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 & 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 & 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 -

enter image description here

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;
}

Can you please show the method you are calling in the background? And make sure the object `docids` is retained. — Rog, Aug 14 '11 at 07:02
yes, `docids` are `retain`. I have put it in `.h` as `@property (nonatomic, retain) NSMutableArray *docids;` — Srikar Appalaraju, Aug 14 '11 at 07:09
Don't prefix methods with `get`; that should just be `resultSetFromDB:` — bbum, Aug 14 '11 at 18:01

score 272 · Accepted Answer · edited May 12 '19 at 12:15

272

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.

edited May 12 '19 at 12:15

Cœur

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answered Aug 14 '11 at 07:22

Scott Forbes

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cool! didn't know this. Does this apply to `[NSThread detachNewThreadSelector:@selector....` also? – Srikar Appalaraju Aug 14 '11 at 07:25
Yes. Per the Apple docs, calling `performSelectorInBackground:withObject:` "is the same as if you called the `detachNewThreadSelector:toTarget:withObject:` method of `NSThread` with the current object, selector, and parameter object as parameters." – Scott Forbes Aug 14 '11 at 07:29
Is there a difference between `(unsigned long)NULL` and `0` in this matter? – Sti Mar 09 '14 at 01:12
4

@Sti from apple Dev [Docs](https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1): Note: The second argument to the dispatch_get_global_queue function is reserved for future expansion. For now, you should always pass 0 for this argument. – Jawad Al Shaikh May 07 '14 at 13:30
Should I then use performSelectorOnMainThread to update UI with operation results or there is more consistent way to update UI with GCD? – Ilya Denisov Feb 03 '15 at 10:53
Also, you can change the flag from *DISPATCH_QUEUE_PRIORITY_DEFAULT* to *DISPATCH_QUEUE_PRIORITY_BACKGROUND* – OhadM Jul 21 '15 at 11:50

score 11 · Answer 2 · answered Aug 10 '17 at 09:24

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

score 4 · Answer 3 · edited May 23 '17 at 10:31

4

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.

edited May 23 '17 at 10:31

Community

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answered Aug 14 '11 at 07:04

Nicolas S

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Please copy the line you used that run smoothly on main thread. – Nicolas S Aug 14 '11 at 07:06
I use this from main thread & atleast it hits that method instead of abruptly crashing - `[self getResultSetFromDB:docids];` – Srikar Appalaraju Aug 14 '11 at 07:11
have you enabled what i told you? – Nicolas S Aug 14 '11 at 07:12
Put a breakpoint in this line: _SpotMain *mirror = [[SpotMain alloc] init];_ and tell me if its hit and, if tehn, which line crashes. Enable zombies please so we can get a clear error log. – Nicolas S Aug 14 '11 at 07:16
yes, I have enabled zombies. I get this - ` 2011-08-14 12:49:42.697 FLO[16211:707] *** -[FMResultSet release]: message sent to deallocated instance 0x2bff80 2011-08-14 12:49:42.697 FLO[16211:1607] *** __NSAutoreleaseNoPool(): Object 0x2c0cc0 of class __NSCFData autoreleased with no pool in place - just leaking`. Also when I try to call this method from background thread I do not reach `SpotMain *mirror...`, It crashes soon after entering the background thread... – Srikar Appalaraju Aug 14 '11 at 07:20
This is not a zombie-related problem. The problem is that, if you create a new thread using `performSelectorInBackground:withObject`, your new thread doesn't have an autorelease pool (unless you create one manually). Hence the error message about autoreleasing an object with no pool in place. – Scott Forbes Aug 14 '11 at 07:27

score 2 · Answer 4 · answered Jun 08 '12 at 06:59

2

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.

answered Jun 08 '12 at 06:59

Mugunth

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score 2 · Answer 5 · answered May 11 '16 at 00:44

2

Swift 2.x answer:

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        self.getResultSetFromDB(docids)
    }

answered May 11 '16 at 00:44

Crashalot

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iOS start Background Thread

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