When iOS GCD threading is applied, parsing of xml stops working - ios

Question

I'm working on an app where user sends data to a central database using a web service. The web service sends back an XML file with the primary key from the central database so the local database on the app is updated with this primary key. When I don't use GCD, everything works perfectly. As soon as I introduce threading, I don't seem to get the XML file to parse. It seems like the code sends the XML to the web service, but nothing happens thereafter. Is there something wrong with the way i implement threading? Here's the code sample:
-(void) viewDidLoad
{
dispatch_queue_t saveCentralDB = dispatch_queue_create("Writing Database", NULL);
dispatch_async(saveCentralDB, ^ {
NSLog(#"Created NEW THREAD to send info to CENTRAL DB");
NSString *soapMsg = [NSString stringWithFormat:
#"<?xml version=\"1.0\" encoding=\"utf-8\"?>"
"<soap12:Envelope "
"xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" "
"xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" "
"xmlns:soap12=\"http://www.w3.org/2003/05/soap-envelope\">"
"<soap12:Body>"
"<InsertPurchase xmlns=\"http://tempuri.org/\">"
"<RequestObject xsi:type = \"SpazaPurchaseRequest\">"
"<PurchaseID>%#</PurchaseID>"
"<RemoteSpazaPurchaseID>%#</RemoteSpazaPurchaseID>"
"<UserID>%d</UserID>"
"<RetailerID>%#</RetailerID>"
"<ItemID>%#</ItemID>"
"<CostPrice>%#</CostPrice>"
"<Longitude>%#</Longitude>"
"<Latitude>%#</Latitude>"
"<DatePurchased>%#</DatePurchased>"
"<Barcode>%#</Barcode>"
"<BasketID>%#</BasketID>"
"</RequestObject>"
"</InsertPurchase>"
"</soap12:Body>"
"</soap12:Envelope>",#"0",pklPurchaseID1,fklUserID,fklRetailerID1,fklItemID1, lCostPrice1, sLongitude1, sLatitude1,dtPurchase1,sBarcode1,fklBasketID1];
//---print of the XML to examine---
NSLog(#"%#", soapMsg);
NSURL *url = [NSURL URLWithString:#"http://www.myapp.com/purchases/ProviderWS.asmx?op=InsertPurchase"];
NSMutableURLRequest *req = [NSMutableURLRequest requestWithURL:url];
NSString *msgLength = [NSString stringWithFormat:#"%d", [soapMsg length]];
[req addValue:#"text/xml; charset=utf-8" forHTTPHeaderField:#"Content-Type"];
[req addValue:msgLength forHTTPHeaderField:#"Content-Length"];
[req setHTTPMethod:#"POST"];
[req setHTTPBody: [soapMsg dataUsingEncoding:NSUTF8StringEncoding]];
conn = [[NSURLConnection alloc] initWithRequest:req delegate:self];
if (conn)
{
webData = [NSMutableData data];
}
});
}
I then implement the following methods to deal with the response from the Web Service.
/************************Processing the feedback XML returned by webservice*****************/
-(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 {
}
-(void) connectionDidFinishLoading:(NSURLConnection *) connection {
NSLog(#"DONE. Received Bytes: %d", [webData length]);
NSString *theXML = [[NSString alloc] initWithBytes:[webData mutableBytes]
length:[webData length]
encoding:NSUTF8StringEncoding];
//---prints the XML received---
NSLog(#"%#", theXML);
xmlParser = [[NSXMLParser alloc] initWithData: webData];
[xmlParser setDelegate: self];
[xmlParser setShouldResolveExternalEntities: YES];
[xmlParser parse];
}
The normal delegate methods are then implemented:
- (void) parser:(NSXMLParser *)parser didStartElement:(NSString *)elementname namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName attributes:(NSDictionary *)attributeDict
{
NSLog(#"XML Parsing Method: didStartElement");
//This is the first node that we search for. The information we want is contained within this node.
if ([elementname isEqualToString:#"ResponseMessage"])
{
currentCentralDBPurchase = [parsingCentralDBPurchaseXML alloc];
//Flag to indicate that we are within the ResponseMessage node/tag.
isStatus = YES;
}
}
- (void) parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
NSLog(#"XML Parsing Method: foundCharacters");
currentNodeContent = (NSMutableString *) [string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
}
- (void) parser:(NSXMLParser *)parser didEndElement:(NSString *)elementname namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{
if (isStatus)
{
if ([elementname isEqualToString:#"PurchaseID"])
{
currentCentralDBPurchase.centralPurchaseID = currentNodeContent;
}
if ([elementname isEqualToString:#"RemotePurchaseID"])
{
currentCentralDBPurchase.localPurchaseID = currentNodeContent;
}
}
if ([elementname isEqualToString:#"ResponseMessage"])
{
//Update local database with the PurchaseID from the central database. This is how we will identify records that must still be sent to the central database.
//Now update the local database with purchases that have been sent to central database
// Get the DBAccess object;
DBAccess *dbAccess = [[DBAccess alloc] init];
[dbAccess UpdateCentralPurchaseID: [currentCentralDBPurchase.localPurchaseID integerValue] :[currentCentralDBPurchase.centralPurchaseID integerValue] ];
// Close the database because we are finished with it
[dbAccess closeDatabase];
currentCentralDBPurchase = nil;
//Clear the currentNodeContent node so we are ready to process the next one.
currentNodeContent = nil;
}
}

Answer 1

NSURLConnection initWithRequest: should be called on the main thread in this situation.
dispatch_async(dispatch_get_main_queue(), ^{
conn = [[NSURLConnection alloc] initWithRequest:req delegate:self];
if (conn)
{
webData = [NSMutableData data];
}
});
It relies on RunLoop and Runloop is automatically working on the main thread. You can call initWithRequest on the other thread but you should execute RunLoop on the thread. However it is kind of difficult on a dispatch queue, thus using the main queue is helpful for the situation.