iOS start Background Thread

Question

I have a small sqlitedb in my iOS device  When a user presses a button  I fetch the data from sqlite  amp  show it to user   This fetching part I want to do it in a background thread  to not block the UI main thread   I do this like so -    self performSelectorInBackground  selector getResultSetFromDB   withObject docids    After the fetching  amp  a little bit of processing  I need to update the UI  But since  as a good practice  we should not perform UI updation from background threads  I call a selector on mainthread like so -    self performSelectorOnMainThread  selector showResults  withObject nil waitUntilDone NO    But my App crashes in the first step  i e  starting a background thread  Is this not a way to start background threads in iOS    UPDATE 1  After  self performSelectorInBackground     I get this stacktrace  no info what so ever -     UPDATE 2  I even tried  starting a background thread like so -   NSThread detachNewThreadSelector  selector getResultSetFromDB   toTarget self withObject docids   but still I get same stacktrace   Just so that I clarify  when I perform this operation on main thread everything runs smooth     UPDATE 3 This is the method I am trying to run from background   -  void getResultSetFromDB  NSMutableArray   toProceessDocids       SpotMain  mirror     SpotMain alloc  init       NSMutableArray  filteredDocids   toProceessDocids       if   gMediaBucket isEqualToString               filteredDocids    mirror FetchDocIdsForMediaBucketWithDocID filteredDocids mBucket gMediaBucket numRes -1       if   gMediaType isEqualToString               filteredDocids    mirror FetchDocIdsForMediaType filteredDocids mediaType gMediaType numRes -1       if   gPlatform isEqualToString               filteredDocids    mirror FetchDocIdsForPlatformID filteredDocids platformId   1  numRes -1        self resultSet    mirror FetchObjectFromDocid filteredDocids        filteredDocids release        mirror release         self performSelectorOnMainThread  selector showResults  withObject nil waitUntilDone NO       return

User · Answer

The default sqlite library that comes with iOS is not compiled using the SQLITE THREADSAFE macro on  This could be a reason why your code crashes

User · Answer

Enable NSZombieEnabled to know which object is being released and then accessed  Then check if the getResultSetFromDB  has anything to do with that  Also check if docids has anything inside and if it is being retained   This way you can be sure there is nothing wrong

User · Answer

If you use performSelectorInBackground withObject  to spawn a new thread  then the performed selector is responsible for setting up the new thread s autorelease pool  run loop and other configuration details     see  Using NSObject to Spawn a Thread  in Apple s Threading Programming Guide   You d probably be better off using Grand Central Dispatch  though   dispatch async dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0           self getResultSetFromDB docids         GCD is a newer technology  and is more efficient in terms of memory overhead and lines of code     Updated with a hat tip to Chris Nolet  who suggested a change that makes the above code simpler and keeps up with Apple s latest GCD code examples

User · Answer

Well that s pretty easy actually with GCD  A typical workflow would be something like this   dispatch queue t queue   dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0ul       dispatch async queue                Perform async operation            Call your method function here            Example             NSString  result    anObject calculateSomething                   dispatch sync dispatch get main queue                              Update UI                        Example                         self myLabel text   result                                For more on GCD you can take a look into Apple s documentation here

User · Answer

Swift 2 x answer       dispatch async dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0             self getResultSetFromDB docids

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