I am dispatching a queue to download some flickr photos on a separate thread (in viewWillAppear). When I log the contents of the array inside the block, it shows everything perfectly:
dispatch_queue_t photoDowonload=dispatch_queue_create("photoDownload", NULL);
dispatch_async(photoDowonload, ^{
NSArray *photoList=[FlickrFetcher topPlaces]; //downloads flickr data
self.listOfCities=photoList;
NSLog(#"inside block: %#", self.listOfCities); //shows contents
});
but when I try to log the array that was set inside the block outside the block, it returns null.
dispatch_queue_t photoDowonload=dispatch_queue_create("photoDownload", NULL);
dispatch_async(photoDowonload, ^{
NSArray *photoList=[FlickrFetcher topPlaces];
self.listOfCities=photoList;
});
NSLog(#"after block: %#", self.listOfCities); //returns null
What's the problem here? self.listOfCities is set up as NSArray property so once it's set in the block, it should be accessible outside of it.
The code in the block is run asynchronously. So the code after the block is run before the code in the block has had a chance to run (or certainly complete at least).
I've just started learning Objective-c, and I can be blind for some kind of issues but I'm wondering what is the impact of the _dispatch_asynch_ on executing block of code shown above.
Docs says
The dispatch_async() and dispatch_sync() functions schedule blocks for concurrent execution within the dispatch framework.
Maybe NSLog is called before execution of code block and variable is not initialized yet.
#rmaddy You was faster.
Ok I figured this out. My goal was to update the tableView with the info returned by block.
The block execution was changing the array variable but that change was not getting shown.
The trick was to detect this change in the getter for the array as follows:
-(void) setListOfCities:(NSArray *)listOfCities
{
if (_listOfCities!=listOfCities)
{
_listOfCities=listOfCities;
[self.tableView reloadData]; //<-- reloads table after change
}
}
Related
I am trying to debug something going on in a completion block. I put a breakpoint in the completion block, but the code is not breaking. Is it possible to put a breakpoint in a completion block? I recall it being possible but cannot seem to find any confirmation in the docs or on the Internet.
dispatch_async(dispatch_get_main_queue(), ^{
LogDebug(#"ready to save to database if this was new to server");
Items *object = [self.managedObjectContext objectRegisteredForID:myMoID];
//TRIED PUTTING BREAKPOINT RIGHT HERE BUT NOT STOPPING
if (successInt==1) {
object.needsync=#0;
}
});
You should always put a breakpoint on a line where some code is, not an empty line, it works better.
If your log is not printed out in a console, it means your block is never called.
If your code is not breaking this means that your completion block is not being called. Try to print log in completion block to check that if it is being called or not.
I need to perform a lot of calculations every time a getter is called from my app. The data returned from the getter is constantly changing based on the environment, and it has to do a lot of calculations to compute what it should return. Therefore, I don't want the code in the getter running on the main thread. This is what I have so far:
#interface Calculator ()
#property (nonatomic, strong) dispatch_queue_t calculationThread;
#end
- (dispatch_queue_t)calculationThread {
if (!_calculationThread) {
_calculationThread = dispatch_queue_create("calculation_thread", NULL);
}
return _calculationThread;
}
- (NSArray *)calculation {
// perform calculation in calculationThread, which should not be on main thread and be asynchronous
return arrayContainingCalculations;
}
I basically want to know how to use GCD to replace the comment. I have tried using dispatch_queue_t and dispatch_group_notify, but I don't seem to be implementing it correctly.
I think using a callback is probably the simplest and most efficient solution to this problem.
It is simply impossible to use only a single getter to do an asynchronous calculation without blocking the thread it was called on, as you expect code called after it to continue executing while it does the calculation.
You just have to create a new method with a callback, for example:
-(void) doCalculation:(void(^)(NSArray* result))callback {
dispatch_async(self.calculationQueue, ^{
NSArray* result = self.calculation; // make sure this is doing a synchronous calculation. If it's asynchronous, you'll have to use a semaphore (or another callback!).
if (callback) {
dispatch_async(dispatch_get_main_queue(), ^{ // return to main thread
callback(result);
});
}
});
}
Then you can simply invoke it on your main thread like so:
[calculator doCalculation:^(NSArray* result) {
textView.text = [result[0] stringValue]; // update UI with new info.
