iphone-uialertview-thred - iphone

Question

in iphone,
i call one webservice For login checking...
When Application Is Underprocess ,, I Show UIAlertview With UIActivityIndicatorView
using thread,,
now i want to enable cancel button ,, means during the process if i want to cancel that process,, then my apps teminates webservice calling
but when i enable cancel button then ERROR OCccur,
Any One Can Help
My COde Is
-(NSMutableString*) getLoginMessage:(NSString*) UserName : (NSString *) Password
{
[NSThread detachNewThreadSelector:#selector(showAlertMethod) toTarget:self withObject:nil];
NSArray *Keys =[[NSArray alloc] initWithObjects:#"LoginName",#"PassWord",nil];
NSArray *KeyValue =[[NSArray alloc] initWithObjects:UserName,Password,nil];
operationName=[[NSString alloc] init];
operationName =#"ClientLogin";
NSString *StrService=[[NSUserDefaults standardUserDefaults] objectForKey:#"WebService"];
NSURL *WebServiceUrl=[WebServiceHelper generateWebServiceHTTPGetURL:StrService : operationName : Keys :KeyValue];
NSXMLParser *parser = [[NSXMLParser alloc] initWithContentsOfURL:WebServiceUrl];
[parser setShouldReportNamespacePrefixes:NO];
[parser setShouldResolveExternalEntities:NO];
[parser setDelegate:self];
[parser parse];
[Keys release];
[KeyValue release];
[StrService release];
[WebServiceUrl release];
//[parser release];
[NSThread detachNewThreadSelector:#selector(dismissAlertMethod) toTarget:self withObject:nil];
return Result;
}
-(void)showAlertMethod
{
NSAutoreleasePool *pool1=[[NSAutoreleasePool alloc]init];
progressAlert = [[UIAlertView alloc] initWithTitle:#"Loging in...\nPlease wait...\n" message:#"" delegate:nil cancelButtonTitle:nil otherButtonTitles:nil];
CGRect alertFrame = progressAlert.frame;
UIActivityIndicatorView* activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
activityIndicator.frame = CGRectMake(135,alertFrame.size.height+75, alertFrame.size.width,30);
activityIndicator.hidden = NO;
activityIndicator.contentMode = UIViewContentModeCenter;
[activityIndicator startAnimating];
[progressAlert addSubview:activityIndicator];
[activityIndicator release];
[progressAlert show];
[pool1 release];
}
-(void)dismissAlertMethod
{
NSAutoreleasePool *pool2=[[NSAutoreleasePool alloc]init];
[progressAlert dismissWithClickedButtonIndex:0 animated:YES];
[pool2 release];
}

Answer 1

There are some major flaws in how you attack the problem. Firstly you should not detach new threads to show and hide the alert view, all UIKit classes must be called form the main thread (only few documented exceptions exist).
What you want is an API designed to be asynchronous for dispatching the login request. I would suggest you use an Sync-Async pattern for this. I have written a longer blog post on this topic here: http://blog.jayway.com/2011/04/28/sync-asyn-pair-pattern-easy-concurrency-on-ios/
In essence I believe you want two public methods:
-(NSString*)loginMessageWithName:(NSString*)name
password:(NSString*)password
error:(NSError**)error;
-(NSOperation*)loginMessageWithName:(NSString*)name
password:(NSString*)password
delegate:(id<LoginMessageDelegate>)delegate;
The first method is synchronous, implement it as straightforward as you like, no threads on any thing, just make it work.
The second method is a wrapper that instantiates a NSOperation objects and puts it on some queue. Returning the operation allows you to cancel it, but the result will be returned on the delegate. The delegate will probably need to look something like this:
#protocol LogonMessageDelegate <NSObject>
-(void)didReceiveLoginMessage:(NSString*)message;
-(void)failedLoginMessageWithError:(NSError*)error;
#end
The implementation of loginMessageWithName:password:delegate: is very straight forward:
NSOperation* op = [[LoginMessageOperation alloc] initWithName:name
password:password
delegate:delegate];
[myOperationQueue addOperation:op];
return [op autorelease];
Most of the work will be done in your NSOperation subclass' main method. This is where you call the synchronious implementation, check for cancelation, and call back to the delegate if needed. Probably something like this:
-(void)main {
NSError* error = nil;
NSString* message = [logonMessageManager logonWithName:name
password:password:
error:&error];
if (![self isCancelled]) {
if (message) {
[delegate performSelectorOnMainThread:#selector(didReceiveLoginMessage:)
withObject:message
waitUntilDone:NO];
} else {
[delegate performSelectorOnMainThread:#selector(didReceiveLoginMessage:)
withObject:error
waitUntilDone:NO];
}
}
}
Then setup and handle the alert view on the main thread. Call [operation cancel] if the user cancels, or dismiss the alert when the delegate receives a callback.