Use dispatch_get_global_queue() in dispatch_get_main_queue - ios

I have started learning "GCD".
I found that when we use the FirstWay, the alertView will be called after all the NSLog functions are done with print.
But when we use the SecondWay, the alertView will be called before the NSLog function.
Why do these two methods run different results?
Can these two methods not be asynchronous methods?
My English is so poor, I hope somebody can understand my description.
Thank you!
/*
*FirstWay
*/
// dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
// dispatch_async(globalQueue, ^{
// for (int i = 0; i < 10000 ; i++) {
// NSLog(#"i = %d", i);
// }
//
// dispatch_async(dispatch_get_main_queue(), ^{
// NSLog(#"i = %d", i);
// UIAlertView *alertView = [[UIAlertView alloc]initWithTitle:#"Title " message:#"Message" delegate:self cancelButtonTitle:#"Cancel" otherButtonTitles:#"Yes", nil];
// [alertView show];
// });
// });
/*
*SecondWay
*/
dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *alertView = [[UIAlertView alloc]initWithTitle:#"Title " message:#"Message" delegate:self cancelButtonTitle:#"Cancel" otherButtonTitles:#"Yes", nil];
[alertView show];
dispatch_async(globalQueue, ^{
for (int i = 0; i < 10000 ; i++) {
NSLog(#"i = %d", i);
}
});
});

Here you are submitting a job (task) to a queue by calling the dispatch_async function. Feature of dispatch_async is that it returns immediately, and the submitted block will execute asynchronously in the background.Because UI related tasks run only in the main queue, so you need to return to the main queue if you are in the background queue. And you simply call the dispatch_get_main_queue() method to update the UI.
Here in your case:
In the first case you are submitting the task to the global_queue whereas in the second case you have submitted your task to the main queue.
In the first one , for-loop code is executed from the global_queue and task inside inner dispatch handler is dispatched to the main_queue where it updates the UI (i.e shows the alert view).
In the second one , you are simply submitting to main_queue and you are updating the UI (showing alert) and then you are dispatching to global queue to do for-loop task.
I hope now it helps if not ,feel free to comment.
More Info:
Don't get confused, first just look at the outer dispatch. You are dispatching something to work in the background.You are willing to do that task in the other thread so that your main thread won't have to wait for some long task.(if your main thread takes much time waiting for something then the OS kills your application so GCD is here to take us out of that easily).
Secondly just think the code inside of same outer dispatch as a instructions (line by line codes only). Don't mingle with the inner dispatch handler. Third Go through those line by line.Code upto the start of inner dispatch is the block of code you wish to do in the background.And the reason you nest the inner dispatch is to come out of that background thread(in most cases) in your first case you have done it to show the alert view after your for-loop code completes.

Related

Issues related to calling UIKit methods from non-main thread

I implemented login method in this way:
[KVNProgress show];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//some error handling like:
if ([_usernameField.text length] < 4) {
[KVNProgress showErrorWithStatus:#"Username too short!"];
_passwordField.text = #"";
return;
}
//Then I call login web service synchronously here:
result = [ServerRequests login];
dispatch_async(dispatch_get_main_queue(), ^{
if(!result)
{
[KVNProgress showErrorWithStatus:#"problem!" completion:NULL];
_passwordField.text = #"";
}
else if([result.successful boolValue])
{
[KVNProgress showSuccessWithStatus:result.message];
}
});
});
It crashed mostly and by surrounding blocks with only Main Queue (no priority default one) that solved! but the problem is:KVNProgress is only showing in error handling area not the next part that we call web service. It's not user friendly at all! Any idea is welcomed :)
You MUST call methods that update the user interface in any way from the main thread, as per the UIKit documentation:
For the most part, use UIKit classes only from your app’s main thread. This is particularly true for classes derived from UIResponder or that involve manipulating your app’s user interface in any way.
I suggest you try to limit the number of callbacks you make to the main thread, so therefore you want to batch as much user interface updates together as you can.
Then all you have to do, as you correctly say, is to use a dispatch_async to callback to your main thread whenever you need to update the UI, from within your background processing.
Because it's asynchronous, it won't interrupt your background processing, and should have a minimal interruption on the main thread itself as updating values on most UIKit components is fairly cheap, they'll just update their value and trigger their setNeedsDisplay so that they'll get re-drawn at the next run loop.
From your code, it looks like your issue is that you're calling this from the background thread:
if ([_usernameField.text length] < 4) {
[KVNProgress showErrorWithStatus:#"Username too short!"];
_passwordField.text = #"";
return;
}
This is 100% UI updating code, and should therefore take place on the main thread.
Although, I have no idea about the thread safety of KVNProgress, I assume it should also be called on the main thread as it's presenting an error to the user.
Your code therefore should look something like this (assuming it's taking place on the main thread to begin with):
[KVNProgress show];
//some error handling like:
if ([_usernameField.text length] < 4) {
[KVNProgress showErrorWithStatus:#"Username too short!"];
_passwordField.text = #"";
return;
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Then I call login web service synchronously here:
result = [ServerRequests login];
dispatch_async(dispatch_get_main_queue(), ^{
if(!result) {
[KVNProgress showErrorWithStatus:#"problem!" completion:NULL];
_passwordField.text = #"";
} else if([result.successful boolValue]) {
[KVNProgress showSuccessWithStatus:result.message];
}
});
});

