What's calling CFStringCreateFromExternalRepresentation? - objective-c

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.

Answer 2

Ken Thomases's answer seems to have it exactly right—for some reason, for very short input (particularly the empty string), -objectByApplyingXSLTString:arguments:error: with the given XSLT returns an NSXMLDocument (even though I don't think it should). To fix it, I first detect whether we got an NSXMLDocument and if so, turn it into a string representation of the XML and feed that back into the method; otherwise assume we got the NSData that we'd originally expected.
Replacing the last 3 lines of the given method (from NSData *theData =... on) with the code below seems to have fixed the issue.
// Had a report of an exception that seemed to indicate objectByApplyingXSLTString:arguments:error: was returning an NSXMLDocument objectinstead of an NSData object, so let's guard against that. (discussed at https://stackoverflow.com/q/10669479/291280 )
NSObject *XSTLresult = [theDocument objectByApplyingXSLTString:theXSLTString arguments:NULL error:&theError];
[theDocument release];
if ([XSTLresult isKindOfClass:[NSXMLDocument class]]) {
// If the result is an NSXMLDocument, call XMLData to get an NSData object, turn it into a string, and feed that back into this method...
return [self stripAllHtmlFromString:[[[NSString alloc]
initWithData:[(NSXMLDocument *)XSTLresult XMLData]
encoding:NSUTF8StringEncoding]
autorelease]
];
} else {
// Otherwise, assume we have an NSData object.
return [[[NSString alloc] initWithData:(NSData *)XSTLresult encoding:NSUTF8StringEncoding] autorelease];
}