Perform block inside a NSOperation

Question

I have a method in some class which performs some task using a block. When I execute that method using NSInvocationOperation then control never goes to the block. I tried logging inside the block but that is never called actually. But if I simply call that method with instance of that class then everything works as expected.

Don’t blocks run inside NSOperation?

NSInvocationOperation *op = [[NSInvocationOperation alloc] initWithTarget:myClassObj selector:@selector(myClassMethod:) object:obj1];
[[AppDelegate sharedOpQueue] addOperation:op];
[op release];


- (void)myClassMethod:(id)obj
{
    AnotherClass *otherClass = [[AnotherClass allco] init]
    [otherClass fetchXMLWithCompletionHandler:^(WACloudURLRequest* request, xmlDocPtr doc, NSError* error)
     {
         if(error){
             if([_delegate respondsToSelector:@selector(handleFail:)]){
                 [_delegate handleFail:error];
             }
             return;
         }

         if([_delegate respondsToSelector:@selector(doSomeAction)]){
             [_delegate doSomeAction];
         }
     }];

}

- (void) fetchXMLWithCompletionHandler:(WAFetchXMLHandler)block
{
    _xmlBlock = [block copy];
    [NSURLConnection connectionWithRequest:request delegate:self];
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    if(_xmlBlock) {
        const char *baseURL = NULL;
        const char *encoding = NULL;

        xmlDocPtr doc = xmlReadMemory([_data bytes], (int)[_data length], baseURL, encoding, (XML_PARSE_NOCDATA | XML_PARSE_NOBLANKS)); 

        NSError* error = [WAXMLHelper checkForError:doc];

        if(error){
            _xmlBlock(self, nil, error);
        } else {
            _xmlBlock(self, doc, nil);
        }

        xmlFreeDoc(doc);
    }
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    if(_xmlBlock) {
        _xmlBlock(self, nil, error);
    }
}

Is the `-parseXMLWithCompletionHandler:` method on AnotherClass asynchronous? You might want to post the code for that method, as it looks like whatever it does for the callback isn't calling, and that's not described in your above code. — Brad Larson, Nov 19 '11 at 18:21
@BradLarson thanks for response, I have edited question with the details of the method -parseXMLWithCompletionHandler: — Jitendra Singh, Nov 20 '11 at 16:41
Your code calls `parseXMLWithCompletionHandler:` but shows the implementation for `fetchXMLWithCompletionHandler:`. — titaniumdecoy, Nov 21 '11 at 07:20
@Bavarious I didn't add the block tag. I just replace the iOS tag. — vikingosegundo, Nov 21 '11 at 11:53
@vikingosegundo I know; it’s just a note: when you edit questions, feel free to replace block/blocks with objective-c-blocks, too! — , Nov 23 '11 at 01:15
ok, most times I am just focus on the annoying iOS and the most annoying Xcode tag — vikingosegundo, Nov 23 '11 at 02:41

score 0 · Answer 1 · answered Nov 23 '11 at 13:23

You are performing your NSConnection asynchronously (which you don't need to do in an NSOperation because you should already be on a background thread).

After your call to fetchXMLWithCompletionHandler, your method ends. This signals that the NSOperation is finished and it gets released and it's thread gets either reused for something else or, more likely, released as well. This means that by the time you get your callbacks, your initial object doesn't exist anymore!

There are two solutions :

1) Use NSURLConnection synchronously. This will wait in your myClassMethod until it has got a response.

2) Learn about NSOperations's concurrent mode. I don't know if this will work with NSInvocationOperation though :( And it's fairly complicated compared to option (1).

I would use method (1) - you have already created a background thread to perform your operation in, why bother creating another one to do your connection request?

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

There are two ways of fixing your problem:

The easy way out

is — as Dean suggests — using +[NSURLConnection sendSynchronousRequest:returningResponse:error:], as you already are on a different thread. This has you covered — I'd say — 80-90% of the time, is really simple to implement and Just Works™.

The other way

is only slightly more complicated and has you covered for all the cases where the first method does not suffice — by visiting the root of your problem:

NSURLConnection works in conjunction with the runloop — and the threads managed by NSOperationQueue don't necessarily use (or even have!) an associated runloop.

While calling +[NSURLConnection connectionWithRequest:delegate:] will implicitly create a runloop, if needed, it does not cause the runloop to actually run!

This is your responsibility, when the NSOperationQueue you use is not the queue associated with the main thread.

To do so, change your implementation of fetchXMLWithCompletionHandler: to look similar to the following:

- (void)fetchXMLWithCompletionHandler:(WAFetchXMLHandler)block
{
    self.xmlHandler = block; // Declare a @property for the block with the copy attribute set

    self.mutableXMLData = [NSMutableData data]; // again, you should have a property for this...

    self.currentConnection = [NSURLConnection connectionWithRequest:request delegate:self]; // having a @property for the connection allows you to cancel it, if needed.

    self.connectionShouldBeRunning = YES; // ...and have a BOOL like this one, setting it to NO in connectionDidFinishLoad: and connection:didFailWithError:

    NSRunLoop *loop = [NSRunLoop currentRunLoop];
    NSDate *neverExpire = [NSDate distantFuture];

    BOOL runLoopDidIterateRegularly = YES;
    while( self.connectionShouldBeRunning && runLoopDidIterateRegularly ) {
        runLoopDidIterateRegularly = [loop runMode:NSDefaultRunLoopMode beforeDate:neverExpire];
    }
}

With these small changes, you're good to go. Bonus: this is really flexible and (eventually) reusable throughout all your code — if you move the XML-parsing out of that class and make your handler simply take an NSData, an NSError and (optionally) an NSURLResponse.

Since you probably don't want the clients of your loader to see and possibly mess with the properties I just suggested you should add, you can declare them in a class continuation.

Perform block inside a NSOperation

2 Answers2

The easy way out

The other way