Intangible Order of Execution (dispatch_semaphore_t, dispatch_group_async) and the Use of Them in Combination with Different Dispatch Queue Types

I just took some time in the evening to play around with GCD, especially with dispatch_semaphore_t because I never used it. Never had the need to.
So I wrote the following as a test:
- (void)viewDidLoad
{
UIView *firstView = [[UIView alloc] initWithFrame:(CGRect){{0, 0}, self.view.frame.size.width/4, self.view.frame.size.width/5}];
firstView.backgroundColor = [UIColor purpleColor];
[self.view addSubview:firstView];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
{
for (long i = 0; i < 1000; i++)
{
sleep(5);
dispatch_async(dispatch_get_main_queue(), ^
{
firstView.layer.opacity = ((i%2) ? 0: 1);
});
}
});
dispatch_queue_t queue1 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group1 = dispatch_group_create();
dispatch_group_async(group1, queue1, ^
{
sleep(3);
NSLog(#"dispatch group 1");
});
dispatch_group_notify(group1, queue1, ^
{
NSLog(#"dispatch notify 1");
});
dispatch_async(myQueue, ^
{
for(int z = 0; z < 10; z++)
{
NSLog(#"%i", z);
sleep(1);
}
dispatch_semaphore_signal(mySemaphore);
});
dispatch_semaphore_wait(mySemaphore, DISPATCH_TIME_FOREVER);
NSLog(#"Loop is Done");
}
If I ran the above, the output would be:
0
1
2
dispatch group 1
dispatch notify 1
3
4
5
6
7
8
9
Loop is Done
After the above, firstView appears on the screen (before semaphore the whole screen was black) and finally this gets executed:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
{
for (long i = 0; i < 1000; i++)
{
sleep(5);
dispatch_async(dispatch_get_main_queue(), ^
{
firstView.layer.opacity = ((i%2) ? 0: 1);
});
}
});
Which only alternates the opacity as the loop runs after semaphore is done.
1.)
So, it seems that I have to wait until dispatch_semaphore finish to do its work before any UI thing takes place.
BUT:
It seems like dispatch_group_t runs concurrently with dispatch_semaphore as shown from the output above (i.e., 1, 2, 3, ....).
???
2.)
And if I change the for loop in the above to using: dispatch_async(dispatch_get_main_queue(), ^
instead of:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^,
nothing gets shown on the screen even after semaphore is done.
How so???
3.)
Furthermore if I change semaphore to the following as opposed to using a global queue like in the above:
dispatch_async(dispatch_get_main_queue(), ^
{
for(int z = 0; z < 10; z++)
{
NSLog(#"%i", z);
sleep(1);
}
dispatch_semaphore_signal(mySemaphore);
});
Only dispatch_group takes place; nothing else take places / get executed, not the for loop in the above, including UI. Nothing.
4.)
So besides what I pointed out in the above, what can I do, in order to make semaphore not blocking my UI and my other process and just let my UI and the other processes do their thing?
And as mentioned above, why changing the type of queue for semaphore from global to main would cause nothing to be shown on screen and even the loop would not execute except dispatch_group?
5.)
If I change semaphore to:
dispatch_semaphore_t mySemaphore = dispatch_semaphore_create(1);
//1 instead of 0 (zero)
Everything (i.e., both for loop and UI) runs immediately and NSLog(#"Loop is Done"); is displayed also immediately, which tells me that semaphore didn't wait here:
dispatch_semaphore_wait(mySemaphore, DISPATCH_TIME_FOREVER);
NSLog(#"Loop is Done");
???
I spent the whole evening trying to figure this out, but to no avail.
I hope someone with great GCD knowledge can enlighten me on this.
First things first: As a general rule, one should never block the main queue. This rule about not blocking the main queue applies to both dispatch_semaphore_wait() and sleep() (as well as any of the synchronous dispatches, any group wait, etc.). You should never do any of these potentially blocking calls on the main queue. And if you follow this rule, your UI should never become non-responsive.
Your code sample and subsequent questions might seem to suggest a confusion between groups and semaphores. Dispatch groups are a way of keeping track of a group of dispatched blocks. But you're not taking advantage of the features of dispatch groups here, so I might suggest excising them from the discussion as it's irrelevant to the discussion about semaphores.
