Trying to send process to background without UI lockup - ios

Question

I am trying to call a method in which I send to the background making use of dispatch_async.
It should be something that is simple, but for some reasons the UI is still blocked until the method returns.
Here is what I have:
dispatch_queue_t privateQueue = dispatch_queue_create("com", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(privateQueue, ^
{
__block UIImageView *imgView = [[UIImageView alloc] initWithFrame:self.view.frame];
dispatch_async(dispatch_get_main_queue(), ^
{
imgView = [controllerB startProcess];
controllerC.imageView = imgView;
});
});
I still have to wait for startProcess returns before UI is free again.
Then I tried to move imgView = [controllerB startProcess]; outside of dispatch_get_main_queue():
dispatch_async(privateQueue, ^
{
__block UIImageView *imgView = [[UIImageView alloc] initWithFrame:self.view.frame];
imgView = [controllerB startProcess];
dispatch_async(dispatch_get_main_queue(), ^
{
controllerC.imageView = imgView;
});
});
In this case, the UI is never updated with imgView but UI is not locked up.
I have tried to use a global queue, but the result is the same (UI has to wait):
dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
I think I am missing something very obvious here. Either that or it has been long day for me.
EDIT:
In [controllerB startProcess];
I am making use of:
UIGraphicsBeginImageContextWithOptions(self.frame.size, NO, 0.0);
UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
I am not sure if these methods have anything to do with GCD that causes my problem. The image is just .png.
Been thinking hard on this. Am running out of ideas. The only way I can update the UI with the image is to place the method call within dispatch_get_main_queue(), which beats the purpose of using GCD because all UI is blocked until the image is ready and method returns.
Any suggestion would be greatly greatly appreciated.

Answer 1

Use the second approach. Modify startProcess to use completion blocks and update your imageView inside the completion block. This ensures that imageView is updated after startProcess is complete.

Answer 2

Is it possible, that -in your 2nd example- when you try to set the imageView on the main queue the asynchronous calculation of the imageView in the background has not finished yet, so it can't display the imageView?
In that case a dispatch group might help:
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_queue_create("com.name.queue”, DISPATCH_QUEUE_CONCURRENT);
dispatch_group_async(group, queue, ^{
//Do work
imgView = [controllerB startProcess];
});
dispatch_group_notify(group,queue,^{
//This code runs as soon as the code in the dispatch group is done.
controllerC.imageView = imgView;
});