NSOperation Causing Crash when Passed to Delegate

Question

For an iPhone app, I'm using a NSOperationQueue to limit access to the SQLite database to one query at a time. I created a subclass of NSOperation and in the main function I have the following:

- (void)main
{    
    // ... other code here ...

    if( [_delegate respondsToSelector:@selector(queryCompleted:)] )
    {
        [_delegate performSelectorOnMainThread:@selector(queryCompleted:) 
                                    withObject:self
                                 waitUntilDone:NO];
    }
}

Delegate side:

- (void)queryCompleted:(QueryOperation*)aQueryOperation
{
     // Breakpoint here allows me to explore and see the members of aQueryOperation

     id results = [aQueryOperation resultSet]; // Crashes here

     // ... more code here ...
}

The reason I am passing self is to allow the delegate to access the ID for the query operation (in the case where there is more than one request open per delegate) and the results from the query.

In the documentation for performSelectorOnMainThread:withObject:waitUntilDone:, it clearly states:

"This method retains the receiver and the arg parameter until after the selector is performed."

However when the delegate method tries to access the argument, an "EXC_BAD_ACCESS" exception is thrown. Any thoughts on why?

Oddly enough, if I set a breakpoint before the crashing reference to the NSOperation object, the debugger permits me to see the object instance and the values of all the parameters.

Answer 1

Try setting the waitUntilDone: parameter to YES. Possibly there is a race condition which is allowing the NSOperation to deallocate itself.