Dispatch semaphores are, on the other hand, simply a mechanism for one thread to send a signal to another thread that is waiting for the signal. Needless to say, the fact that you've created a semaphore and sent signals via that semaphore will not affect any of your dispatched tasks (whether to group or not) unless the code in question happens to call dispatch_semaphore_wait.
Finally, in some of your later examples you tried have the semaphore send multiple signals or changing the initial count to supplied when creating the semaphore. For each signal, you generally want a corresponding wait. If you have ten signals, you want ten waits.
So, let's illustrate semaphores in a way where your main queue (and thus the UI) will never be blocked. Here, we can send ten signals between two separate concurrently running tasks, having the latter one update the UI:
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
// send 10 signals from one background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
for (NSInteger i = 0; i < 10; i++) {
NSLog(#"Sleeping %d", i);
sleep(3);
NSLog(#"Sending signal %d", i);
dispatch_semaphore_signal(semaphore);
}
NSLog(#"Done signaling");
});
// and on another thread, wait for those 10 signals ...
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
for (NSInteger i = 0; i < 10; i++) {
NSLog(#"Waiting for signal %d", i);
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(#"Got signal %d", i);
// if you want to update your UI, then dispatch that back to the main queue
dispatch_async(dispatch_get_main_queue(), ^{
// update your UI here
});
}
NSLog(#"Done waiting");
});
This is, admittedly, not a terribly useful example of semaphores, but it illustrates how theoretically you could use them. In practice, it's rare that you have to use semaphores, as for most business problems, there are other, more elegant coding patterns. If you describe what you're trying to do, we can show you how to best achieve it.
As for an example with non-zero value passed to dispatch_semaphore_create, that's used to control access to some finite resource. In this example, let's assume that you had 100 tasks to run, but you didn't want more than 5 to run at any given time (e.g. you're using network connections (which are limited), or the each operation takes up so much memory that you want to avoid having more than five running at any given time). Then you could do something like:
// we only want five to run at any given time
dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);
// send this to background queue, so that when we wait, it doesn't block main queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for (NSInteger i = 0; i < 100; i++)
{
// wait until one of our five "slots" are available
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
// when it is, dispatch code to background queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(#"starting %d", i);
// to simulate something slow happening in the background, we'll just sleep
sleep(5);
NSLog(#"Finishing %d", i);
// when done, signal that this "slot" is free (please note, this is done
// inside the dispatched block of code)
dispatch_semaphore_signal(semaphore);
});
}
});
Again, this isn't a great example of semaphores (in this case, I'd generally use an NSOperationQueue with a maxConcurrentOperationCount), but it illustrates an example of why you'd use a non-zero value for dispatch_source_create.
You've asked a number of questions about groups. I contend that groups are unrelated to your own semaphores. You might use a group, for example, if you want to run a block of code when all of the tasks are complete. So here is a variation of the above example, but using a dispatch_group_notify to do something when all of the other tasks in that group are complete.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); // or create your own concurrent queue
dispatch_semaphore_t semaphore = dispatch_semaphore_create(5);
dispatch_group_t group = dispatch_group_create();
// send this to background queue, so that when we wait, it doesn't block main queue
dispatch_async(queue, ^{
for (NSInteger i = 0; i < 100; i++)
{
// wait until one of our five "slots" are available
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
// when it is, dispatch code to background queue
dispatch_group_async(group, queue, ^{
NSLog(#"starting %d", i);
// to simulate something slow happening in the background, we'll just sleep
sleep(5);
NSLog(#"Finishing %d", i);
dispatch_semaphore_signal(semaphore);
});
}
dispatch_group_notify(group, queue, ^{
NSLog(#"All done");
});
});

