Question

Im using a threading class (.h/.m below) where the subclass is UIViewcontroller works without any issues.

@interface myFirstClass : UIViewController <MyOperationDelegate>{

However when I use it where the subclass is a NSobject to call a reachability class checking for internet connection, the App crashes when calling performSelectorOnMainThread? I dont understand why, there are no error when I build the App and when it crashes all i get is EXC_BAS_ACCESS. Is it not possible to do this when dealing with an NSObject? Any suggestion will be helpful for me.

@interface AppController : NSObject <MyOperationDelegate>{

myThreading.h

@protocol MyOperationDelegate
    @required
    -(void) updatedStatus:(NSArray*)items;
    -(void) failedStatusWithError:(NSError*)error;
@end

@interface MyOperation : NSObject {    
    NSObject<MyOperationDelegate> * delegate;
    NSOperationQueue *queue;
}

@property (retain) NSObject<MyOperationDelegate> *delegate;

-(void)load: (NSString *)stringUrlPath:(NSString *)functionAction;

@end

myThreading.m

@interface MyOperation (NSObject)
    -(void)dispatchLoadingOperation:(NSDictionary *)aParameters;        
@end


@implementation MyOperation

@synthesize delegate;

-(id)init   
{
    if ([super init]!=nil) {
        queue = [NSOperationQueue new];
        [queue setMaxConcurrentOperationCount:1];
    }
    return self;
}

-(void)load: (NSString *)stringUrlPath: (NSString *)functionAction {

    [self dispatchLoadingOperation:[NSDictionary dictionaryWithObjectsAndKeys:
                                        stringUrlPath, @"urlString", functionAction, @"action", nil]];
}

-(void)dealloc {
        [queue cancelAllOperations];
        self.delegate = nil;
        [super dealloc];
}

-(void)dispatchLoadingOperation:(NSDictionary *)aParameters {

    if([aParameters objectForKey:@"action"] == @"getStatus"){
        @synchronized(self) {
            NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self
                                                                                        selector:@selector(fetchCheckStatus:)
                                                                                          object:aParameters];
            [queue addOperation:operation];
            [operation release];
        }
    }
}


-(void) fetchCheckStatus:(NSDictionary *)aParameters
{   

    NSData* data = [[NSMutableData alloc] initWithContentsOfURL:[NSURL URLWithString:[aParameters objectForKey:@"urlString"]] ];

    NSError *error;
    NSString *responseString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

    if (responseString != nil) {

        NSMutableArray *rssItems;

        [self.delegate performSelectorOnMainThread:@selector(updatedStatus:) withObject:[NSArray arrayWithObjects:rssItems, nil] waitUntilDone:NO];


    } else {
            [queue cancelAllOperations];
            [self.delegate performSelectorOnMainThread:@selector(failedStatusWithError:) withObject:error waitUntilDone:NO];
    }

    [responseString autorelease];
    [data release];
}
@end 

Answer 1

The problem are these lines:

NSMutableArray *rssItems;

[self.delegate performSelectorOnMainThread:@selector(updatedStatus:) withObject:[NSArray arrayWithObjects:rssItems, nil] waitUntilDone:NO];

You declare a variable rssItems but don’t set it. It will contain random garbage from the stack which will then be interpreted as a pointer. Maybe sometimes you’re lucky and the value is actually a pointer to a living object, but more likely dereferencing it causes your crash.

You need to actually initialize the variable, e.g.:

NSMutableArray *rssItems = nil;

but I guess you really want:

NSMutableArray *rssItems = [NSMutableArray array];