Method in stack trace is shown to be invoked from block, while it is not true in code

Question

I have stack trace for EXC_BAD_ACCESS KERN_INVALID_ADDRESS, and i am currently investigating this issue. However what surprises me most is that stack trace shows block invocation, while method is not called from block but rather from URLSession delegate method.

    Crashed: com.apple.main-thread
0  libobjc.A.dylib                0x1814d01a0 objc_retain + 16
1  libobjc.A.dylib                0x1814d0218 objc_storeStrong + 44
2  App                        0x1004644e8 -[FCProgressIndicator animateProgressViewToProgress:] (FCProgressIndicator.m:85)
3  App                        0x10046a560 -[FCSeriesListCollectionViewCell updateForItem:] (FCSeriesListCollectionViewCell.m:156)
4  App                        0x1003da4a8 __54-[FCFilmDownloadService notifyListenersForItemUpdate:]_block_invoke (FCFilmDownloadService.m:295)
5  libdispatch.dylib              0x181becaa0 _dispatch_call_block_and_release + 24
6  libdispatch.dylib              0x181beca60 _dispatch_client_callout + 16
7  libdispatch.dylib              0x181bf965c _dispatch_main_queue_callback_4CF$VARIANT$mp + 1012
8  CoreFoundation                 0x1822a3070 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
9  CoreFoundation                 0x1822a0bc8 __CFRunLoopRun + 2272
10 CoreFoundation                 0x1821c0da8 CFRunLoopRunSpecific + 552
11 GraphicsServices               0x1841a6020 GSEventRunModal + 100
12 UIKit                          0x18c1e0758 UIApplicationMain + 236
13 App                        0x1004a707c main (main.m:14)
14 libdyld.dylib                  0x181c51fc0 start + 4

Actual code is:

- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {

    NSString *downloadUrl = downloadTask.originalRequest.URL.absoluteString;
    FCDownloadableFilm *item = self.downloads[downloadUrl];

    /*Processing data */

    [self notifyListenersForItemUpdate:item];
}

- (void)notifyListenersForItemUpdate:(FCDownloadableFilm *) item {

    dispatch_async(dispatch_get_main_queue(), ^{
        NSSet *listeners = [self.listeners[item.url] copy];
        for (id <FCFilmDownloadServiceProtocol> listener in listeners) {
            [listener updateForItem: item];
        }
    });
}

What can cause stack trace to show that method notifyListenersForItemUpdate is called from block while it is not?

The call stack includes all of the runtime code, mostly stuff whose source you never see. The way to diagnose this is to set an exception breakpoint and rerun. https://stackoverflow.com/questions/17802662/exception-breakpoint-in-xcode — danh, Aug 04 '18 at 21:22

Sulthan · Accepted Answer · 2018-08-04T21:23:43.617

0

The stack trace does not show that notifyListenersForItemUpdate has been called from a block. It is a trace for updateForItem which was called from a block inside notifyListenersForItemUpdate:

dispatch_async(dispatch_get_main_queue(), ^{

(this one)

The stack trace says that a main loop is being processed and on it there is a block that is declared in [FCFilmDownloadService notifyListenersForItemUpdate:].

edited Aug 04 '18 at 21:23

answered Aug 04 '18 at 21:13

Sulthan

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Yes, i see now. – IeN Aug 04 '18 at 21:48

Method in stack trace is shown to be invoked from block, while it is not true in code

1 Answers1