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First I create a serial queue like this
static dispatch_queue_t queue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_SERIAL);
then, at some unknown point in time a task gets added to the queue like this
dispatch_async(queue, ^{
// do something, which takes some time
});
If the first task hasn't finished yet, the new task will wait until the first completes (that's of course what a serial queue is for).
But if I add 5 new tasks to the queue, while the original first one is still running, I don't want to execute new task no.1, then no.2, then no.3 and so on, but want to get rid of tasks 1 to 4 and directly start executing task no.5 after the original first task has finished.
In other words, I want to pop any waiting task (not the one that is currently running) off the queue, if I add a new one.
Is there a build in mechanism for this or do I have to implement this myself? And for the latter, how would I identify single tasks inside a queue and remove them?
Once a block has been submitted to a GCD dispatch queue, it will run. There is no way to cancel it. You can, as you know, implement your own mechanism to "abort" the block execution early.
An easier way to do this would be to use NSOperationQueue, as it already provides an implementation for canceling pending operations (i.e., those not yet running), and you can easily enqueue a block with the new-ish addOperationWithBlock method.
Though NSOperationQueue is implemented using GCD, I find GCD much easier to use in most cases. However, in this case, I would seriously consider using NSOperationQueue because it already handles canceling pending operations.
With Davids answer getting me on track I succeeded in doing this like so
taskCounter++;
dispatch_async(queue, ^{
if (taskCounter > 1) {
taskCounter--;
NSLog(#"%#", #"skip");
return;
}
NSLog(#"%#", #"start");
// do stuff
sleep(3);
taskCounter--;
NSLog(#"%#", #"done");
});
taskCounter has to be either an ivar or a property (initialize it with 0). In that case it doesn't even need the __block attribute.
The way you handle this is to use an ivar that indicates to the queued blocks they should just return:
^{
if(!canceled) {
... do work
}
}
You don't need to use a simple boolean either - you can make this more complex - but the general idea is to use one or more ivars that the block queries before doing anything.
I use this technique (but did not invent it) with great success.
If instead of adding a closure in you add a DispatchWorkItem, you can cancel it as long as it hasn't started executing yet.
In the following code, backgroundWorkItem will never run, because it is cancelled before it starts executing.
let backgroundWorkItem = DispatchWorkItem {
print("Background work item executed")
}
DispatchQueue.main.async(execute: backgroundWorkItem)
backgroundWorkItem.cancel()
I ran into a scenario where I had a delegate callback which could occur on either the main thread or another thread, and I wouldn't know which until runtime (using StoreKit.framework).
I also had UI code that I needed to update in that callback which needed to happen before the function executed, so my initial thought was to have a function like this:
-(void) someDelegateCallback:(id) sender
{
dispatch_sync(dispatch_get_main_queue(), ^{
// ui update code here
});
// code here that depends upon the UI getting updated
}
That works great, when it is executed on the background thread. However, when executed on the main thread, the program comes to a deadlock.
That alone seems interesting to me, if I read the docs for dispatch_sync right, then I would expect it to just execute the block outright, not worrying about scheduling it into the runloop, as said here:
As an optimization, this function invokes the block on the current thread when possible.
But, that's not too big of a deal, it simply means a bit more typing, which lead me to this approach:
-(void) someDelegateCallBack:(id) sender
{
dispatch_block_t onMain = ^{
// update UI code here
};
if (dispatch_get_current_queue() == dispatch_get_main_queue())
onMain();
else
dispatch_sync(dispatch_get_main_queue(), onMain);
}
However, this seems a bit backwards. Was this a bug in the making of GCD, or is there something that I am missing in the docs?
dispatch_sync does two things:
queue a block
blocks the current thread until the block has finished running
Given that the main thread is a serial queue (which means it uses only one thread), if you run the following statement on the main queue:
dispatch_sync(dispatch_get_main_queue(), ^(){/*...*/});
the following events will happen:
dispatch_sync queues the block in the main queue.
dispatch_sync blocks the thread of the main queue until the block finishes executing.
dispatch_sync waits forever because the thread where the block is supposed to run is blocked.
The key to understanding this issue is that dispatch_sync does not execute blocks, it only queues them. Execution will happen on a future iteration of the run loop.
