-(void)GrabbingProcess:(void (^)())block;
{
AssertNonMainThread;
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
if (self.OtherGrabbingIndicator == 0)
{
self.isStillLoading = true;
}
self.OtherGrabbingIndicator ++;
AssertMainThread
}];
block();
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
[self updateStatusAtList];
self.OtherGrabbingIndicator --;
if (self.OtherGrabbingIndicator ==0)
{
self.isStillLoading = false;
}
AssertMainThread
}];
}
Basically, the code is set up so that self.OtherGrabbingIndicator++ will always be balanced with self.OtherGrabbingIndicator--
I couldn't think of any reason why it would fail. It doesn't use to fail.
Now, once in a while it fails. self.OtherGrabbingIndicator would hover at 2 or 1 and all the threads have stopped. Somehow some ++ are not balanced by the --. But how can that be?
I have no idea how.
I checked around that there is no way self.OtherGrabbingIndicator is changed anywhere else.
One thing I plan to do is to add the value of a block in some array and remove those blocks appropriately. The problem is if I know the block how do I know what code that blocks represent?
It used to work fine and now it doesn't. This drives me nuts.
The only way I can think of is that somehow block() fails to complete but which block fails to complete and how can that happens?
If I press pause, all the threads are empty.
Update: Adding #synchronized() to the ++ and -- solves the problem. However, it DOESN'T make sense.
It should still work without #synchronized because mainQueue has only 1 maximum thread and execute things one at a time.
Unfortunately, I have no final answer for you. However, I did experience something similar recently with NSOperations and I will share it with you.
A few questions about your code:
Does the first block in addOperationWithBlock need to complete before the "block()" call ca be executed ?
Does the second block require both other operation to be completed before being executed ?
In my case, the operations where added in the proper order but based on the thread scheduling of the operation queue (which was lower than some other thread that needed the operation result) they were executed too late and made the code fail.
I do not know how time critical is your grabbing process, but in my case the use of NSOperation in a time critical part of my application was never working 100% of the time. Therefore, I redesigned to use an NSThread of which we could control the priority and since then everything works fine.
My conclusion after this experience is that NSOperation is a great tool because it is simple to use but is not of much help in time critical situation because it is not really meant for that use case.
Hope this helps, best of luck to you.
Related
I have a chain of many NSBlockOperations with dependencies. If one operation early in the chain fails - I want the other operations to not run. According to docs, this should be easy to do from the outside - if I cancel an operation, all dependent operations should automatically be cancelled.
However - if only the execution-block of my operation "knows" that it failed, while executing - can it cancel its own work?
I tried the following:
NSBlockOperation *op = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakOpRef = op;
[takeScreenShot addExecutionBlock:^{
LOGInfo(#"Say Cheese...");
if (some_condition == NO) { // for some reason we can't take a photo
[weakOpRef cancel];
LOGError(#"Photo failed");
}
else {
// take photo, process it, etc.
LOGInfo(#"Photo taken");
}
}];
However, when I run this, other operations dependent on op are executed even though op was cancelled. Since they are dependent - surely they're not starting before op finished, and I verified (in debugger and using logs) that isCancelled state of op is YES before the block returns. Still the queue executes them as if op finished successfully.
I then further challenged the docs, like thus:
NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
NSBlockOperation *op = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakOpRef = takeScreenShot;
[takeScreenShot addExecutionBlock:^{
NSLog(#"Say Cheese...");
if (weakOpRef.isCancelled) { // Fail every once in a while...
NSLog(#"Photo failed");
}
else {
[NSThread sleepForTimeInterval:0.3f];
NSLog(#"Photo taken");
}
}];
NSOperation *processPhoto = [NSBlockOperation blockOperationWithBlock:^{
NSLog(#"Processing Photo...");
[NSThread sleepForTimeInterval:0.1f]; // Process
NSLog(#"Processing Finished.");
}];
// setup dependencies for the operations.
[processPhoto addDependency: op];
[op cancel]; // cancelled even before dispatching!!!