}];
That way you can easily keep your resulting code in-line with the call to the method.
It's also worth noting that your calculationQueue's getter (I renamed it, as the word thread is misleading when you're working with queues) isn't thread-safe. I would advise you use a dispatch_once to make it thread-safe:
-(dispatch_queue_t) calculationQueue {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_calculationQueue = dispatch_queue_create("calculation_queue", DISPATCH_QUEUE_SERIAL);
});
return _calculationQueue;
}
You can use the following to put it on your queue asynchronously. The problem however is that the method is going to return immediately.
dispatch_async(your_queue, ^{
// Code to be executed on background thread
});
What you probably want is to have some kind of method calculateWithCompletion where the caller can define a block that you can invoke once the completion is finished.
As you said in your comment to Peter, you want to keep it so you can call self.calculation and get your logic executed and return the calculation synchronously.
However because you want to avoid locking the UI while this logic is executing, you would like it to execute on a background thread.
Therefore, all you should need to do is use dispatch_sync instead of dispatch_async inside of your calculate method.
What dispatch_sync does is it places a task (the block that contains your logic) onto a specified queue (probably should pick a global concurrent queue), which then executes your task on a thread the OS picks for you (not the main thread). dispatch_async does the same, Except that dispatch_async will continue execution immediately after dispatching your task onto a queue.
dispatch_sync on the other hand, will block execution in the current run loop until your tasks returns.
This will allow you to execute your expensive logic on a background thread, while still remaining synchronous so that you can continue using self.calculation
I am getting EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) on dispatch_semaphore_dispose but don't really know how to track down the root cause of this. My code makes use of dispatch_async, dispatch_group_enter and so on.
UPDATE:
The cause of the crash is due to the fact that the webserviceCall (see code below) never calls onCompletion and when the code is run again, I got the error EXC_BAD_INSTRUCTION. I verified this is indeed the case, but not sure why or how to prevent this.
Code:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();
for (...) {
if (...) {
dispatch_group_enter(group);
dispatch_async(queue, ^{
[self webserviceCall:url onCompletion:^{
dispatch_group_leave(group);
}];
});
}
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)));
dispatch_sync(queue, ^{
// call completion handler passed in by caller
});
});
From your stack trace, EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) occurred because dispatch_group_t was released while it was still locking (waiting for dispatch_group_leave).
According to what you found, this was what happened :
dispatch_group_t group was created. group's retain count = 1.
-[self webservice:onCompletion:] captured the group. group's retain count = 2.
dispatch_async(...., ^{ dispatch_group_wait(group, ...) ... }); captured the group again. group's retain count = 3.
Exit the current scope. group was released. group's retain count = 2.
dispatch_group_leave was never called.
dispatch_group_wait was timeout. The dispatch_async block was completed. group was released. group's retain count = 1.
You called this method again. When -[self webservice:onCompletion:] was called again, the old onCompletion block was replaced with the new one. So, the old group was released. group's retain count = 0. group was deallocated. That resulted to EXC_BAD_INSTRUCTION.
To fix this, I suggest you should find out why -[self webservice:onCompletion:] didn't call onCompletion block, and fix it. Then make sure the next call to the method will happen after the previous call did finish.
In case you allow the method to be called many times whether the previous calls did finish or not, you might find someone to hold group for you :
You can change the timeout from 2 seconds to DISPATCH_TIME_FOREVER or a reasonable amount of time that all -[self webservice:onCompletion] should call their onCompletion blocks by the time. So that the block in dispatch_async(...) will hold it for you.
OR
You can add group into a collection, such as NSMutableArray.
I think it is the best approach to create a dedicate class for this action. When you want to make calls to webservice, you then create an object of the class, call the method on it with the completion block passing to it that will release the object. In the class, there is an ivar of dispatch_group_t or dispatch_semaphore_t.
I had a different issue that brought me to this question, which will probably be more common than the overrelease issue in the accepted answer.
Root cause was our completion block being called twice due to bad if/else fallthrough in the network handler, leading to two calls of dispatch_group_leave for every one call to dispatch_group_enter.