Objective C- Trouble updating UI on main thread

I am having some trouble updating my UI using performSelectorOnMainThread. Here is my situation. In my viewDidLoad I set up an activity indicator and a label. Then I call a selector to retrieve some data from a server. Then I call a selector to update the UI after a delay. Here's the code:
- (void)viewDidLoad
{
[super viewDidLoad];
self.reloadSchools = [[UIAlertView alloc] init];
self.reloadSchools.message = #"There was an error loading the schools. Please try again.";
self.reloadSchools.title = #"We're Sorry";
self.schoolPickerLabel = [[UILabel alloc]init];
self.schoolPicker = [[UIPickerView alloc] init];
self.schoolPicker.delegate = self;
self.schoolPicker.dataSource = self;
self.server = [[Server alloc]init];
schoolList = NO;
_activityIndicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
[self.view addSubview:_activityIndicator];
[self.view bringSubviewToFront:_activityIndicator];
[_activityIndicator startAnimating];
[NSThread detachNewThreadSelector: #selector(getSchoolList) toTarget: self withObject: nil];
[self performSelector:#selector(updateUI) withObject:nil afterDelay:20.0];
}
The selector updateUI checks to see if the data was retrieved, and calls a selector on the main thread to update the UI accordingly. Here is the code for these parts:
-(void)updateUI
{
self.schools = [_server returnData];
if(!(self.schools == nil)) {
[self performSelectorOnMainThread:#selector(fillPickerView) withObject:nil waitUntilDone:YES];
}
else {
[self performSelectorOnMainThread:#selector(showError) withObject:nil waitUntilDone:YES];
}
}
-(void)showError {
NSLog(#"show error");
[_activityIndicator stopAnimating];
[self.reloadSchools show];
}
-(void)fillPickerView {
NSLog(#"fill picker view");
schoolList = YES;
NSString *schoolString = [[NSString alloc] initWithData:self.schools encoding:NSUTF8StringEncoding];
self.schoolPickerLabel.text = #"Please select your school:";
self.shoolArray = [[schoolString componentsSeparatedByString:#"#"] mutableCopy];
[self.schoolPicker reloadAllComponents];
[_activityIndicator stopAnimating];
}
When the selector fillPickerView is called the activity indicator keeps spinning, the label text doesn't change, and the picker view doesn't reload its content. Can someone explain to me why the method I am using isn't working to update my ui on the main thread?
dispatch_async(dispatch_get_global_queue(0, 0), ^{
//load your data here.
dispatch_async(dispatch_get_main_queue(), ^{
//update UI in main thread.
});
});
First of all you should not be using detachNewThreadSelector. You should use GCD and submit your background task to an async queue. Threads are costly to create. GCD does a much better job of managing system resources.
Ignoring that, your code doesn't make a lot of sense to me. You submit a method, getSchoolList, to run on a background thread. You don't show the code that you are running in the background.
Then use performSelector:withObject:afterDelay to run the method updateUI on the main thread after a fixed delay of 20 seconds.
updateUI checks for self.schools, which presumably was set up by your background thread, and may or may not be done. If self.schools IS nil, you call fillPickerView using performSelectorOnMainThread. That doesn't make sense because if self.schools is nil, there is no data to fill the picker.
If self.schools is not nil, you display an error, again using performSelectorOnMainThread.
It seems to me that the logic on your check of self.schools is backwards. If it is nil you should display an error and if it is NOT nil you should fill the picker.
Next problem: In both cases you're calling performSelectorOnMainThread:withObject:waitUntilDone: from the main thread. Calling that method from the main thread doesn't make sense.
Third problem: It doesn't make sense to wait an arbitrary amount of time for a background task to run to completion, and then either succeed or fail. You won't have any idea what's going on for the full 20 seconds. If the background task finishes sooner, you'll never know.
Instead, you should have your background task notify the main thread once the task is done. That would be a valid use of performSelectorOnMainThread:withObject:waitUntilDone:, while calling it from the main thread is not. (Again, though, you should refactor this code to use GCD, not using threads directly.
It seems pretty clear that you are in over your head. The code you posted needs to be rewritten completely.