The following approach:
if (queueA == dispatch_get_current_queue()){
block();
} else {
dispatch_sync(queueA, block);
}
is perfectly fine, but be aware that it won't protect you from complex scenarios involving a hierarchy of queues. In such case, the current queue may be different than a previously blocked queue where you are trying to send your block. Example:
dispatch_sync(queueA, ^{
dispatch_sync(queueB, ^{
// dispatch_get_current_queue() is B, but A is blocked,
// so a dispatch_sync(A,b) will deadlock.
dispatch_sync(queueA, ^{
// some task
});
});
});
For complex cases, read/write key-value data in the dispatch queue:
dispatch_queue_t workerQ = dispatch_queue_create("com.meh.sometask", NULL);
dispatch_queue_t funnelQ = dispatch_queue_create("com.meh.funnel", NULL);
dispatch_set_target_queue(workerQ,funnelQ);
static int kKey;
// saves string "funnel" in funnelQ
CFStringRef tag = CFSTR("funnel");
dispatch_queue_set_specific(funnelQ,
&kKey,
(void*)tag,
(dispatch_function_t)CFRelease);
dispatch_sync(workerQ, ^{
// is funnelQ in the hierarchy of workerQ?
CFStringRef tag = dispatch_get_specific(&kKey);
if (tag){
dispatch_sync(funnelQ, ^{
// some task
});
} else {
// some task
}
});
Explanation:
I create a workerQ queue that points to a funnelQ queue. In real code this is useful if you have several “worker” queues and you want to resume/suspend all at once (which is achieved by resuming/updating their target funnelQ queue).
I may funnel my worker queues at any point in time, so to know if they are funneled or not, I tag funnelQ with the word "funnel".
Down the road I dispatch_sync something to workerQ, and for whatever reason I want to dispatch_sync to funnelQ, but avoiding a dispatch_sync to the current queue, so I check for the tag and act accordingly. Because the get walks up the hierarchy, the value won't be found in workerQ but it will be found in funnelQ. This is a way of finding out if any queue in the hierarchy is the one where we stored the value. And therefore, to prevent a dispatch_sync to the current queue.
If you are wondering about the functions that read/write context data, there are three:
dispatch_queue_set_specific: Write to a queue.
dispatch_queue_get_specific: Read from a queue.
dispatch_get_specific: Convenience function to read from the current queue.
The key is compared by pointer, and never dereferenced. The last parameter in the setter is a destructor to release the key.
If you are wondering about “pointing one queue to another”, it means exactly that. For example, I can point a queue A to the main queue, and it will cause all blocks in the queue A to run in the main queue (usually this is done for UI updates).
I found this in the documentation (last chapter):
Do not call the dispatch_sync function from a task that is executing
on the same queue that you pass to your function call. Doing so will
deadlock the queue. If you need to dispatch to the current queue, do
so asynchronously using the dispatch_async function.
Also, I followed the link that you provided and in the description of dispatch_sync I read this:
Calling this function and targeting the current queue results in deadlock.
So I don't think it's a problem with GCD, I think the only sensible approach is the one you invented after discovering the problem.
I know where your confusion comes from:
As an optimization, this function invokes the block on the current
thread when possible.
Careful, it says current thread.
Thread != Queue
A queue doesn't own a thread and a thread is not bound to a queue. There are threads and there are queues. Whenever a queue wants to run a block, it needs a thread but that won't always be the same thread. It just needs any thread for it (this may be a different one each time) and when it's done running blocks (for the moment), the same thread can now be used by a different queue.
The optimization this sentence talks about is about threads, not about queues. E.g. consider you have two serial queues, QueueA and QueueB and now you do the following:
dispatch_async(QueueA, ^{
someFunctionA(...);
dispatch_sync(QueueB, ^{
someFunctionB(...);
});
});
When QueueA runs the block, it will temporarily own a thread, any thread. someFunctionA(...) will execute on that thread. Now while doing the synchronous dispatch, QueueA cannot do anything else, it has to wait for the dispatch to finish. QueueB on the other hand, will also need a thread to run its block and execute someFunctionB(...). So either QueueA temporarily suspends its thread and QueueB uses some other thread to run the block or QueueA hands its thread over to QueueB (after all it won't need it anyway until the synchronous dispatch has finished) and QueueB directly uses the current thread of QueueA.
Needless to say that the last option is much faster as no thread switch is required. And this is the optimization the sentence talks about. So a dispatch_sync() to a different queue may not always cause a thread switch (different queue, maybe same thread).