Completion block called multiple times:
dispatch_group_enter(group);
[self badMethodThatCallsMULTIPLECompletions:^(NSString *completion) {
// this block is called multiple times
// one `enter` but multiple `leave`
dispatch_group_leave(group);
}];
Debug via the dispatch_group's count
Upon the EXC_BAD_INSTRUCTION, you should still have access to your dispatch_group in the debugger. DispatchGroup: check how many "entered"
Print out the dispatch_group and you'll see:
<OS_dispatch_group: group[0x60800008bf40] = { xrefcnt = 0x2, refcnt = 0x1, port = 0x0, count = -1, waiters = 0 }>
When you see count = -1 it indicates that you've over-left the dispatch_group. Be sure to dispatch_enter and dispatch_leave the group in matched pairs.
My problem was took IBOutlet but didn't connect with interface builder and using in swift file.
Sometimes all it takes to get a EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) is a missing return statement.
It certainly was my case.
My issue was that I was creating objects that I wanted to be stored in a NSMutableDictionary but I never initialized the dictionary. Therefore the objects were getting deleted by garbage collection and breaking later. Check that you have at least one strong reference to the objects youre interacting with.
In my case:
PHImageRequestOptions *requestOptions = [PHImageRequestOptions new];
requestOptions.synchronous = NO;
Was trying to do this with dispatch_group
I landed here because of an XCTestCase, in which I'd disabled most of the tests by prefixing them with 'no_' as in no_testBackgroundAdding. Once I noticed that most of the answers had something to do with locks and threading, I realized the test contained a few instances of XCTestExpectation with corresponding waitForExpectations. They were all in the disabled tests, but apparently Xcode was still evaluating them at some level.
In the end I found an XCTestExpectation that was defined as #property but lacked the #synthesize. Once I added the synthesize directive, the EXC_BAD_INSTRUCTION disappeared.
My issue was that it was in my init().
Probably the "weak self" killed him while the init wasn't finished.
I moved it from the init and it solved my issue.
I have a method that at times can be invoked throughout my code. Below is a very basic example, as the code processes images and files off of the iphone photo gallery and marks them already processed when done with the method.
#property (nonatomic, assign) dispatch_queue_t serialQueue;
....
-(void)processImages
{
dispatch_async(self.serialQueue, ^{
//block to process images
NSLog(#"In processImages");
....
NSLog(#"Done with processImages");
});
}
I would think that each time this method is called I would get the below output...
"In processImages"
"Done with processImages"
"In processImages"
"Done with processImages"
etc...
but I always get
"In processImages"
"In processImages"
"Done with processImages"
"Done with processImages"
etc...
I thought a serial queue would wait till the first block is done, then start. To me it seems it is starting the method, then it gets called again and starts up before the first call even finishes, creating duplicates of images that normally would not be processed due to the fact that if it really executed serially the method would know they were already processed. Maybe my understanding of serial queues is not concrete. Any input? Thank you.
EDIT:MORE Context below, this is what is going on in the block...Could this cause the issue???
#property (nonatomic, assign) dispatch_queue_t serialQueue;
....
-(void)processImages
{
dispatch_async(self.serialQueue, ^{
//library is a reference to ALAssetsLibrary object
[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop)
{
[group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop)
{
....
//Process the photos here
}];
failureBlock:^(NSError *error) { NSLog(#"Error loading images from library");
}];
});
}
-(id)init
{
self = [super init];
if(self)
{
_serialQueue = dispatch_queue_create("com.image.queue",NULL);
}
return self;
}
this object is only created once, and as far as I can tell can never be created again based off my code...I will run tests to make sure though.
UPDATE 2: WHAT I THINK IS HAPPENING, please comment on this if you agree/disagree....
Obviously my main issue is that it seems this block of code is being executed concurrently, creating duplicate entries (importing the same photo twice) when it wouldn't normally do this if it was run serially. When a photo is processed a "dirty" bit is applied to it ensuring the next time the method is invoked it skips this image, but this is not happening and some images are processed twice. Could this be due to the fact I am enumerating the objects in a second queue using enumerategroupswithtypes: within that serialQueue?
call processImages
enumerateObjects
immediately return from enumerateObjects since it is async itself
end call to processImages
processImages is not really done though due to the fact that enumerategroups is probably still running but the queue might thing it is done since it reaches the end of the block before enumerategroups is finished working. This seems like a possibility to me?
