Main Thread Conflict / Timing Issue - GCD - iPhone - iphone

Question

I have the following dispatch queue my app :
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^ {
[activeModel freeUpMallocedData];
// UI Updates have to be made on the main thread, so request from GCD.
dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_async(queue, ^ {
[mainViewController removeTidyUpScreen];
[mainViewController showSceneList];
[activeModel release];
});
});
The freeUpMallocedData method updates a UI Progress View :
- (void) freeUpMallocedData {
// Calculate the percentage increase for each item in the pointerStorageArray as it is released so we can update the Progress Screen.
float arrayCount = [pointerStorageArray count];
float incrementCounter = 1 / arrayCount; // Caculates 1% of pointerStorageArray
float newValue = incrementCounter;
int stepCounter = 0;
NSString * message;
// Now iterate through the pointerStorageArray and free all malloced memory.
for (NSValue * value in pointerStorageArray) {
stepCounter ++;
newValue = newValue + incrementCounter;
message = [NSString stringWithFormat:#"Freeing Up Memory (%d of %d) ...", stepCounter, (int)arrayCount];
free(value);
[self tidyUpProgress:message amount:newValue];
}
}
The tidyUpProgress method then executes on the main thread.
- (void) tidyUpProgress: (NSString *) progressMsg amount: (float) amount {
if (tidyUpMonitorDelegate) {
tidyUpProgressMsg = progressMsg;
tidyUpProgressAmount = amount;
[tidyUpMonitorDelegate performSelectorOnMainThread:#selector(model3DTidyUpProgressUpdate) withObject:nil waitUntilDone:NO];
}
}
- (void) model3DTidyUpProgressUpdate {
progressView.progress = app.activeModel.tidyUpProgressAmount;
loadingStatus.text = app.activeModel.tidyUpProgressMsg;
}
The problem is that the app crashes when the freeUpMallocedData method completes. The reason for this is that my initial dispatch queue moves on to request the main queue which then releases activeView. This seems to hijack the thread from the tidyUpMonitorDelegate before it can perform its last update - when it gets the main thread back the activeView has been released and therefore the app crashes as the model3DTidyUpProgresUpdate method is requesting access to variable in a class which has now been dealloced.
Can anyone advise on how to fix this timing issue ?
Thank you in advance.

Answer 1

Just a thought - try renaming the variable inside the dispatch:
dispatch_queue_t mainqueue = dispatch_get_main_queue();
dispatch_async(mainqueue, ^ {

Answer 2

You use two different mechanisms to schedule tasks in the main thread : dispatch_asyc and performSelectorInMainThread:withObject:waitUntilDone:. Each mechanism uses its own queue, which is read by the main run loop.
The order in which those queue are read is undefined. Thus, a task scheduled by performSelectorInMainThread:withObject:waitUntilDone may be performed after (or before) a task scheduled with dispatch_async regardless of which was scheduled first.
You should update tidyUpProgress: to use dispatch_async. Then the order will be guaranteed.
Moreover, after releasing an object, you should always nullify the variable that holds the reference to that object.

Answer 3

this is wrong way:
float arrayCount = [pointerStorageArray count];
-correct way:
NSUinteger arrayCountInt = [pointerStorageArray count];
NSNumber *arrayCountNumber = [NSNumber numberWithUnsignedInteger:arrayCountInt]
float arrayCount = [arrayCountNumber floatValue];