How to get an array out of Grand Central Dispatch? - ios

Question

I am a newbie. I am using Grand Central Dispatch to populate an array (student_temp) on another thread. That part is working fine. The problem is I cannot pass the array to a class property (student_Array) where it is used throughout the class. I can't get the array back on the main thread.
it works fine until I get back tot he main thread and I can't pass student_temp into student_Array (the property) either inside or outside of GCD.
What am I doing wrong, or is there a better to populate the array property using GCD?
Thank you for your help. And please try to explain in non-technical language if possible I am new at this.
- (void)viewDidLoad
{
[super viewDidLoad];
R2LFetcher *studentFetch = [[R2LFetcher alloc] init];
__block NSMutableArray *student_temp = [[NSMutableArray alloc] init];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
//long-running code goes here…
student_temp = [studentFetch fetchToStudentArray];
dispatch_async(dispatch_get_main_queue(), ^{
// code the updates the main thread (UI) here...
student_Array = student_temp;
});
});
student_Array = student_temp;

Answer 1

A couple of reactions:
In the last line of your code, you're setting student_Array to student_temp. Clearly that line makes no sense because you're populating student_temp asynchronously. And you're opening yourself up to synchronization issues if you're trying to simultaneously access the save variable in two queues. Don't bother to assign student_Array to student_temp at the end of viewDidLoad, but rather just do it inside the nested dispatch_async calls.
Inside the block, you're populating and setting student_temp. It probably makes more sense to make that variable scoped within that block, avoiding temptation to access it from outside that block as well as simplifying your code because the __block qualifier is no longer needed.
This block is running asynchronously, so when you update student_Array in the main queue, you might want to update your UI at the same time (e.g. reload the tableview or whatever). Perhaps you're doing that already and just removed it for the sake of brevity, but I just wanted to make sure.
Thus:
- (void)viewDidLoad
{
[super viewDidLoad];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);
dispatch_async(queue, ^{
R2LFetcher *studentFetch = [[R2LFetcher alloc] init];
// long-running code goes here, for example ...
NSMutableArray *student_temp = [studentFetch fetchToStudentArray];
dispatch_async(dispatch_get_main_queue(), ^{
student_Array = student_temp;
// code the updates the main thread (UI) here, for example...
[self.tableView reloadData];
});
});
}

Answer 2

You should be able to add objects to student_Array directly from your block. Unlike stack variables, properties and ivars don't get copied when used inside a block. Instead, self gets retained in the block, and the property is referenced through it.
Of course, you need to be aware of concurrency issues, e.g. if you need to access the data from the main thread as well. For that, you probably still want to have this at the end of your async GCD block:
// done populating the data
dispatch_async(dispatch_get_main_queue(), ^{
// update the UI
}