How to parse xml using Objective C

There will always be a need to parse xml, especially if you are dealing with web services. In this post i will show how to use NSXMLParser to parse the xml you get back from Amazon Web Service when you attach a device to a running EC2 instance.
Here is what the xml response looks like:

<AttachVolumeResponse xmlns="http://ec2.amazonaws.com/doc/2010-08-31/">
  <volumeId>vol-4d826724</volumeId>
  <instanceId>i-6058a509</instanceId>
  <device>/dev/sdh</device>
  <status>attaching</status>
  <attachTime>2008-05-07T11:51:50.000Z</attachTime>
</AttachVolumeResponse>

To do the parsing , we need to use NSXMLParserDelegate protocol and we will implement three of its methods:

• – parser:didStartElement:namespaceURI:qualifiedName:attributes:
• – parser:didEndElement:namespaceURI:qualifiedName:
• – parser:foundCharacters:

Here is what the header file looks like:

@interface AttachVolumeParser : NSObject {

    NSMutableArray *items;

    AttachVolumeInfo *volumeInfo;

    NSString *keyInProgress;

    NSMutableString *textInProgress;

bool isItemInProgress;

}

@property(nonatomic, assign) bool isItemInProgress;

- (BOOL)parseData:(NSData *)d;

- (NSArray *)items;

@end

items is the array that will hold the objects resulting from parsing the xml.

you need to create an object that corresponds to the xml above and that i refer to in above code snippet as AttachVolumeInfo.

And the implementation looks something like this:

#import "AttachVolumeParser.h"

static NSSet *interestingKeys;

@implementation AttachVolumeParser

@synthesize isItemInProgress;

+ (void)initialize

{

    if (!interestingKeys) {

        interestingKeys = [[NSSet alloc] initWithObjects:@"volumeId", @"instanceId", @"device", @"status", @"attachTime", nil];

    }

}

- (void)dealloc

{

    [items release];

    [super dealloc];

}

- (BOOL)parseData:(NSData *)d

{

    [items release];

    items = [[NSMutableArray alloc] init];

    NSXMLParser *parser = [[NSXMLParser alloc] initWithData:d];

    [parser setDelegate:self];

    [parser parse];

    [parser release];

    return YES;

}

- (NSArray *)items

{

    return items;

}

#pragma mark Delegate calls

- (void)parser:(NSXMLParser *)parser

didStartElement:(NSString *)elementName

  namespaceURI:(NSString *)namespaceURI

 qualifiedName:(NSString *)qName

    attributes:(NSDictionary *)attributeDict

{

    if ([elementName isEqual:@"AttachVolumeResponse"]) {

isItemInProgress = true;

volumeInfo = [[AttachVolumeInfo alloc] init];

        return;

    }

    if ([interestingKeys containsObject:elementName]) {

        keyInProgress = [elementName copy];

        textInProgress = [[NSMutableString alloc] init];

    }

}

- (void)parser:(NSXMLParser *)parser

 didEndElement:(NSString *)elementName

  namespaceURI:(NSString *)namespaceURI

 qualifiedName:(NSString *)qName

{

    if ([elementName isEqual:@"AttachVolumeResponse"]) {

[items addObject:volumeInfo];

[volumeInfo release];

volumeInfo = nil;

isItemInProgress = false;

        return;

    }

    // Is the current key complete?

    if ([elementName isEqual:keyInProgress]) {

        if([elementName isEqual:@"volumeId"]) {

            [volumeInfo setVolumeId:textInProgress];

        } else if([elementName isEqual:@"instanceId"]) {

            [volumeInfo setInstanceId:textInProgress];

        } else if([elementName isEqual:@"device"]) {

            [volumeInfo setDevice:textInProgress];

        }  else if([elementName isEqual:@"status"]) {

            [volumeInfo setStatus:textInProgress];

        } else if([elementName isEqual:@"attachTime"]) {

            [volumeInfo setAttachTime:textInProgress];

        }

        // Clear the text and key

        [textInProgress release];

        textInProgress = nil;

        [keyInProgress release];

        keyInProgress = nil;

    }

}

// This method can get called multiple times for the

// text in a single element

- (void)parser:(NSXMLParser *)parser

foundCharacters:(NSString *)string

{

    [textInProgress appendString:string];

}

@end

What i like about this method is that it is fast due to its event-based nature, but on the other hand i need to write a new parser every time i have a new xml schema. 

I have already tried to use ObjectiveResource to parse amazon xml schemas but it was a little bit challenging, hopefully i will find some time soon to grab the source code and modify it to do that.

If you have done any amazon xml parsing using a better method, you are welcome to share.