Objective c: Bad access error when using performSelectorOnMainThread

Question

Here is the problem.

I have a method called -(void)searchingInBackground which is running in background (performSelectorInBackground).

In this method, I have couple of different threads which are running in background too (performSelectorInBackground). Like this:

-(void)searchingInBackground
{
  @autoreleasepool {
    [self performSelectorInBackground:@selector(getDuplicatedPictures:) withObject:copyArray];
  }

  @autoreleasepool {
    [self performSelectorInBackground:@selector(getLocationsOfPhotos:) withObject:copyArray];
  }
... (and so on)
}

In each of functions in threads (ie. getDuplicatedPictures, getLocationsOfPhotos...) they will generate NSStrings at the end and I will use those strings to update my text field GUI.

In order to update my text field GUI. I created a function called UpdateGUI which will use to help me update all of my NSStrings. Like this,

-(void)UpdateUI
{
   [_NumDupPhotosLabel(label for GUI)  setStringValue: resultDupPhotos(string from thread function which is getDuplicatedPictures in this case)];
    ....(includes all of my strings from threads)
}

Here is the problem, when I call this UpdateGUI using performSelectorOnMainThread in each of threads function. It will give me EXC_BAD_ACCESS. Here is what I did. For example:
```
-(void)getDupicatedPictures
{
     resultDupPhotos = .....;
     [self performSelectorOnMainThread:@selector(UpdateUI) withObject:nil waitUntilDone:YES];
}
```

If I do not use performSelectorOnMainThread, just set the values directly in those functions it works fine. I just want to better organize the code.

-(void)getDuplicatedPictures
{
     resultDupPhotos = .....;
     [_NumDupPhotosLabel  setStringValue: resultDupPhotos]; (works good and it will set the value to the GUI label)
}

Could you guys tell me how to fix this? Thanks!!!

Maybe is a typo here, but you are using getDupicatedPictures instead getDup(l)icatedPictures and UpdateUI instead Update(G)UI — Vertig0, May 22 '13 at 15:18
If those spelling errors are corrected as Patricio mentioned and the error still happens, try enabling NSZombies to track it down. http://stackoverflow.com/questions/5386160/how-to-enable-nszombie-in-xcode — rmooney, May 22 '13 at 15:23
BTW: Methods shouldn't be prefixed with `get` unless they are of a very specific type, which these aren't. Also, preoperties and ivars should be prefixed with lowercase letters. Method names should always start with a lowercase letter. — bbum, May 22 '13 at 15:51

bbum · Accepted Answer · 2013-05-22T17:04:33.700

ARC or no?
if you have a crash, post the backtrace
surrounding a performInBackground:... call with an @autoreleasepool does nothing (NSAutoreleasePool isn't going to help, either -- you need the autorelease pool to be in the thread of execution)
if a variable is involved in a crash, show the variable's declaration and initialization
spawning a bunch of threads simultaneously to do a bunch of work is likely to be slower than doing the work sequentially. Concurrency should always be controlled. If you have a long running task, you might likely want to spin up a second thread. Or you might want to re-order operations. The issue, though, is that running multiple threads at once, especially if those threads are doing a lot of I/O, is just going to increase contention and may likely make things slower, often a lot slower.

More likely than not, one of the objects calculated on a background thread is being released before the main thread tries to use it. How do you ensure that resultDupPhotos is valid between threads?

Thanks. I keep only one performInBackground thread and get rid of others. But I heard if one function runs like more than 5 second or so, we suppose to open a new thread, don't we? Also if the @autoreleasepool does nothing. What should I do? Should I create NSAutoreleasePool *autoreleasepool = [[NSAutoreleasePool alloc] init]; in the function instead? — YU FENG, May 22 '13 at 16:43

Objective c: Bad access error when using performSelectorOnMainThread

1 Answers1