NSURLConnection - Is it possible to wait/block on

Question

I need to wait for a response from a SOAP webservice, I am calling via a NSURLConnection as I need to manipulate the data being returned and then return it from my class to the calling class..

Here is my code:

#import <Foundation/Foundation.h>


@interface UsersBLL : NSObject {

 NSMutableData *webData;
 NSMutableString *soapResults;
 NSXMLParser *xmlParser;
 BOOL *recordResults;
 NSNumber *EmailCount;
}

@property(nonatomic, retain) NSMutableData *webData;
@property(nonatomic, retain) NSMutableString *soapResults;
@property(nonatomic, retain) NSXMLParser *xmlParser;



-(int)checkEmailAddress:(NSString*)emailAddress;
@end

#import "UsersBLL.h"


@implementation UsersBLL
@synthesize webData;
@synthesize soapResults;
@synthesize xmlParser;

-(id)init {
 self = [super init];
 return self;
}

-(int)checkEmailAddress:(NSString*)emailAddress {
 // Build the SOAP envelope
 NSString *soapMessage = [NSString stringWithFormat:
        @"<?xml version=\"1.0\" encoding=\"utf-8\"?>\n"
        "<soap:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\">\n"
        "<soap:Body>\n"
        "<CheckEmailAddress xmlns=\"http://tempuri.org/\">\n"
        "<EmailAddress>%@</EmailAddress>\n"
        "</CheckEmailAddress>\n"
        "</soap:Body>\n"
        "</soap:Envelope>\n", emailAddress];

 NSLog(soapMessage);

 NSURL *url = [NSURL URLWithString:@"http://photoswapper.mick-walker.co.uk/UsersService.asmx?op=CheckEmailAddress"];
 NSMutableURLRequest *theRequest = [NSMutableURLRequest requestWithURL:url];
 NSString *msgLength = [NSString stringWithFormat:@"%d", [soapMessage length]];

 [theRequest addValue: @"text/xml; charset=utf-8" forHTTPHeaderField:@"Content-Type"];
 [theRequest addValue: @"http://tempuri.org/CheckEmailAddress" forHTTPHeaderField:@"SOAPAction"];
 [theRequest addValue: msgLength forHTTPHeaderField:@"Content-Length"];
 [theRequest setHTTPMethod:@"POST"];
 [theRequest setHTTPBody: [soapMessage dataUsingEncoding:NSUTF8StringEncoding]];

 NSURLConnection *theConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];

 if( theConnection )
 {
  webData = [[NSMutableData data] retain];
 }
 else
 {
  NSLog(@"theConnection is NULL");
 }
 NSLog(@"%@", EmailCount);
}

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
 [webData setLength: 0];
}
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
 [webData appendData:data];
}
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
 NSLog(@"ERROR with theConenction");
 [connection release];
 [webData release];
}
-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
 NSLog(@"DONE. Received Bytes: %d", [webData length]);
 NSString *theXML = [[NSString alloc] initWithBytes: [webData mutableBytes] length:[webData length] encoding:NSUTF8StringEncoding];
 NSLog(theXML);
 [theXML release];

 if( xmlParser )
 {
  [xmlParser release];
 }

 xmlParser = [[NSXMLParser alloc] initWithData: webData];
 [xmlParser setDelegate: self];
 [xmlParser setShouldResolveExternalEntities: YES];
 [xmlParser parse];

 [connection release];
 [webData release];
}

-(void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *) namespaceURI qualifiedName:(NSString *)qName
   attributes: (NSDictionary *)attributeDict
{
 if( [elementName isEqualToString:@"CheckEmailAddressResult"])
 {
  if(!soapResults)
  {
   soapResults = [[NSMutableString alloc] init];
  }
  recordResults = TRUE;
 }
}
-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
 if( recordResults )
 {
  [soapResults appendString: string];
 }
}
-(void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{
 if( [elementName isEqualToString:@"CheckEmailAddressResult"])
 {
  recordResults = FALSE;
  NSNumberFormatter *formatter = [[NSNumberFormatter alloc]init];
  EmailCount = [formatter numberFromString:soapResults];
            [formatter release];
  [soapResults release];
  soapResults = nil;
 }
}

@end

CheckEmailAddress is declared as returning an integer value (I know it returns nothing in the sample above).

What I ideally want, is through the CheckEmailAddress method, return the value retrieved from the web service. However as the call NSURLConnection does not wait until the request has completed, I cannot do it.

I would be grateful if anyone could give me any potential ideas for workarounds.

Answer 1

I have just posted a solution which wraps an asynchronous NSURLConnection to be able to block the calling thread. In case you need more control than the standard [NSURLConnection sendSynchronousRequest:returningResponse:error:] you can check out this link on StackOverflow:

NSURLConnection blocking wrapper implemented with semaphores

Answer 2

The simplest solution would be using [NSURLConnection sendSynchronousRequest:returningResponse:error:].

It does not allow as much control as the approach you've taken, but is usually enough for most applications.

Answer 3

You have two choices:

• Use +[NSURLConnection sendSynchronousRequest:returningResponse:error:]

• Schedule the connection in a custom runloop mode, and run the loop in that mode until the data arrives or you have need to cancel the connection

Answer 4

It all depends on the level of asynchronism you need:

• If it's OK to stay blocked during the whole request you may want to use

    +[NSURLConnection sendSynchronousRequest:returningResponse:error:]
  

  But, as suggested by Wade, be careful to add a timeout to your NSURLRequest, otherwise the connection might blocks and your application will hang.

• If not, you can simply use the NSNotificationCenter. But you must be careful with race conditions over your data, specially if you are handling multiple requests