View is not displayed immediately - ios

Question

What i want to achieve :
Create processing screen with some custom icons, label etc. with following behaviour.
Add view over window which will not allow user to touch anything in application until it is removed. (like processing/loading screen)
When this view is displayed all other operation like adding subview, performing segue etc should work as they work normally but below my loading view.
Want method showProcessingScreen to work on any thread (Whatever thread switching code etc should be in respective show/hide method).
It should be displayed/removed immediately in after calling respective methods.
Code :
-(void) showProcessingScreen
{
dispatch_async(dispatch_get_main_queue(),
^{
UIStoryboard *mystoryboard = [UIStoryboard storyboardWithName:#"Main_iPhone" bundle:nil];
processingScreen = [mystoryboard instantiateViewControllerWithIdentifier:#"loadingViewController"];
UIWindow* mainWindow = [[UIApplication sharedApplication] keyWindow];
[mainWindow addSubview: processingScreen.view];
[mainWindow bringSubviewToFront:processingScreen.view];
});
}
-(void) hideProcessingScreen
{
dispatch_async(dispatch_get_main_queue(),
^{
[processingScreen.view removeFromSuperview];
});
}
Issue:
I want code above to work with showing/hiding loading screen immediately.
- (IBAction)proceedBtnPressed:(id)sender
{
[[GUIUtilities sharedObj] showProcessingScreen];
//Some other code here
}
When i call showProcessingScreen like above processing screen takes around 2-3 sec to show.
But when i remove other code below it (//Some other code) it shows screen immediately.
What i have tried:
Putting code in showProcessingScreen in other method and calling that on main thread using performSelectorOnMainThread.
Calling showProcessingScreen on background and executing show code on main thread using performSelector.
This works
//code
-(IBAction)proceedBtnPressed:(id)sender
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
^{
[[GUIUtilities sharedObj] showProcessingScreen];
//Some other code here
});
}
But i don't want any thread switching mechanism outside of showProcessingScreen.
This is common screen almost used in every application. I used similar codes with xib, custom views in my previous apps which were not using storyboard etc.,
I know this is related to threading, what i am doing wrong here ? what is best practice to achieve this ?
Any help will be aprreciatead.

Answer 1

The problem is that when you're dispatching, you put the creation of your loading view at the end of the run loop. Whatever your "other code" is is blocking the main thread for a few seconds.
If you move this "other code" to a different thread, this will resolve your issue. (This solution is ideal.)
You could also only switch to the main thread conditionally, which would resolve the issue if you call the loading view from the main thread:
-(void) showProcessingScreen
{
if (![NSThread isMainThread]) {
[self performSelectorOnMainThread:#selector(showProcessingScreen) withObject:nil waitUntilDone:FALSE];
return;
}
UIStoryboard *mystoryboard = [UIStoryboard storyboardWithName:#"Main_iPhone" bundle:nil];
processingScreen = [mystoryboard instantiateViewControllerWithIdentifier:#"loadingViewController"];
UIWindow* mainWindow = [[UIApplication sharedApplication] keyWindow];
[mainWindow addSubview: processingScreen.view];
[mainWindow bringSubviewToFront:processingScreen.view];
}

Answer 2

Instead of adding an overlay view that intercepts touch events, you can just call
– beginIgnoringInteractionEvents
– endIgnoringInteractionEvents

Answer 3

You could try with this:
- (void)doSomeOtherCode {
//Some other code here
}
- (IBAction) showProcessingScreenAndDoSomeOtherCode:(id)sender
{
[[GUIUtilities sharedObj] showProcessingScreen];
[self performSelector:#selector(doSomeOtherCode) withObject:nil afterDelay:0.0];
}
- (IBAction)proceedBtnPressed:(id)sender
{
[self showProcessingScreenAndDoSomeOtherCode:self];
}
and it will work. About your remark:
But i don't want any thread switching mechanism outside of showProcessingScreen.
The above solution will not cause any thread switching outside of showProcessingScreen. performSelector will just add an entry in the event loop queue.
EDIT:
If you are using blocks, the same result can be achieved through:
dispatch_async(dispatch_get_main_queue(),
^{
[[GUIUtilities sharedObj] showProcessingScreen];
dispatch_async(dispatch_get_main_queue(), ^{
//Some other code here
});
});
or:
dispatch_async(dispatch_get_main_queue(),
^{
[[GUIUtilities sharedObj] showProcessingScreen];
});
dispatch_async(dispatch_get_main_queue(), ^{
//Some other code here
});
Since the main queue is a serial queue, showProcessingScreen and "Some other code here" would be executed serially.