asLJ                                0x0000000100013828 +[stripHTML stripAllHtmlFromString:] + 212

In particular, I don’t understand where the call to CFStringCreateFromExternalRepresentation is happening, so I don’t know what part of my code (+[stripHTML stripAllHtmlFromString:]) is causing the exception. What’s causing the call to CFStringCreateFromExternalRepresentation? If it’s obvious, what is it that I’m doing wrong that’s causing the exception? In the future, how can I go about determining what’s calling CFStringCreateFromExternalRepresentation?

Here’s +[stripHTML stripAllHtmlFromString:]:

+ (NSString *)stripAllHtmlFromString:(NSString *)inputString
{
    // based on code from http://sugarmaplesoftware.com/25/strip-html-tags/#comment-71

    NSError *theError = NULL;
    NSString *modifiedInputString = [NSString stringWithFormat:@"%@\n\n\n\n\n\n\n\n\n\n\n\n",inputString]; // adding some spare newlines at the end to ensure that things will work even with a short non-HTML string
    NSXMLDocument *theDocument = [[NSXMLDocument alloc] initWithXMLString:modifiedInputString
                                                                  options:NSXMLDocumentTidyHTML
                                                                    error:&theError];

    NSString *theXSLTString = @"<?xml version='1.0' encoding='utf-8'?>"
                                "<xsl:stylesheet version='1.0' xmlns:xsl='http://www.w3.org/1999/XSL/Transform' xmlns:xhtml='http://www.w3.org/1999/xhtml'>"
                                "<xsl:output method='text'/>"
                                "<xsl:template match='xhtml:head'></xsl:template>"
                                "<xsl:template match='xhtml:script'></xsl:template>"
                                "</xsl:stylesheet>";    
    NSData *theData = [theDocument objectByApplyingXSLTString:theXSLTString arguments:NULL error:&theError];
    [theDocument release];
    return [[[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding] autorelease];
}

Answer 1

Oh, actually, probably -objectByApplyingXSLTString:arguments:error: returned an NSXMLDocument and not an NSData. So, the call to -[NSString initWithData:encoding:] is invoking -length on what it thinks is an NSData, but NSXMLDocument doesn’t recognize that.