After I create a new session data task with URLSession.dataTask(with:completionHandler:) and start the task by calling its resume() method, given that the app doesn't crash while the request is running, is it safe for me to assume that completionHandler (passed to URLSession.dataTask(with:completionHandler:) above) will always eventually get called only once, even if something weird happens with the network request (like if the connection drops) or with the app (like if it goes into the background)?
Note: I'm not explicitly calling cancel() or suspend() on the task. Just resume().
I want to know the answer to this question because (from my app's main thread) I'm creating and starting (one after the other) multiple asynchronous network requests and want to know when the last one has finished.
Specifically, I'm working on an app that has a custom class called Account. On launch, the app (assuming it finds an account access token stored in UserDefaults) creates only one instance of that class and stores it to a global variable (across the entire app) called account, which represents the app's currently-logged-in account.
I've added a stored var (instance) property to Account called pendingGetFooRequestCount (for example) and set it to 0 by default. Every time I make a call to Account.getFoo() (an instance method), I add 1 to pendingGetFooRequestCount (right before calling resume()). Inside completionHandler (passed to URLSession.dataTask(with:completionHandler:) and (to be safe) inside a closure passed to DispatchQueue.main.async(), I first subtract 1 from pendingGetFooRequestCount and then check if pendingGetFooRequestCount is equal to 0. If so, I know the last get-foo request has finished, and I can call another method to continue the flow.
How's my logic? Will this work as expected? Should I be doing this another way? Also, do I even need to decrement pendingGetFooRequestCount on the main thread?
URLRequest has a timeoutInterval property, its default value is 60 seconds. If there is no response by then, the completion is called with non-nil error.
Related
When calling the method
- (void)readPacketsWithCompletionHandler:(void (^)(
NSArray<NSData *> *packets, NSArray<NSNumber *> *protocols))completionHandler;
the completionHandler is either called directly, in case packets are available at call time, or it is called at a later tim when packets become available.
Yet what is nowhere documented is: What happens if I call this method again before the prior set completionHandler has ever been called?
Will the new handler replace the prior set one and the prior set one won't get called at all anymore?
Are both handler scheduled and called as data arrives? And if so, will they be called in the order I passed them, in reverse order, or in random order?
Has anyone any insights on how that method is implemented?
Of course, I can make a demo project, create a test setup, and see what results I get through testing but that is very time consuming and not necessarily reliable. The problem with unspecified behavior is that it may change at will without letting anyone know. This method may behave differently on macOS and iOS, it may behave differently with every new OS release, or depending on the day of the week.
Or does the fact that nothing is documented is by intention? Do I have to interpret that as: You may call this method once and after your callback was executed, you may call it again with the same or a new callback. Everything else is undefined behavior and you cannot and should not rely on any specific behavior if use that API in a different manner.
As nobody has replied so far, I tried my best to figure it out myself. As testing is not good enough for me, here is what I did:
First I extracted the NetworkExtension framework binary from the dyld cache of macOS Big Sur using this utility.
Then I ran otool -Vt over the resulting binary file to get a disassembler dump of the binary.
My assembly skills are a bit rusty but from what I see the completionHandler is stored in a property named packetHandler, replacing any previous stored value there. Also a callback is created in that method and stored on an object obtained by calling the method interface.
When looking at the code of this created callback, it obtains the value of the packetHandler property and sets it to NULL after the value was obtained. Then it creates NSData and NSNumber objects, adds those to NSArray objects and calls the obtained handler with those arrays.
So it seems that calling the method again just replaces the previous completionHandler which is never be called in that case. So you must not rely that a scheduled handler will eventually be called at some time in the future if the tunnel is not teared down if the possibility exists that your code might replace it. Also calling the method multiple times to schedule multiple callbacks has no effect as as only the last one will be kept and eventually be called.
I am able to observe the CNContactStoreDidChangeNotification when the contact database is changed while the app is in background state. I am pretty sure that only one observer was added to NSNotificationCenter.
The problem is NSNotificationCenter posts MULTIPLE times (2, 3, 5, and even more times) even if I only add one new contact.
Where is the problem?
Make certain you aren't adding the observer multiple times. This can happen without you realizing it if (for example) you call -addObserver from -viewDidLoad or -viewDidAppear in your view controller (as these might get called more than once throughout the life of your application), or from any of the application state callbacks in your app delegate (-applicationDidBecomeActive, -applicationWillResignActive, -applicationDidEnterBackground, -applicationWillEnterForeground, etc).