But a dispatch_sync() still cannot happen to the same queue (same thread, yes, same queue, no). That's because a queue will execute block after block and when it currently executes a block, it won't execute another one until the currently executed is done. So it executes BlockA and BlockA does a dispatch_sync() of BlockB on the same queue. The queue won't run BlockB as long as it still runs BlockA, but running BlockA won't continue until BlockB has ran. See the problem? It's a classical deadlock.
The documentation clearly states that passing the current queue will cause a deadlock.
Now they don’t say why they designed things that way (except that it would actually take extra code to make it work), but I suspect the reason for doing things this way is because in this special case, blocks would be “jumping” the queue, i.e. in normal cases your block ends up running after all the other blocks on the queue have run but in this case it would run before.
This problem arises when you are trying to use GCD as a mutual exclusion mechanism, and this particular case is equivalent to using a recursive mutex. I don’t want to get into the argument about whether it’s better to use GCD or a traditional mutual exclusion API such as pthreads mutexes, or even whether it’s a good idea to use recursive mutexes; I’ll let others argue about that, but there is certainly a demand for this, particularly when it’s the main queue that you’re dealing with.
Personally, I think that dispatch_sync would be more useful if it supported this or if there was another function that provided the alternate behaviour. I would urge others that think so to file a bug report with Apple (as I have done, ID: 12668073).
You can write your own function to do the same, but it’s a bit of a hack:
// Like dispatch_sync but works on current queue
static inline void dispatch_synchronized (dispatch_queue_t queue,
dispatch_block_t block)
{
dispatch_queue_set_specific (queue, queue, (void *)1, NULL);
if (dispatch_get_specific (queue))
block ();
else
dispatch_sync (queue, block);
}
N.B. Previously, I had an example that used dispatch_get_current_queue() but that has now been deprecated.
Both dispatch_async and dispatch_sync perform push their action onto the desired queue. The action does not happen immediately; it happens on some future iteration of the run loop of the queue. The difference between dispatch_async and dispatch_sync is that dispatch_sync blocks the current queue until the action finishes.
Think about what happens when you execute something asynchronously on the current queue. Again, it does not happen immediately; it puts it in a FIFO queue, and it has to wait until after the current iteration of the run loop is done (and possibly also wait for other actions that were in the queue before you put this new action on).
Now you might ask, when performing an action on the current queue asynchronously, why not always just call the function directly, instead of wait until some future time. The answer is that there is a big difference between the two. A lot of times, you need to perform an action, but it needs to be performed after whatever side effects are performed by functions up the stack in the current iteration of the run loop; or you need to perform your action after some animation action that is already scheduled on the run loop, etc. That's why a lot of times you will see the code [obj performSelector:selector withObject:foo afterDelay:0] (yes, it's different from [obj performSelector:selector withObject:foo]).
As we said before, dispatch_sync is the same as dispatch_async, except that it blocks until the action is completed. So it's obvious why it would deadlock -- the block cannot execute until at least after the current iteration of the run loop is finished; but we are waiting for it to finish before continuing.
In theory it would be possible to make a special case for dispatch_sync for when it is the current thread, to execute it immediately. (Such a special case exists for performSelector:onThread:withObject:waitUntilDone:, when the thread is the current thread and waitUntilDone: is YES, it executes it immediately.) However, I guess Apple decided that it was better to have consistent behavior here regardless of queue.
Found from the following documentation.
https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/index.html#//apple_ref/c/func/dispatch_sync
Unlike dispatch_async, "dispatch_sync" function does not return until the block has finished. Calling this function and targeting the current queue results in deadlock.
Unlike with dispatch_async, no retain is performed on the target queue. Because calls to this function are synchronous, it "borrows" the reference of the caller. Moreover, no Block_copy is performed on the block.
As an optimization, this function invokes the block on the current thread when possible.
Let's say I have a serial dispatch queue and I enqueue several operations on it. I've read that I can't cancel operations once they are dispatched. Is it possible to at the very least view what GCD blocks I've dispatched to maybe make a decision if I want to dispatch another one?
Example, I dispatch Operation A to the queue but soon after my application decides to enqueue another Operation A, so now there are 2 of these operations queued up.
As Kevin Ballard said, you need to elaborate on what exactly you are trying to do. One thing you could do is set a flag, like valid_ and then you can effectively cancel all but the current item in the queue by doing something like this:
dispatch_async(queue, ^{
if (valid_) {
// perform your task here
}
});
Then whenever you want to "cancel" the queue, just set your valid_ flag to NO.