[myQueue addOperation: op];
[myQueue addOperation: processPhoto];
NSLog(#">>> Operations Dispatched, Wait for processing");
[eventQueue waitUntilAllOperationsAreFinished];
NSLog(#">>> Work Finished");
But was horrified to see the following output in the log:
2020-11-05 16:18:03.803341 >>> Operations Dispatched, Wait for processing
2020-11-05 16:18:03.803427 Processing Photo...
2020-11-05 16:18:03.813557 Processing Finished.
2020-11-05 16:18:03.813638+0200 TesterApp[6887:111445] >>> Work Finished
Pay attention: the cancelled op was never run - but the dependent processPhoto was executed, despite its dependency on op.
Ideas anyone?
OK. I think I solved the mystery. I just misunderstood the [NSOperation cancel] documentation.
it says:
In macOS 10.6 and later, if an operation is in a queue but waiting on
unfinished dependent operations, those operations are subsequently
ignored. Because it is already cancelled, this behavior allows the
operation queue to call the operation’s start method sooner and clear
the object out of the queue. If you cancel an operation that is not in
a queue, this method immediately marks the object as finished. In each
case, marking the object as ready or finished results in the
generation of the appropriate KVO notifications.
I thought if operation B depends on operation A - it implies that if A is canceled (hence - A didn't finish its work) then B should be cancelled as well, because semantically it can't start until A completes its work.
Apparently, that was just wishful thinking...
What documentation says is different. When you cancel operation B (which depends on operation A), then despite being dependent on A - it won't wait for A to finish before it's removed from the queue. If operation A started, but hasn't finished yet - canceling B will remove it (B) immediately from the queue - because it will now ignore dependencies (the completion of A).
Soooo... to accomplish my scheme, I will need to introduce my own "dependencies" mechanism. The straightforward way is by introducing a set of boolean properties like isPhotoTaken, isPhotoProcessed, isPhotoColorAnalyzed etc. Then, an operation dependent on these pre-processing actions, will need to check in its preamble (of execution block) whether all required previous operations actually finished successfully, else cancel itself.
However, it may be worth subclassing NSBlockOperation, overriding the logic that calls 'start' to skip to finished if any of the 'dependencies' has been cancelled!
Initially I thought this is a long shot and may be hard to implement, but fortunately, I wrote this quick subclass, and it seems to work fine. Of course deeper inspection and stress tests are due:
#interface MYBlockOperation : NSBlockOperation {
}
#end
#implementation MYBlockOperation
- (void)start {
if ([[self valueForKeyPath:#"dependencies.#sum.cancelled"] intValue] > 0)
[self cancel];
[super start];
}
#end
When I substitute NSBlockOperation with MYBlockOperation in the original question (and my other tests, the behaviour is the one I described and expected.
If you cancel an operation you just hint that it is done, especially in long running tasks you have to implement the logic yourself. If you cancel something the dependencies will consider the task finished and run no problem.
So what you need to do is have some kind of a global synced variable that you set and get in a synced fashion and that should capture your logic. Your running operations should check that variable periodically and at critical points and exit themselves. Please don't use actual global but use some common variable that all processes can access - I presume you will be comfortable in implementing this?
Cancel is not a magic bullet that stop the operation from running, it is merely a hint to the scheduler that allows it to optimise stuff. Cancel you must do yourself.
This is explanation, I can give sample implementation of it but I think you are able to do that on your own looking at the code?
EDIT
If you have a lot of blocks that are dependent and execute sequentially you do not even need an operation queue or you only need a serial (1 operation at a time) queue. If the blocks execute sequentially but are very different then you need to rather work on the logic of NOT adding new blocks once the condition fails.
EDIT 2
Just some idea on how I suggest you tackle this. Of course detail matters but this is also a nice and direct way of doing it. This is sort of pseudo code so don't get lost in the syntax.
// Do it all in a class if possible, not subclass of NSOpQueue
class A
// Members
queue
// job1
synced state cancel1 // eg triggered by UI
synced state counter1
state calc1 that job 1 calculates (and job 2 needs)
synced state cancel2
synced state counter2
state calc2 that job 2 calculated (and job 3 needs)
...
start
start on queue
schedule job1.1 on (any) queue
periodically check cancel1 and exit
update calc1
when done or exit increase counter1
schedule job1.2 on (any) queue
same
schedule job1.3
same
wait on counter1 to reach 0
check cancel1 and exit early
// When you get here nothing has been cancelled and
// all you need for job2 is calculated and ready as
// state1 in the class.