Serial Queues ABSOLUTELY will perform serially. They are not guaranteed to perform on the same thread however.
Assuming you are using the same serial queue, the problems is that NSLog is NOT guaranteed to output results in the proper order when called near simultaneously from different threads.
here is an example:
SQ runs on thread X, sends "In processImages"
log prints "In proc"
SQ on thread X, sends "Done with processImages"
SQ runs on thread Y, sends "In processImages"
log prints "essImages\n"
After 5., NSLog doesn't necessarily know which to print, 3. or 4.
If you absolutely need time ordered logging, You need a dedicated queue for logging. In practice, I've had no problems with just using the main queue:
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"whatever");
});
If all NSlog calls are the on the same queue, you shouldn't have this problem.
enumerateGroupsWithTypes:usingBlock:failureBlock: does its work asynchronously on another thread and calls the blocks passed in when it's done (on the main thread I think). Looking at it from another perspective, if it completed all the synchronously by the time the method call was complete, it could just return an enumerator object of the groups instead, for instance, for a simpler API.
From the documentation:
This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately. You should perform whatever work you want with the assets in enumerationBlock.
I'm not sure why you're trying to accomplish by using the serial queue, but if you just want to prevent simultaneous access, then you could just add a variable somewhere that keeps track of whether we're currently enumerating or not and check that at first, if you don't have to worry about synchronization issues. (If you do, perhaps you should look into using a GCD group, but it's probably overkill for this situation.)
If the question is "Can serial queue perform tasks asynchronously?" then the answer is no.
If you think that it can, you should make sure that all tasks are really performing on the same queue. You can add the following line in the block and compare the output:
dispatch_async(self.serialQueue, ^{
NSLog(#"current queue:%p current thread:%#",dispatch_get_current_queue(),[NSThread currentThread]);
Make sure that you write NSLog in the block that performs on your queue and not in the enumerateGroupsWithTypes:usingBlock:failureBlock:
Also you can try to create your queue like this
dispatch_queue_create("label", DISPATCH_QUEUE_SERIAL);
but I don't think that will change anything
EDIT:
By the way, method
enumerateGroupsWithTypes:usingBlock:failureBlock:
is asynchronous, why do you call it on another queue?
UPDATE 2:
I can suggest something like this:
dispatch_async(queue, ^{
NSLog(#"queue");
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER, *pmutex = &mutex;
pthread_mutex_lock(pmutex);
ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) {
NSLog(#"block");
if (group) {
[groups addObject:group];
} else {
[self.tableView performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:NO];
dispatch_async(dispatch_get_current_queue(), ^{
pthread_mutex_unlock(pmutex);
});
}
NSLog(#"block end");
};
[assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock];
pthread_mutex_lock(pmutex);
pthread_mutex_unlock(pmutex);
pthread_mutex_destroy(pmutex);
NSLog(#"queue end");
});
I hit an issue like this, and the answer for me was to realize that asynchronous calls from a method on the serialized queue goes to another queue for processing -- one that is not serialized.
So you have to wrap all the calls inside the main method with explicit dispatch_async(serializedQueue, ^{}) to ensure that everything is done in the correct order...
Using Swift and semaphores to illustrate an approach to serialization:
Given: a class with an asynchronous ‘run’ method that will be run on multiple objects at once, and the objective is that each not run until the one before it completes.
The issue is that the run method allocates a lot of memory and uses a lot of system resources that can cause memory pressure among other issues if too many are run at once.
So the idea is: if a serial queue is used then only one will run at a time, one after the other.
Create a serial queue in the global space by the class:
let serialGeneratorQueue: DispatchQueue = DispatchQueue(label: "com.limit-point.serialGeneratorQueue", autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.workItem)
class Generator {
func run() {
asynchronous_method()
}
func start() {
serialGeneratorQueue.async {
self.run()
}
}
func completed() {
// to be called by the asynchronous_method() when done
}
}
The ‘run’ method of this class for which very many objects will be created and run will be processed on the serial queue:
serialGeneratorQueue.async {
self.run()
}
In this case an autoreleaseFrequency is .workItem to clean up memory after each run.