Again though, give more info on what you are trying to do and I can give you a better answer.
Since NSOperation is now built on top of GCD, you can now use addOperationWithBlock: to put your block on an NSOperationQueue, then you can invoke operations on the NSOperationQueue to get an NSArray of unfinished operations.
The problem with this, is that this is more than two operations and is not atomic, so it's entirely possible that your operation will finish in between the time you get the operations array and see if it's contained there.
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/NSOperationQueue_class/Reference/Reference.html
NSOperations have a prerequisite API, however, so you can enqueue another operation which will only run if your first NSOperation finishes, and use this to keep track of when you should try to enqueue your first NSOperation again.
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/NSOperation_class/Reference/Reference.html#//apple_ref/doc/uid/TP40004591
Does the following code run on the main thread? Does "main queue" refer to the main thread?
dispatch_async(dispatch_get_main_queue(),
^{
// Some code
});
The async part of dispatch async vs sync is different than concurrent vs serial. Async means that the function returns immediately, sync means that it'll wait until the block is executed. Since the main thread/queue is serial, things are going to get executed in order - I believe this means that since you're asking it to async dispatch on the same thread you're dispatching from, it'll return immediately, wait till the end of the current run loop and anything else in the queue, and then execute your block.
This is more useful for inside a queue than it is on the main thread - you can process your data, let the UI know to update, and continue processing without waiting for everything to redraw, etc. That's why you'll often see a dispatch_async call to the main thread inside another dispatch_async(concurrent queue) instead of just a dispatch_sync.
Yes. From Apple developer site:
The dispatch framework provides a default serial queue for the
application to use. This queue is accessed via
dispatch_get_main_queue().
This is documented in multiple places, including the docs for dispatch_get_main_queue() itself. The Concurrency Programming Guide says:
The main dispatch queue is a globally available serial queue that executes tasks on the application’s main thread.
I'm trying to enforce a specific order for tasks to complete using Grand Central Dispatch but I'm having a bit of trouble understanding the correct way to do it. I tried using groups in the following way:
Initialization:
startup = dispatch_group_create();
Tasks that need to wait:
//Don't want to wait on the main thread, so dispatch async to a concurrent queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0L),^{
//Wait until we're finished starting up
dispatch_group_wait(startup,DISPATCH_TIME_FOREVER);
//Now we can do this stuff back on the main queue
dispatch_async(dispatch_get_main_queue(),^{
//Do work
});
});
Work that I need to wait for:
dispatch_group_async(startup,dispatch_get_main_queue(),^{ // work });
Due to the nature of my app, the tasks that need to wait can occur BEFORE the work that I need to wait for. What I really want is the ability to wait on a condition that way when the condition is done, it's done, and all future threads can do their thing. Does GCD have this?
I'm not sure of all the details of your implementation, so forgive me if I'm repeating what you already know.
Create a dispatch group
Use dispatch_group_async to a serial queue. Using the serial queue, you are assured that your tasks are processed in the order you wish. Use a concurrent queue if you the order doesn't matter, but your question said that they had to complete in a specific order.
After you are done dispatching all of your tasks, use dispatch_group_notify. This will execute a block on the queue you specify once all the tasks assigned to the group have finished processing.
dispatch_group_notify(startup, dispatch_get_main_queue(), ^{
// Don't forget to release the dispatch group!
dispatch_release(startup)
// perform work block;
});
For a particular parsing activity, I need to do some processing of data that can be done while the rest of the parsing activity is ongoing. I assign the processing to a group on a concurrent queue. Then when my parsing is done, I check to see if the group is done. If it is done, I clean things up. If not, utilize the dispatch_group_notify() and execute the work afterwards. Something along these lines:
double delayInSeconds = 2.0;
dispatch_time_t groupWaitTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
if (dispatch_group_wait(myDispatchGroup, groupWaitTime)==0){
NSLog(#"dispatch group completed in time");
dispatch_release(myDispatchGroup);
[self parsingCompleteWithActivity:activity];
}else{
NSLog(#"dispatch group did not complete in time");
dispatch_group_notify(myDispatchGroup, dispatch_get_main_queue(), ^{
dispatch_release(myDispatchGroup);
[self parsingCompleteWithActivity:activity];
});
}
Good luck!
Works if I use a semaphore and then signal after each call to wait.
Also works if I call dispatch_group_enter and dispatch_group_leave.