Objective c: Bad access error when using performSelectorOnMainThread

Question

Here is the problem.

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

2. 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)
   }
   

3. 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.

4. 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)
   }
   

5. 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];
   }
   

6. 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!!!

Answer 1

• 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?