AFHTTPRequestOperation subclass for simple XML parsing

MRSimpleXMLRequestOperation.h

//
//  MRSimpleXMLRequestOperation.h
//  Marked
//
//  Created by Zachary Waldowski on 10/22/11.
//  Copyright (c) 2011 Dizzy Technology. All rights reserved.
//

#import "AFHTTPRequestOperation.h"
#import "SMXMLDocument.h"

@interface MRSimpleXMLRequestOperation : AFHTTPRequestOperation

@property (readonly, nonatomic, strong) SMXMLElement *responseXML;

+ (MRSimpleXMLRequestOperation *)XMLRequestOperationWithRequest:(NSURLRequest *)urlRequest
                                                        success:(void (^)(NSHTTPURLResponse *response, SMXMLElement *XML))success
                                                        failure:(void (^)(NSHTTPURLResponse *response, NSError *error))failure;

@end

MRSimpleXMLRequestOperation.m

//
//  MRSimpleXMLRequestOperation.m
//  Marked
//
//  Created by Donald Waldowski on 10/22/11.
//  Copyright (c) 2011 Dizzy Technology. All rights reserved.
//

#import "MRSimpleXMLRequestOperation.h"

static dispatch_queue_t MRSimpleXMLRequestOperationQueue;
static dispatch_queue_t MRSimpleXMLRequestOperationProcessingQueue() {
    if (MRSimpleXMLRequestOperationQueue == NULL) {
        MRSimpleXMLRequestOperationQueue = dispatch_queue_create("com.dizzytechology.marked.simple-xml-request", 0);
    }
    
    return MRSimpleXMLRequestOperationQueue;
}

@interface MRSimpleXMLRequestOperation ()
@property (readwrite, nonatomic, strong) id responseXML;
@property (readwrite, nonatomic, strong) NSError *error;

+ (NSSet *)defaultAcceptableContentTypes;
@end

@implementation MRSimpleXMLRequestOperation

@synthesize responseXML, error;

+ (MRSimpleXMLRequestOperation *)XMLRequestOperationWithRequest:(NSURLRequest *)urlRequest
                                                        success:(void (^)(NSHTTPURLResponse *response, SMXMLElement *XML))success
                                                        failure:(void (^)(NSHTTPURLResponse *response, NSError *error))failure {
    MRSimpleXMLRequestOperation *operation = [[self alloc] initWithRequest:urlRequest];
    __block MRSimpleXMLRequestOperation *safeOperation = operation;
    operation.completionBlock = ^{
        if (safeOperation.isCancelled)
            return;
        
        if (safeOperation.error && failure) {
            dispatch_async(dispatch_get_main_queue(), ^{
                failure(safeOperation.response, safeOperation.error);
            });
        } else if (!safeOperation.error && success) {
            dispatch_async(MRSimpleXMLRequestOperationProcessingQueue(), ^{
                id XML = safeOperation.responseXML;
                dispatch_async(dispatch_get_main_queue(), ^(void) {
                    if (safeOperation.error && failure) {
                        failure(safeOperation.response, safeOperation.error);
                    } else if (!safeOperation.error && success) {
                            success(safeOperation.response, XML);
                    }
                });
                
            });
        }
    };
    return operation;
}

- (id)responseXML {
    if (!responseXML && [self isFinished]) {
        NSError *parseError = nil;
        self.responseXML = [SMXMLElement rootElementForData:self.responseData error:&parseError];
        self.error = parseError;
    }
    
    return responseXML;
}

+ (NSSet *)defaultAcceptableContentTypes {
    return [NSSet setWithObjects:@"application/xml", @"text/xml", nil];
}

- (id)initWithRequest:(NSURLRequest *)urlRequest {
    if ((self = [super initWithRequest:urlRequest])) {
        self.acceptableContentTypes = [[self class] defaultAcceptableContentTypes];
    }
    
    return self;
}

+ (BOOL)canProcessRequest:(NSURLRequest *)request {
    return [[self defaultAcceptableContentTypes] containsObject:[request valueForHTTPHeaderField:@"Accept"]];
}

@end