iOS: dispatch_async and UIImageWriteToSavedPhotosAlbum

Just learning how to allocate tasks among threads, or dispatch asynchronously. I understand that any operation that "touches" a view must be done on the main thread. What about: UIImageWriteToSavedPhotosAlbum? I would assume this could be done on a background thread, but am I mistaken?
Also, if it should be done on a background thread, is there a difference between these two calls below, as one saves a UIImage and the other saves a UIImage from a view?
UIImageWriteToSavedPhotosAlbum(_someUIImage ,nil,nil,nil);
UIImageWriteToSavedPhotosAlbum(_imageView.image ,nil,nil,nil);
By the way I am using this setup to run an HUD in the main thread and to tasks in the background, that is my intention.
[HUD_code showMessage:#"saving image"];
dispatch_queue_t concurrentQueue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
UIImageWriteToSavedPhotosAlbum(someUIImage ,nil,nil,nil);
dispatch_async(dispatch_get_main_queue(), ^{
[HUD_code dismiss];
});
});
UIKit classes are documented to be usable from the main thread only, except where documented otherwise. (For example, UIFont is documented to be thread-safe.)
There's no explicit blanket statement about the thread safety of UIKit functions (as distinct from classes), so it's not safe to assume they generally thread-safe. The fact that some UIKit functions, like UIGraphicsBeginImageContext, are explicitly documented to be thread-safe, implies that UIKit functions are not generally thread-safe.
Since UIImageWriteToSavedPhotosAlbum can send an asynchronous completion message, you should just call it on the main thread and use its completion support to perform your [HUD_code dismiss].
Here is my latest code after reading the answers, if anyone cares to know, or to comment (appreciated).
-(void)saveToLibrary {
if (_imageView.image != NULL) {
messageHUD = #"Saving Image...";
[SVProgressHUD showWithStatus:messageHUD];
UIImageWriteToSavedPhotosAlbum(_imageView.image, self, #selector(image:didFinishSavingWithError:contextInfo:), nil);
}
}
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo
{
UIAlertView *alert;
// Unable to save the image
if (error) {
alert = [[UIAlertView alloc] initWithTitle:#"Error"
message:#"Unable to save image to Photo Album."
delegate:self cancelButtonTitle:#"Ok"
otherButtonTitles:nil];
}else {// All is well
messageHUD = #"Success!\nImage Saved.";
[SVProgressHUD showSuccessWithStatus:messageHUD];
[self myPerformBlock:^{[SVProgressHUD dismiss];} afterDelay:0.5];
}
}
The myPerformBlock is from the following link https://gist.github.com/955123
I would assume this could be done on a background thread, but am I mistaken?
Honestly, I would assume it too, since this has absolutely nothing to do with updating the UI, it's just some file operation. However, Apple's documentation says that every call to UIKit needs to be performed on the main thread (Except where something else is explicitly stated). This function is no exception, you have to call it on the main thread.
By the way, this function is asynchronous itself. It will notify the callback object/selector supplied as its 2nd and 3rd arguments when the image is saved, and thus it doesn't block the UI.

problems with UIViewController switching

I have a little problem with an application.
When I switch the Views there will be a time delay about 3 to 10 seconds, after this app run two sorting algorythmen. (Quick and Bubblesort) in an extra thread. I dont know where the problem exists.
Here the code snippet:
[alert show];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long)NULL), ^(void) {
time = [Sort sortRuns:(int)slider_durchlaeufe.value WithCapacity:(int)slider_ArraySize.value Reference:alert]; //time is a global reference to my Time object
[self setDataForStatistik]; // a Method to set the statisik for the next view
[alert dismissWithClickedButtonIndex:0 animated:true];
});
I think the failure is in this code snippet.
I hope you can help me.
Robybyte
You need to make UI updates (or any changes that could trigger UI updates) on the main queue. You will see a good deal of this nested "async to background queue then async to main queue" when looking at code samples.
[alert show];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long)NULL), ^(void) {
time = [Sort sortRuns:(int)slider_durchlaeufe.value WithCapacity:(int)slider_ArraySize.value Reference:alert]; //time is a global reference to my Time object
dispatch_async(dispatch_get_main_queue(), ^{
[self setDataForStatistik]; // a Method to set the statisik for the next view
[alert dismissWithClickedButtonIndex:0 animated:true];
});
});
Hope that helps.

Resources