Question

I have been struggling with this piece of code for a while and I just don’t know why it happens that when running in the device … the app crashes with an EXC_BAD_ACCESS Error but when running in the simulator it runs fine.

The scenario: A subclass of NSOperation that makes an async connection with NSURLConnection and gets custom data. When finished, it calls the block with the downloaded data.

Here is the .h file:

@interface FileDownloader : NSOperation <NSURLConnectionDataDelegate>

typedef void (^CompletionBlockForFile)(NSData *);

- (id)initWithCompletionBlock:(CompletionBlockForFile)block;

@end

and the .m file:

@interface FileDownloader ()

@property (strong, nonatomic) CompletionBlockForFile completionBlock;

@property (strong, nonatomic) NSMutableData *downloadedData;

- (void)downloadFileWithCompletionBlock:(CompletionBlockForFile)block;

@end


@implementation FileDownloader

@synthesize downloadedData = _downloadedData;
@synthesize completionBlock = _completionBlock;

- (id)initWithCompletionBlock:(CompletionBlockForFile)block
{
    self = [super init];
    if (self) {
        _completionBlock = block;
    }
    return self;
}

- (void)main
{
    if (self.isCancelled) return;
    if (_completionBlock) {
        [self downloadFileWithCompletionBlock:_completionBlock];
    }
}

- (void)downloadFileWithCompletionBlock:(CompletionBlockForFile)block
{   
    NSURL *url = [NSURL URLWithString:@"http://www.google.com"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    dispatch_async(dispatch_get_main_queue(), ^{
        NSURLConnection *connection = [NSURLConnection connectionWithRequest:request delegate:self];
        [connection start];
    });
}

... delegate methods of NSURLConnection

@end

And Finally, the method that adds the operation object to the queue at the MainViewController Class:

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    FileDownloader *fileDownloader = [[FileDownloader alloc] initWithCompletionBlock:^(NSData *data){      // <----- HERE IS THE EXC_BAD_ACCES ERROR JUST WHEN RUNNING IN THE DEVICE !!! :S
        NSLog(@"%@", data);
    }];

    [queue addOperation:fileDownloader];
}

Can anybody explain me what am I doing wrong? And is it correct to put strong for the block var in the property? Why not assign? Or weak ?

Thanks in advance 🙂

Answer 1

Blocks should be copied and not retained.
Change

@property (**strong**, nonatomic) CompletionBlockForFile completionBlock;

to

@property (**copy**, nonatomic) CompletionBlockForFile completionBlock;

and it should work