NSOperation and NSOperationQueue to perform task in background - ios

Question

I want to program a NSOperationQueue to perform tasks on the other thread.
So I created NSOperationQueue and class that derive from NSOperation . I added there a protocol to know if the task finished without errors and so on. But if I add an instance in my other class and set the delegate there then the main thread waits for async task to exectute the delegate code. How to prevent from it. I want to do something in my app when the background task is running.
AsyncTask.h
#protocol AsyncTaskEventsProtocol <NSObject>
-(void)OnFinishedSuccess:(NSObject *)sender information:(NSString *)inf;
-(void)OnFinishedFailed:(NSObject *)sender information:(NSString *)inf exception:(NSException *) ex;
#end
#interface AsyncOperation : NSOperation
#property id<AsyncTaskEventsProtocol> delegate;
#end
Async.m
#import "AsyncOperation.h"
#implementation AsyncOperation
#synthesize delegate;
-(void)main{
NSException * ex;
#try {
NSLog([NSThread isMainThread] ? #"This is the main thread" : #"This is not the main thread");
}
#catch (NSException *exception) {
ex = exception;
}
if(delegate != nil)
{
if(ex == nil){
[delegate OnFinishedSuccess:nil information:#"Data has been sent"];
}
else
{
[delegate OnFinishedFailed:nil information:#"Data has not been sent." exception:ex];
}
}
}
#end
In other class where create a queue
AsyncOperation * op = [[AsyncOperation alloc] init];
op.delegate = self;
NSOperationQueue * queue = [[NSOperationQueue alloc] init];
[queue addOperation:op]
And the handler
-(void)OnFinishedSuccess:(NSObject *)sender information:(NSString *)inf
{
UIAlertView * view = [[UIAlertView alloc] initWithTitle:#"Test" message:#"Message" delegate:nil cancelButtonTitle:#"ok" otherButtonTitles:nil];
[view show];
}