The run method is of some general form:
func run() {
asynchronous_method()
}
The problem with this: the run method exits before the asynchronous_method completes, and the next run method in the queue will run, etc. So the objective is not being achieved because each asynchronous_method is running in parallel, not serially after all.
Use a semaphore to fix. In the class declare
let running = DispatchSemaphore(value: 0)
Now the asynchronous_method completes it calls the ‘completed’ method:
func completed() {
// some cleanup work etc.
}
The semaphore can be used to serialized the chain of asynchronous_method’s by add ‘running.wait()’ to the ‘run’ method:
func run() {
asynchronous_method()
running.wait()
}
And then in the completed() method add ‘running.signal()’
func completed() {
// some cleanup work etc.
running.signal()
}
The running.wait() in ‘run’ will prevent it from exiting until signaled by the completed method using running.signal(), which in turn prevents the serial queue from starting the next run method in the queue. This way the chain of asynchronous methods will indeed be run serially.
So now the class is of the form:
class Generator {
let running = DispatchSemaphore(value: 0)
func run() {
asynchronous_method()
running.wait()
}
func start() {
serialGeneratorQueue.async {
self.run()
}
}
func completed() {
// to be called by the asynchronous_method() when done
running.signal()
}
}
I thought a serial queue would wait [until] the first block is done ...
It does. But your first block simply calls enumerateGroupsWithTypes and the documentation warns us that the method runs asynchronously:
This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately.
(FWIW, whenever you see a method that has a block/closure parameter, that’s a red flag that the method is likely performing something asynchronously. You can always refer to the relevant method’s documentation and confirm, like we have here.)
So, bottom line, your queue is serial, but it is only sequentially launching a series of asynchronous tasks, but obviously not waiting for those asynchronous tasks to finish, defeating the intent of the serial queue.
So, if you really need to have each tasks wait for the prior asynchronous task, there are a number of traditional solutions to this problem:
Use recursive pattern. I.e., write a rendition of processImage that takes an array of images to process and:
check to see if there are any images to process;
process first image; and
when done (i.e. in the completion handler block), remove the first image from the array and then call processImage again.
Rather than dispatch queues, consider using operation queues. Then you can implement your task as an “asynchronous” NSOperation subclass. This is a very elegant way of wrapping an asynchronous task This is illustrated in https://stackoverflow.com/a/21205992/1271826.
You can use semaphores to make this asynchronous task behave synchronously. This is also illustrated in https://stackoverflow.com/a/21205992/1271826.
Option 1 is the simplest, option 2 is the most elegant, and option 3 is a fragile solution that should be avoided if you can.
You might have more than one object, each with its own serial queue. Tasks dispatched to any single serial queue are performed serially, but tasks dispatched to different serial queues will absolutely be interleaved.
Another simple bug would be to create not a serial queue, but a concurrent queue...
I'm using below code to download some data from the web. Am I right that I need to retain the data like I have done? Also the NSLog statement from inside the block shows that the array has been populated, but when I run the NSLog outside the block the arrays show as (null). How would I save the data outside dispatch_async method?
__block NSArray *downloadedCareerIds;
__block NSArray *diskCareerIds;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
/* Download stuff */
downloadedCareerIds = [[CareersParser idsFrom:#"web"] retain];
diskCareerIds = [[CareersParser idsFrom:#"disk"] retain];
DLog(#"downloadedCareerIds: %#", downloadedCareerIds);
DLog(#"diskCareerIds: %#", diskCareerIds);
});
DLog(#"downloadedCareerIds: %#", downloadedCareerIds);
DLog(#"diskCareerIds: %#", diskCareerIds);
The idea of dispatch_async is that you give it a block of code to execute asynchronously, therefore giving up any control of when that code gets executed. The call to dispatch_async returns once the block has been enqueued, NOT once the block has finished executing (hence async). Therefore, the log statements inside of the block you're passing to dispatch_async will get executed, almost always, after the log statements below your call to dispatch_async.
dispatch_async is a non blocking method so it will return immediately. So when the DLog statements outside the block are called, they will mostly not have been set. Hence you don't see the values you get from the internal log statements.
If you want to act on the data within the same method, you will have to either send a blocking dispatch_sync which is pointless or you can call the methods within the block.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
....
[self doStuffWithTheArrays];
});
Once the block is executed the objects will be available provided they are instance variables or you will lose the references.