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
consider the following method snippet.
- (void) closeSocket {
...
dispatch_async(dispatch_get_main_queue(), ^{
// last message before actual disconnection:
[self.connectionListenerDelegate connectionDisconnected:self];
}
self.connectionListenerDelegate = nil;
...
}
This method of my "socket" implementation class can be called by external object, in some arbitrary thread (main, or other). I wish to only notify my delegate once, on the main thread, and remove the delegate so that other background events and possible incoming data won't reach it.
In other words, I want to make sure connectionDisconnected: is the last call from the socket to the delegate.
I know code blocks capture their environment's variables etc. But will the block capture and retain the self.connectionListenerDelegate when created?
If closeSocket is being called on some background thread, and dispatches the connectionDisconnected: asynchronously on the main thread, and I nullify my weak reference to my delegate right away - maybe the block will have a nil object and won't send its message?
What is the right way to go about this?
I guess I could use the old
[self.connectionListenerDelegate performSelectorOnMainThread:#selector(connectionDisconnected:) withObject:self waitUntilDone:NO];
which retains both the receiver and parameter object (self), but I prefer GCD dispatch_async and I'd like to better understand blocks.
If closeSocket is being called on some background thread, and dispatches the connectionDisconnected: asynchronously on the main thread, and I nullify my weak reference to my delegate right away - maybe the block will have a nil object and won't send its message?
I think that after self.connectionListenerDelegate = nil; runs, all methods in the dispatch method will get the nil reference when accessing the connectionListenerDelegate.
So, the best way to go about this would be to transfer the delegate reference to a temporary object for use inside the block:
id<ConnectionListenerDelegate> *tempDelegateRef = self.connectionListenerDelegate;
dispatch_async(dispatch_get_main_queue(), ^{
// last message before actual disconnection:
[tempDelegateRef connectionDisconnected:self];
}
self.connectionListenerDelegate = nil;
I'm not sure if you need strong/weak references or something like that.
I am parsing an XML web service and after that parse finishes I want to call another method. But my code calls the method during the parse process. What I want is to wait till the parse process end. Here is my code:
ArsivNoCheck *arsivNoCheck = [ArsivNoCheck alloc];
[arsivNoCheck checkArsivNo:_txtArsivNo.text]; //Here I call parsing method in another class
//Here I call the method
[self performSelectorOnMainThread:#selector(sampleMethod) withObject:nil waitUntilDone:YES];
-(void) sampleMethod
{
//some code
}
You should consider NSOperation, and its method completionBlock.
Then, you would be able to perform your parsing, and at the end of it, execute some code.
Note : If you plan to update the UI, take care, because the completionBlock is not necessarily running on the main thread!
From NSOperation's Doc reference :
completionBlock
Returns the block to execute when the operation’s main
task is complete.
-(void (^)(void))completionBlock
Return Value
The block to execute after the operation’s main task is completed. This block takes no
parameters and has no return value.
Discussion
The completion block you provide is executed when the value
returned by the isFinished method changes to YES. Thus, this block is
executed by the operation object after the operation’s primary task is
finished or cancelled.
Example :
[filterOp setCompletionBlock: ^{
NSLog(#"Finished filtering an image.");
}];
See this tutorial on Ray Wenderlich's site for implementation.
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 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
}
}