Wrap the call to -addObserver in a conditional that ensures it can only be called once (set a flag), and put NSLog statements around it so you can see in the debug console if you are getting there more than once. Search your code for other calls to -addObserver that you might have forgotten about.
Call -removeObserver before adding it, just to be sure (making sure to pass the same name and object as when you added it). Calling -removeObserver on an observer that doesn't exist is okay. Note that this is more of a band-aid than a fix - your code should be smart enough to know whether or not you've already added it - but this might help you diagnose the problem).
I just wrote a quick minimal test program that adds an observer (once!) on CNContactStoreDidChangeNotification and I only get the notification once when I add or change a contact. Write a similar test program for yourself and see if you get the same result. If your test program works correctly, then it is likely that your app is doing something you don't expect (and calling -addObserver multiple times).
I had the same problem, the number of times it fired varied between 2 & 3. The solution that worked for me was to set a semaphore variable, set in the handler and reset the semaphore when finished. Wrap the address book processing in an if statement on the semaphore to ignore further calls.
addressBkSemphore is reset to false in buildFrendsAndContacts
- (void)addressBkChange:(NSNotification *)note
{
if (addressBkSemphore == false)
{
addressBkSemphore = TRUE;
[self buildFrendsAndContacts];
}
}
Hope it helps.
You can start a one time execution timer or a dispatch after few seconds and cancel it in case there's a new contacts update within those seconds, thus ensuring that only the timer or dispatch_after triggered by the last update will actually execute (taking into account that all update calls come one after the other within under a sec. difference, as far as I tested)
And btw, I could reproduce the issue only when making change to contacts on the same device with my app. If I change the contacts on another device linked to the same apple account, there was only one update.
I realize this question sounds contradictory. I have several Async requests going out in an application. The situation is that the first async request is an authentication request, and the rest will use an access token returned by the successful authentication request.
The two obvious solutions would be:
run them all synchronous, and risk UI block. (bad choice)
run them async, and put request 2-N in the completion handler for the first one. (not practical)
The trouble is that the subsequent requests may be handled anywhere in the project, at anytime. The failure case would be if the 2nd request was called immediately after the 1st authentication request was issued, and before the access token was returned.
My question thus is, is there any way to queue up Async requests, or somehow say not to issue them until the first request returns successfully?
EDIT:
Why (2) is not practical: The first is an authentication request, happening when the app loads. The 2nd+ may occur right away, in which case it is practical, but it also may occur in a completely separate class or any other part of a large application. I can't essentially put the entire application in the completion handler. Other accesses to the API requests may occur in other classes, and at anytime. Even 1-2 days away after many other things have occurred.
SOLUTION:
//pseudo code using semaphore lock on authentication call to block all other calls until it is received
// at start of auth
_semaphore = dispatch_semaphore_create(0)
// at start of api calls
if(_accessToken == nil && ![_apiCall isEqualToString:#"auth]){
dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
}
// at end of auth with auth token
dispatch_semaphore_signal([[SFApi Instance] semaphore]);
_accessToken = ...;
This sounds like a case where you'd want to use NSOperation's dependencies
From apple docs:
Operation Dependencies
Dependencies are a convenient way to execute operations in a specific order. You can add and remove dependencies for an operation using the addDependency: and removeDependency: methods. By default, an operation object that has dependencies is not considered ready until all of its dependent operation objects have finished executing. Once the last dependent operation finishes, however, the operation object becomes ready and able to execute.
note that in order for this to work, you must subclass NSOperation "properly" with respect to KVO-compliance
The NSOperation class is key-value coding (KVC) and key-value observing (KVO) compliant for several of its properties. As needed, you can observe these properties to control other parts of your application.
You can't really have it both ways-- there's no built-in serialization for the NSURLConnection stuff. However, you are probably already funneling all of your API requests through some common class anyway (presumably you're not making raw network calls willy-nilly all over the app).
You'll need to build the infrastructure inside that class that prevents the execution of the later requests until the first request has completed. This suggests some sort of serial dispatch queue that all requests (including the initial auth step) are funneled through. You could do this via dependent NSOperations, as is suggested elsewhere, but it doesn't need to be that explicit. Wrapping the requests in a common set of entry points will allow you to do this any way you want behind the scenes.
In cases like this I always find it easiest to write the code synchronously and get it running on the UI thread first, correctly, just for debugging. Then, move the operations to separate threads and make sure you handle concurrency.
