Question

I want to validate XML files against an XSD on iOS. The documentations alludes to using NSXMLDocument to do this, but its not available on iOS =(. Are there any light weight alternatives to do this on iOS?

Answer 1

I ended up using the validation facilities in libxml2 since its a library already included in iOS. Following this sample code

#include <libxml/parser.h>
#include <libxml/xmlschemas.h>

int is_valid(const xmlDocPtr doc, const char *schema_filename)
{
    xmlDocPtr schema_doc = xmlReadFile(schema_filename, NULL, XML_PARSE_NONET);
    if (schema_doc == NULL) {
        /* the schema cannot be loaded or is not well-formed */
        return -1;
    }
    xmlSchemaParserCtxtPtr parser_ctxt = xmlSchemaNewDocParserCtxt(schema_doc);
    if (parser_ctxt == NULL) {
        /* unable to create a parser context for the schema */
        xmlFreeDoc(schema_doc);
        return -2;
    }
    xmlSchemaPtr schema = xmlSchemaParse(parser_ctxt);
    if (schema == NULL) {
        /* the schema itself is not valid */
        xmlSchemaFreeParserCtxt(parser_ctxt);
        xmlFreeDoc(schema_doc);
        return -3;
    }
    xmlSchemaValidCtxtPtr valid_ctxt = xmlSchemaNewValidCtxt(schema);
    if (valid_ctxt == NULL) {
        /* unable to create a validation context for the schema */
        xmlSchemaFree(schema);
        xmlSchemaFreeParserCtxt(parser_ctxt);
        xmlFreeDoc(schema_doc);
        return -4; 
    }
    int is_valid = (xmlSchemaValidateDoc(valid_ctxt, doc) == 0);
    xmlSchemaFreeValidCtxt(valid_ctxt);
    xmlSchemaFree(schema);
    xmlSchemaFreeParserCtxt(parser_ctxt);
    xmlFreeDoc(schema_doc);
    /* force the return value to be non-negative on success */
    return is_valid ? 1 : 0;
}