// This is why state1 need not be synced as it is
// (potentially) written by job1 and read by job2
// so no concurrent access.
schedule job2.1 on (any) queue
and so on
This is to me most direct and ready for future development way of doing it. Easy to maintain and understand and so on.
EDIT 3
Reason I like and prefer this is because it keeps all your interdependent logic in one place and it is easy to later add to it or calibrate it if you need finer control.
Reason I prefer this to e.g. subclassing NSOp is that then you spread out this logic into a number of already complex subclasses and also you loose some control. Here you only schedule stuff after you've tested some condition and know that the next batch needs to run. In the alternative you schedule all at once and need additional logic in all subclasses to monitor progress of the task or state of the cancel so it mushrooms quickly.
Subclassing NSOp I'd do if the specific op that run in that subclass needs calibration, but to subclass it to manage the interdependencies adds complexity I recon.
(Probably final) EDIT 4
If you made it this far I am impressed. Now, looking at my proposed piece of (pseudo) code you might see that it is overkill and that you can simplify it considerably. This is because the way it is presented, the different components of the whole task, being task 1, task 2 and so on, appear to be disconnected. If that is the case there are indeed a number of different and simpler ways in which you can do this. In the reference I give a nice way of doing this if all the tasks are the same or very similar or if you have only a single subsubtask (e.g. 1.1) per subtask (e.g. 1) or only a single (sub or subsub) task running at any point in time.
However, for real problems, you will probably end up with much less of a clean and linear flow between these. In other words, after task 2 say you may kick of task 3.1 which is not required by task 4 or 5 but only needed by task 6. Then the cancel and exit early logic already becomes tricky and the reason I do not break this one up into smaller and simpler bits is really because like here the logic can (easily) also span those subtasks and because this class A represents a bigger whole e.g. clean data or take pictures or whatever your big problem is that you try to solve.
Also, if you work on something that is really slow and you need to squeeze out performance, you can do that by figuring out the dependencies between the (sub and subsub) tasks and kick them off asap. This type of calibration is where (real life) problems that took way too long for the UI becomes doable as you can break them up and (non-linearly) piece them together in such a way that you can traverse them in a most efficient way.
I've had a few such a problems and, one in particular I am thinking know became extremely fragile and the logic difficult to follow, but this way I was able to bring the solution time down from an unacceptable more than a minute to just a few seconds and agreeable to the users.
(This time really almost the final) EDIT 5
Also, the way it is presented here, as you make progress in solving the problem, at those junctures between say task 1 and 2 or between 2 and 3, those are the places where you can update your UI with progress and parts of the full solution as it trickles in from all the various (sub and subsub) tasks.
(The end is coming) EDIT 6
If you work on a single core then, except for the interdependencies between tasks, the order in which you schedule all those sub and subsub tasks do not matter since execution is linear. The moment you have multiple cores you need to break the solution up into as small as possible subtasks and schedule the longer running ones asap for performance. The performance squeeze you get can be significant but comes at the cost of increasingly complex flow between all the small little subtasks and in the way in which you handle the cancel logic.
Here is the problem.
I have a method called -(void)searchingInBackground which is running in background (performSelectorInBackground).
In this method, I have couple of different threads which are running in background too (performSelectorInBackground). Like this:
-(void)searchingInBackground
{
#autoreleasepool {
[self performSelectorInBackground:#selector(getDuplicatedPictures:) withObject:copyArray];
}
#autoreleasepool {
[self performSelectorInBackground:#selector(getLocationsOfPhotos:) withObject:copyArray];
}
... (and so on)
}
In each of functions in threads (ie. getDuplicatedPictures, getLocationsOfPhotos...) they will generate NSStrings at the end and I will use those strings to update my text field GUI.
