使用类别将XML解析为NSManagedObjects，以及如何处理类别中的属性？

Question

Based on the excellent example "Parsing XML with NSXMLParser" in the book "The Big Nerd Ranch Guide" (3rd ed.), I haved added categories to my NSManagedObjects for which I want to add XML parsing. These categories provide only parsing functionality.

This is how I have implemented these categories: .h:

#import "IBCompany.h"
@interface IBCompany (Xml) <NSXMLParserDelegate>

- (void)parseXmlString:(NSString*)xmlStr withCompletion:(void(^)(NSError *error))completionBlock;

@end

.m:

@implementation IBCompany (Xml) 

- (void)parseXmlString:(NSString*)xmlStr withCompletion:(void(^)(NSError *error))completionBlock;
{
    NSData *xmlData = [xmlStr dataUsingEncoding:NSUTF8StringEncoding];
    NSXMLParser *parser = [[NSXMLParser alloc]initWithData:xmlData];
    parser.delegate = self;

    [parser parse];

    xmlData = nil;

    NSError *error;
    completionBlock(error);
    }


- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict
{
if ([elementName isEqualToString:@"Issue"]) {
                IBIssue *issue = [NSEntityDescription insertNewObjectForEntityForName:@"IBIssue" inManagedObjectContext:self.managedObjectContext];
                issue.company = self;          
                issue.parentParserDelegate = self;
                parser.delegate = issue;
}

As you can see in this code snippet, I switch the parser delegate to other subclasses / XML child elements to have them further process the next XML elements, which belong to them until the end of the XML element is reached and the delegate is set back to the parent.

This is why I need to store the parent delegate in the child. However, ivars and properties are not allowed in categories.

I came up with this solution which seems to circumvent this problem:

Child element, h:

#import "IBIssue.h"

@interface IBIssue (Xml) <NSXMLParserDelegate>
@property id parentParserDelegate;
@end


#import "IBIssue+Xml.h"

@implementation IBIssue (Xml)

NSMutableString *currentString;
NSString *currentXmlDocument;

id _parentParserDelegate;

- (id)parentParserDelegate
{
    return _parentParserDelegate;
}

- (void)setParentParserDelegate:(id)parentParserDelegate;
{
    _parentParserDelegate = parentParserDelegate;
}

- (NSDateFormatter*)dateFormatter
{
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

    [dateFormatter setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]];
    [dateFormatter setDateFormat:@"yyy-MM-dd"];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT: 0]];
    return dateFormatter;
}

- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict
{
    if ([elementName isEqualToString:@"IssueID"]) {
        currentString = [[NSMutableString alloc]init];

        if      ([attributeDict[@"Type"] isEqualToString:@"Ticker"])        self.ticker = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"Name"])          self.issueName = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"CUSIP"])         self.cusip = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"ISIN"])          self.isin = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"RIC"])           self.ric = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"SEDOL"])         self.sedol = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"DisplayRIC"])    self.displayRic = currentString;
        else if ([attributeDict[@"Type"] isEqualToString:@"InstrumentPI"]) ; //
        else if ([attributeDict[@"Type"] isEqualToString:@"QuotePI"])      ; //

    } else if ([elementName isEqualToString:@"Exchange"]) {
        currentString = [[NSMutableString alloc]init];

        self.exchangeCode = attributeDict[@"Code"];
        self.exchangeCountry = attributeDict[@"Country"];
        self.exchange = currentString;
    } else if ([elementName isEqualToString:@"MostRecentSplit"]) {
        currentString = [[NSMutableString alloc]init];

        self.mostRecentSplitDate = [self.dateFormatter dateFromString:attributeDict[@"Date"]];
        // self.mostRecentSplit = [NSNumber numberWithFloat: currentString.floatValue];
    }
}

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
    // NSLog(@"appendString: %@", string);
    [currentString appendString:string];
}

-(void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{
    if ([elementName isEqualToString:@"Issue"]) {
        parser.delegate = self.parentParserDelegate;
    } else if ([elementName isEqualToString:@"MostRecentSplit"]) {
        self.mostRecentSplit = [NSNumber numberWithFloat: currentString.floatValue];
    }

    currentString = nil;
}

@end

I save the delegate to the parent in a variable _parentDelegate which is declared outside the ivar declaration block and does not seem to be a real ivar.

This code works well in my tests and I wonder if I missed something which will turn out to become a problem later in the development process or if this design is ok.

What are your thoughts on that?

Thank you!

Answer 1

I'm not sure how the compiler will treat that variable. Could it be allocated so that only one variable is shared by all objects of this type? If your XML is parsed such that more than one IBCompany exists at a point in time it could cause a problem. I'd write a test that allocated two IBCompany objects, cause them both to write different values to _parentDelegate, then assert the values are different.

Or ignore the issue if there is no possibility that two IBCompany objects are parsed in parallel. You'd have to ensure that the XML can't have an IBCompany inside another IBCompany, multiple parts of the XML will not be processed in parallel, and that multiple XML documents will not be processed in parallel.

I don't see the need for a category. Categories are useful when you shouldn't write a subclass to an existing class, such as adding functionality to classes in the Cocoa framework. You are writing a custom subclass, so why not add the ivar to your subclass? You can have additional ivars in managed objects that are not saved in the Core Data backing stores. At most I'd just use an extension to segregate XML parsing code from the rest of the managed object.