access to property of another class from NSOperation object - multithreading

Question

Dear community.
I have NSOperation subclass with property:
#property(readwrite, getter=isCancelled) BOOL cancelled;
from where i was create object of subclass NSObject (from init):
database = [[[MySQLIXC alloc] initWithQuene:iQuene andCarrier:startForCarrier withState:cancelled] retain];
In this custom object i try to declare local iVar:
#interface MySQLIXC : NSObject {
BOOL _currentQueueStatus;
In init:
- (id)initWithQuene:(NSUInteger)quene andCarrier:(NSString *)carrierName withState:(BOOL)currentQueueStatus;
_currentQueueStatus = currentQueueStatus;
But currentQueueStatus allways null.
Can somebody suggest a problem location?

Answer 1

1) you should avoid re-declaring implemented subclass interface for your additions (e.g. -[NSOperation isCancelled] exists)
2) it is very unusual to start with two retain counts:
database = [[MySQLIXC alloc] initWithQuene:iQuene andCarrier:startForCarrier withState:cancelled];
[otherThingThatHoldsAReference setDatabase:database];
instead of:
database = [[[MySQLIXC alloc] initWithQuene:iQuene andCarrier:startForCarrier withState:cancelled] retain];
3) _currentQueueStatus is not null it's a BOOL, which is a signed char. it should be 0 (NO) or 1 (YES).
4) what is currentQueueStatus? more code would help you receive more specific answers.
EDIT: updated for clarification in response to comments
/* things would look different if you subclass MONOperation */
#interface MONOperation : NSOperation
{
#private
MySQLIXC * sqlIxc;
BOOL didCancelDatabaseRequest;
}
/*
do not use isCancelled for your name - NSOperation declares this method, and
its implementation is well defined. this would override the implementation
and likely cause runtime errors.
specifically, NSOperation/NSOperationQueue uses cancel and isCancelled in its
interface and state. if you must cancel, then call cancel from cancelDatabaseRequest.
you may override cancel, but call [super cancel] in your implementation.
*/
#property (readonly) BOOL didCancelDatabaseRequest;
#property (retain) MySQLIXC * sqlIxc;
#end
#implementation MONOperation
/* ... */
- (BOOL)didCancelDatabaseRequest
{
return didCancelDatabaseRequest;
}
- (void)cancelDatabaseRequest /* in this example, there is no sense making cancelDatabaseRequest publicly visible */
{
/* for example */
assert(self.sqlIxc);
self.sqlIxc = nil;
didCancelDatabaseRequest = YES;
[self cancel]; /* do this rather than overriding NSOperation's cancellation interfaces in your subclass */
}
- (void)main
{
NSAutoreleasePool * pool = [NSAutoreleasePool new];
assert(self.sqlIxc);
self.sqlIxc.doStuff;
if (self.sqlIxc.didTimeout || self.sqlIxc.couldNotAccessFile) {
[self cancelDatabaseRequest];
}
else {
/* use the result */
}
[pool release];
}
#end