SVGParser.h

#import <Foundation/Foundation.h>

@class SVGParser;

FOUNDATION_EXPORT NSString * const kSVGParserErrorDomain;

@protocol SVGParserDelegate <NSObject>
@optional
- (void) svgParser: (SVGParser*) parser scannedAttributes: (NSDictionary*) attributes inElement: (NSString*) elementName;
- (void) svgParser: (SVGParser*) parser didFailWithError: (NSError*) error;
@end

@interface SVGParser : NSOperation

+ (instancetype) parserForFile: (NSString*) filePath;   //Handles absolute file paths or filenames inside the main bundle

@property (weak, nonatomic) id<SVGParserDelegate> delegate; //Must be set before this operation starts

@end

SVGParser.m

#import "SVGParser.h"

#import <libxml/parser.h>

NSString * const kSVGFileExtension = @"svg";
NSString * const kSVGCompressedFileExtension = @"svgz";
NSString * const kSVGParserErrorDomain = @"SVGParserError";

static xmlSAXHandler svgSAXHandlerStruct;
static void errorEncounteredSAX(void * ctx, const char * msg, ...);
static void startElementSAX(void *ctx, const xmlChar *localname, const xmlChar *prefix, const xmlChar *URI, int nb_namespaces, const xmlChar **namespaces, int nb_attributes, int nb_defaulted, const xmlChar **attributes);

@interface SVGParser () {
    NSString* _fileName;
}

- (instancetype) initWithSVGNamed: (NSString*) svgFileName;

@property (readonly, nonatomic) BOOL isAbsoluteFilePath;

@end

@implementation SVGParser

+ (instancetype) parserForFile: (NSString*) svgFileName {
    return [[self alloc] initWithSVGNamed: svgFileName];
}

- (instancetype) initWithSVGNamed:(NSString *)svgFileName {
    NSString* extension = [svgFileName pathExtension];
    
    NSAssert([extension isEqualToString: kSVGFileExtension] ||
             [extension isEqualToString: kSVGCompressedFileExtension],
             @"'%@' is not a valid SVG file",
             svgFileName);
    
    self = [super init];
    if( self ) {
        
        //fragile...
        if( [svgFileName characterAtIndex: 0] == '/' ) {
            _isAbsoluteFilePath = YES;
        }
        
        _fileName = svgFileName;
    }
    
    return self;
}

#pragma mark - NSOperation
- (void) main {
    NSParameterAssert(_delegate);
    
    NSString* filePath = _fileName;
    
    if( !self.isAbsoluteFilePath ) {
        NSBundle* bundle = [NSBundle mainBundle];
        filePath = [bundle pathForResource: _fileName ofType: nil];
    }
    
    NSAssert([[NSFileManager defaultManager] fileExistsAtPath: filePath], @"Could not find file at path %@", filePath);
    
    // http://xmlsoft.org/html/libxml-parser.html#xmlSAXUserParseFile
    int result = xmlSAXUserParseFile(&svgSAXHandlerStruct, (__bridge void*)self, [filePath UTF8String]);
    
    NSAssert(result == 0, @"Failed to parse svg file (%d) at path: %@", result, filePath);
    if( result != 0 ) {
        if( [self.delegate respondsToSelector: @selector(svgParser:didFailWithError:)] ) {
            xmlErrorPtr xmlError = xmlGetLastError();
            NSString* errorMsg = [[NSString alloc] initWithCString: xmlError->message encoding: NSUTF8StringEncoding];
            NSError* error = [NSError errorWithDomain: kSVGParserErrorDomain code: xmlError->code userInfo: @{NSLocalizedDescriptionKey: errorMsg}];
            [self.delegate svgParser: self didFailWithError: error];
        }
    }
    
    xmlCleanupParser();
}

@end

#pragma mark - libxml SAX Parser
// http://xmlsoft.org/html/libxml-tree.html#xmlSAXHandler
static xmlSAXHandler svgSAXHandlerStruct = {
    NULL,                       /* internalSubset */
    NULL,                       /* isStandalone   */
    NULL,                       /* hasInternalSubset */
    NULL,                       /* hasExternalSubset */
    NULL,                       /* resolveEntity */
    NULL,                       /* getEntity */
    NULL,                       /* entityDecl */
    NULL,                       /* notationDecl */
    NULL,                       /* attributeDecl */
    NULL,                       /* elementDecl */
    NULL,                       /* unparsedEntityDecl */
    NULL,                       /* setDocumentLocator */
    NULL,                       /* startDocument */
    NULL,                       /* endDocument */
    NULL,                       /* startElement*/
    NULL,                       /* endElement */
    NULL,                       /* reference */
    NULL,                       /* characters */
    NULL,                       /* ignorableWhitespace */
    NULL,                       /* processingInstruction */
    NULL,                       /* comment */
    NULL,                       /* warning */
    errorEncounteredSAX,        /* error */
    NULL,                       /* fatalError //: unused error() get all the errors */
    NULL,                       /* getParameterEntity */
    NULL,                       /* cdataBlock */
    NULL,                       /* externalSubset */
    XML_SAX2_MAGIC,             //
    NULL,
    startElementSAX,            /* startElementNs */
    NULL,              /* endElementNs */
    NULL,                       /* serror */
};

static NSMutableDictionary *NSDictionaryCreateFromLibxmlAttributes (const xmlChar **attrs, int attr_ct) {
  NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
    
    NSUInteger limit = attr_ct * 5;
	for (int i = 0; i < limit/*attr_ct * 5*/; i += 5) {
		const char *begin = (const char *) attrs[i + 3];
		const char *end = (const char *) attrs[i + 4];
		int vlen = strlen(begin) - strlen(end);
		
		char val[vlen + 1];
		strncpy(val, begin, vlen);
		val[vlen] = '\0';
		
		[dict setObject:[NSString stringWithUTF8String:val]
				 forKey:[NSString stringWithUTF8String:(const char*)attrs[i]]];
	}
	
	return dict;
}

static void startElementSAX(void *ctx, const xmlChar *localname, const xmlChar *prefix, const xmlChar *URI,
                            int nb_namespaces, const xmlChar **namespaces, int nb_attributes, int nb_defaulted, const xmlChar **attributes) {
    SVGParser* parser = (__bridge SVGParser*)ctx;
    if( [parser.delegate respondsToSelector: @selector(svgParser:scannedAttributes:inElement:)] ) {
        NSDictionary* attrs =NSDictionaryCreateFromLibxmlAttributes(attributes, nb_attributes);
        NSString* element = [[NSString alloc] initWithCString: (const char*)localname encoding: NSUTF8StringEncoding];
        [parser.delegate svgParser: parser scannedAttributes: attrs inElement: element];
    }
}

static void errorEncounteredSAX(void *ctx, const char *msg, ...) {
    SVGParser* parser = (__bridge SVGParser*)ctx;
    if( [parser.delegate respondsToSelector: @selector(svgParser:didFailWithError:)] ) {
        NSString* errorMsg = [[NSString alloc] initWithCString: msg encoding: NSUTF8StringEncoding];
        NSError* error = [NSError errorWithDomain: kSVGParserErrorDomain code: -1 userInfo: @{NSLocalizedDescriptionKey : errorMsg}];
        [parser.delegate svgParser: parser didFailWithError: error];
    }
}