In order to update my text field GUI. I created a function called UpdateGUI which will use to help me update all of my NSStrings. Like this,
-(void)UpdateUI
{
[_NumDupPhotosLabel(label for GUI) setStringValue: resultDupPhotos(string from thread function which is getDuplicatedPictures in this case)];
....(includes all of my strings from threads)
}
Here is the problem, when I call this UpdateGUI using performSelectorOnMainThread in each of threads function. It will give me EXC_BAD_ACCESS. Here is what I did.
For example:
-(void)getDupicatedPictures
{
resultDupPhotos = .....;
[self performSelectorOnMainThread:#selector(UpdateUI) withObject:nil waitUntilDone:YES];
}
If I do not use performSelectorOnMainThread, just set the values directly in those functions it works fine. I just want to better organize the code.
-(void)getDuplicatedPictures
{
resultDupPhotos = .....;
[_NumDupPhotosLabel setStringValue: resultDupPhotos]; (works good and it will set the value to the GUI label)
}
Could you guys tell me how to fix this? Thanks!!!
ARC or no?
if you have a crash, post the backtrace
surrounding a performInBackground:... call with an #autoreleasepool does nothing (NSAutoreleasePool isn't going to help, either -- you need the autorelease pool to be in the thread of execution)
if a variable is involved in a crash, show the variable's declaration and initialization
spawning a bunch of threads simultaneously to do a bunch of work is likely to be slower than doing the work sequentially. Concurrency should always be controlled. If you have a long running task, you might likely want to spin up a second thread. Or you might want to re-order operations. The issue, though, is that running multiple threads at once, especially if those threads are doing a lot of I/O, is just going to increase contention and may likely make things slower, often a lot slower.
More likely than not, one of the objects calculated on a background thread is being released before the main thread tries to use it. How do you ensure that resultDupPhotos is valid between threads?
FACTS:
I have a method executing on a background thread :
[currentGame performSelectorInBackground:#selector(playOn:) withObject:self];
This method basically contains a while loop that keeps on executing until the user clicks on a Quit button:
-(void) playOn: (UIViewController*) thisViewController
{
while(!quitButtonPressed)
{
// this is my method's main loop
}
}
PROBLEM:
If the user clicks on Quit somewhere in the middle of the above loop
the rest of the loop would have to execute before it checks the BOOL once again and eventually stops. In order to prevent that from
happening and have the while-loop stop as soon as the user clicks on
Quit, I guess I could also add many if(quitButtonPressed) break;
here and there in my while loop in order to semi-constantly check and "immediately" break away if needed. However, this doesn't seem
very clever or practical from a design perspective given the size of
the above main while-loop and the fact that it contains many smaller
while-loops inside of it (the number of if.. break; I would have to
add would be quite big and could make things quite complicated to
figure out..)
POSSIBLE SOLUTION (but is it the right one?) :
So I was thinking that the best way would be to stop/cancel the
background thread on which the above method's while loop is executing,
instead of the while-loop itself, inside the method, the moment the
user clicks on Quit
Is this, or something similar (i.e. a better suggestion), possible and
how exactly could I do this?
POSSIBLE IMPLEMENTATION OF ABOVE SOLUTION:
I could create this new method:
-(void)checkQuitButton
{
while(!quitButtonPressed)
{
//wait
}
if(quitButtonPressed)
{
// stop this thread-->[currentGame performSelectorInBackground:#selector(playOn:) withObject:self];
// this is the method I'm looking for
}
}
And then I could start executing the above and my previous main method concurrently on two separate background threads as follows:
[currentGame performSelectorInBackground:#selector(playOn:) withObject:self];
[currentGame performSelectorInBackground:#selector(checkQuitButton) withObject:nil];
While the game while-loop is being executed another while-loop is checking the QuitButton at the same time. But is there a method that I can actually call in order to cancel what was started here:
[currentGame performSelectorInBackground:#selector(playOn:) withObject:self];
?
The correct solution is to periodically check for a "stop" flag. Abruptly terminating a thread provides no opportunity to clean up resources. In short, you would leak memory terribly.
