is it necessary to use dispatch_queue at here - ios

Question

Hopefully it is an appropriate question to ask. My goal is
1.add a controller into an array name 'arrControllers'
2.access and get current controller from 'arrControllers' to do sth with it
and I just wanna make sure the arrControllers should be ready before I access and use it. That is why I am using serial queue and dispatch_sycn like like following.
-(void)viewDidLoad
{
[super viewDidLoad];
firstViewController = [[FirstViewController alloc] initWithNibName:#"FirstViewController" bundle:nil];
[self setUpWithView:firstViewController];
}
and in setUpWithView: is
-(void)setUpWithView:(UIViewController*)viewController {
dispatch_queue_t queue;
queue = dispatch_queue_create("my queue", NULL);
containerController = [ContainerViewController sharedContainerController];
// What I am taking about is from now on
dispatch_sync(queue, ^{
[containerController initWithRootViewController:viewController];
});
dispatch_sync(queue, ^{
[containerController setUpView];
});
}
and initWithRootViewController: is
- (void)initWithRootViewController:(UIViewController*)rootViewController {
arrControllers = [[NSMutableArray alloc] init];
arrControllers = [NSMutableArray arrayWithObject:rootViewController];
}
and setUpView is
-(void)setUpView {
/* Process to ADD currentController TO hierarchy */
[self addChildViewController:[arrControllers lastObject]];
............................................................
}
As far as I know the compiler will execute codes line by line, it means by do following
-(void)setUpWithView:(UIViewController*)viewController {
containerController = [ContainerViewController sharedContainerController];
// Not using serial_queue and dispatch_sync
[containerController initWithRootViewController:viewController];// line1
[containerController setUpView]; //line2
}
compiler will execute line2 until it finished the task at line1.
My question is
1. is it necessary to using `serial_queue` and `dispatch_sycn`for this situation.
PS: if you think it is a bad question to ask. please comment what you think about it.

Answer 1

First of all, you should have a look at UIView and UIViewController life cycles, this will tell you when a UIVie or a UIViewController is "ready to be used". Second, you should check when the serial ques and GCD is used.
For the second point I can give you a short summary because it's kind of faster that the first point.
Use CGD and queues (other then main queue) if you want to do computing or other stuff, that don't involve UI changes, on a background thread without freezing the UI.
Use GCD with main queue to switch between a background queue and the main (UI) queue.
All UI operations must be performed on the main thread.
So in your case, you want to create a view controller and store it into an array, so as a suggestion, all the UI related calls of your view controllers should be performed on the UI thread, so no need for GCD or background threads.
In case you are doing some complicated stuffs on your view controller's init methods, just put those complicated stuffs on a GCD block on a separated thread.

Answer 2

In this situation if you do not use dispatch_sync, you will get exactly what you need:
-(void)setUpWithView:(UIViewController*)viewController {
containerController = [ContainerViewController sharedContainerController];
// Not using serial_queue and dispatch_sync
[containerController initWithRootViewController:viewController];// line1
[containerController setUpView]; //line2
}
Line 1 will execute initWithRootViewController:, going deeper and executing lines
arrControllers = [[NSMutableArray alloc] init];
arrControllers = [NSMutableArray arrayWithObject:rootViewController];
then it will return from this function one level above and proceed, moving to line 2 and going down to line
[self addChildViewController:[arrControllers lastObject]];
ALSO. Never write code like this
arrControllers = [[NSMutableArray alloc] init];
arrControllers = [NSMutableArray arrayWithObject:rootViewController];
First line creates an empty NSMutableArray and assigns a pointer to it into arrControllers.
Second line creates another array (autoreleased) and assigns a pointer to it in arrControllers again. So a link to array created in the first line is lost and it is leaked under manual memory management.
If you use manual memory management, do the following:
arrControllers = [[NSMutableArray alloc] init];
[arrControllers addObject:rootViewController];
OR
arrControllers = [[NSMutableArray alloc] initWithObjects:rootViewController,nil];
Under ARC leave only second line.
Hope I understood you correctly. Feel free to ask questions.