objective-c - 什么在调用 CFStringCreateFromExternalRepresentation？

Question

我对这个堆栈跟踪感到相当困惑(只显示了令人困惑的部分):

-[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

特别是，我不明白对 CFStringCreateFromExternalRepresentation 的调用发生在哪里，因此我不知道代码的哪一部分 (+[stripHTML stripAllHtmlFromString:]) 导致异常。是什么原因导致调用 CFStringCreateFromExternalRepresentation？如果很明显，我做错了什么导致了异常？将来，我该如何确定正在调用 CFStringCreateFromExternalRepresentation 的内容？

这里是+[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

哦，实际上，可能 -objectByApplyingXSLTString:arguments:error: 返回了 NSXMLDocument 而不是 NSData。因此，对 -[NSString initWithData:encoding:] 的调用是对它认为是 NSData 的内容调用 -length，但是 NSXMLDocument 无法识别这一点。