But the deeper issue is that you almost certainly should not have this kind of thread. It strongly suggests an incorrect design. In iOS, most background operations should take the form of focused operations, implemented either with NSOperation or blocks with Grand Central Dispatch. You should very, very seldom need a long lived thread that is performing many different kinds of functions. Within your operation, it should be fairly straightforward where to put the "check for cancel" statements.
There is also almost no case where you should use performSelectorInBackground:. It is an incredibly dangerous method that gives you very little control. Instead, read the Concurrency Programming Guide for guidance on how to properly implement background operations. Pay special attention to the section "Migrating Away From Threads."
I'm using NSOperationQueue, and NSOperation for running some function on background click.
But I want to be able, when user clicks some button, stop that Operation.
How can I do it?
Something like, [currentoperation stop];
Cancel - won't work me. I want to stop immediately.
Thanks
You should be calling the -cancel method, and the operation itself has to support being cancelled by monitoring the isCancelled property/keypath and safely stopping when its value becomes YES. If the NSOperation is your own, you will probably have to create a custom subclass to implement this functionality. You cannot (safely) force an arbitrary operation to immediately stop. It has to support being cancelled.
You can't stop immediately by using anything Apple provides with NSOperation. You can use -[cancel] as other people have suggested here, but the current operation will still run until completion. One way of getting close to use -[isCancelled] inside of your operation and sprinkle that throughout the code (especially in long running loops). Something like:
- (void)main {
// do a little work
if ([self isCancelled]) { return; }
// do a little more work
if ([self isCancelled]) { return; }
}
This way you'll get things stopped relatively soon.
If you're looking to really force the thread to stop, you may need to look into signal handling. There's a threaded example here. Sending a custom signal to a specific thread, you may be able to then terminate that thread in some way. This will be a lot more work, though, and is probably much more trouble than it's worth.
you use cancel, and test whether self (the NSOperation) has been cancelled during execution.
The Problem
I have an NSOperationQueue called logEntryGeneratorQueue
I want to wait until all operations on the queue have completed
If I use:
[logEntryGeneratorQueue waitUntilAllOperationsAreFinished];
it works fine if the thread adding to the queue is in the background itself.
However, if I'm running this code via a unit test, it'll be running on the main thread. So I
came up with this "solution", which I really don't like:
if ([NSThread isMainThread]) {
while ([[logEntryGeneratorQueue operations] count] > 0) {
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
}
} else {
[logEntryGeneratorQueue waitUntilAllOperationsAreFinished];
}
This was always less than ideal, but has always worked fine on 10.5. However, now I've upgraded my project to using the 10.6 SDK, and this breaks.
On one test, it actually quit the test before it completed. I've no idea why - I assume it's something to do with the way NSOperationQueues work differently in 10.6 - they now use GCD.
What I've Tried
I've tried replacing the runUntilDate with sleep, which, as I thought, means every test pauses forever when it gets here.
My Question
Is there a better way to wait for an NSOperationQueue to finish on a main thread? If not, how can I get this code working under 10.6?
The Solution
I realised that my code was in an eternal loop because I was calling mergeChangesFromContextDidSaveNotification on the main thread whilst also waiting for the queue to finish on the main thread. And since the merge changes was called after waitUntilAllOperationsAreFinished, it never got executed.
I think the answer is to change where I run NSOperationQueues from. I shouldn't run an NSOperationQueue that deals with core data stuff on the main thread. And I shouldn't really be running this intensive stuff on the main thread for performance reasons anyway I guess.
I'd say that waitUntilAllOperationsAreFinished should work as expected on 10.6, no matter from what thread it's called. Since operation queues in 10.6 no longer use the run loop, there is no point in not blocking and having the loop run. Did you try to just call waitUntilAllOperationsAreFinished=
I agree with Max: -waitUntilAllOperationsAreFinished should work. Is your queue -suspended ?
IMHO you need to consider the possibility that waitUntilAllOperationsAreFinished may hang if one (or all) of its operations are progressing using main app thread as a carrier. Example: your nsoperation is not concurrent and uses glkview auto-update loop for animation and updating own state and your operation is only done (and operation is marked finished) only if main thread has a chance to work. But it can't as it is blocked in waiting for finishing these operations.