In this case the perfect mechanism for concurrency is a semaphore; the authentication operation clears the semaphore when it is done, and all the other operations are blocking on it. Once authentication is done, floodgates are open.
The relevant functions are dispatch_semaphore_create() and dispatch_semaphore_wait() from the Grand Central Dispatch documentation: https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/doc/uid/TP40008079-CH2-SW2
Another excellent solution is to create a queue with a barrier:
A dispatch barrier allows you to create a synchronization point within a concurrent dispatch queue. When it encounters a barrier, a concurrent queue delays the execution of the barrier block (or any further blocks) until all blocks submitted before the barrier finish executing. At that point, the barrier block executes by itself. Upon completion, the queue resumes its normal execution behavior.
Looks like you got it running with a semaphore, nicely done!
Use blocks... 2 ways that I do it:
First, a block inside of a block...
[myCommKit getPlayerInfoWithCallback:^(ReturnCode returnCode, NSDictionary *playerInfo) {
if (playerInfo) {
// this won't run until the first one has finished
[myCommKit adjustSomething: thingToAdjust withCallback:^(ReturnCode returnCode, NSDictionary *successCode) {
if (successCode) {
// this won't run until both the first and then the second one finished
}
}];
}
}];
// don't be confused.. anything down here will run instantly!!!!
Second way is a method inside of a block
[myCommKit getPlayerInfoWithCallback:^(ReturnCode returnCode, NSDictionary *playerInfo) {
if (playerInfo) {
[self doNextThingAlsoUsingBlocks];
}
}];
Either way, any time I do async communication with my server I use blocks. You have to think differently when writing code that communicates with a server. You have to force things to go in the order you want and wait for the return success/fail before doing the next thing. And getting used to blocks is the right way to think about it. It could be 15 seconds between when you start the block and when it gets to the callback and executes the code inside. It could never come back if they're not online or there's a server outage.
Bonus way.. I've also sometimes done things using stages:
switch (serverCommunicationStage) {
case FIRST_STAGE:
{
serverCommunicationStage = FIRST_STAGE_WAITING;
// either have a block in here or call a method that has a block
[ block {
// in call back of this async call
serverCommunicationStage = SECOND_STAGE;
}];
break;
}
case FIRST_STAGE_WAITING:
{
// this just waits for the first step to complete
break;
}
case SECOND_STAGE:
{
// either have a block in here or call a method that has a block
break;
}
}
Then in your draw loop or somewhere keep calling this method. Or set up a timer to call it every 2 seconds or whatever makes sense for your application. Just make sure to manage the stages properly. You don't want to accidentally keep calling the request over and over. So make sure to set the stage to waiting before you enter the block for the server call.
I know this might seem like an older school method. But it works fine.
After calling ReadDirectoryChangesW (in overlapped mode) it returns 1 (true) in two opposite situations: 1) no files changes, 2) one or more file changed; But in first situation no data passed to IOCP and in second situation passed pointer to my overlapped struct.
How to determine if it passed my overlapped struct or not to IOCP while calling ReadDirectoryChangesW ? In another words how to determine does ReadDirectoryChangesW found changes or not?
In some part of my code I call GetQueuedCompletionStatus and get full information about changed files, but before it I wanna to know only fact: was changes or not;
If you're using ReadDirectoryChangesW() with an IOCP then you're using it in asynchronous mode and so after calling it you should wait for it to report the next change as it occurs.
Once you get a completion notification from the IOCP you can process it and then call ReadDirectoryChangesW() again to get more notifications.
On iOS, if there is a single view app, and a new thread is created using:
[NSThread detachNewThreadSelector:#selector(consumeData:)
toTarget:self.consumer withObject:self.queue];
where the consumer is a Consumer object that will process data inside the method consumeData, and the queue is a Queue object, which is where the data comes from for the consumer to process.
But what if the thread needs to check whether a Switch on the main view is set to on or off? That is to toggle whether the Consumer object should do the work or pause at the moment. Should the withObject:self be used instead, so that the whole ViewController reference is passed to the thread, and then the thread will use viewController.view.______ to access the switch's value, and use viewController.queue to access the queue, or is there a better or alternative method?
Absolutely not. Nothing UI-related can ever be touched from another thread. It's simply not safe. If the other thread needs to know the switch's current value, then it needs to call back to the main thread before asking for it.
If you create a subclass, you could store your state in variables in the object, then access these variables from any thread; provided of course these operations do not call methods defined by UIKit.