I got challenged to use the NSURLSession delegates to update a UI element (just a label) with the status of the download, but I can only use NSOperation and not dispatch_get_main_queue
I'm not even sure this is possible to call the UI thread with NSOperation directly (and not a completion block), but I thought of ask here to see if anyone knows if this is possible.
In a nutshell, this is what I have with the C calls to dispatch_async:
- (void)URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
long percentage = (totalBytesWritten * 100) / totalBytesExpectedToWrite;
dispatch_async(dispatch_get_main_queue(),
^{
self.label_bytesExpected.text = [NSString stringWithFormat:#"Expected = %lld", totalBytesExpectedToWrite];
self.label_bytesWritten.text = [NSString stringWithFormat:#"Received = %lld (%ld%%)", totalBytesWritten, percentage];
});
}
Is it possible to call NSOperation (queue or block or anything else with it) to display this on the UI instead of using dispatch_async(dispatch_get_main_queue(), block)?
Thank you!
Yes. It is possible. You should use [NSOperationQueue mainQueue]
+ (NSOperationQueue *)mainQueue
Returns the operation queue associated with the main thread.
So your code could be:
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
self.label_bytesExpected.text = [NSString stringWithFormat:#"Expected = %lld", totalBytesExpectedToWrite];
self.label_bytesWritten.text = [NSString stringWithFormat:#"Received = %lld (%ld%%)", totalBytesWritten, percentage];
}];
NSURLSession has a property called delegateQueue which indicates on which queue delegate's methods will be called. It can be set to [NSOperationQueue mainQueue] via factory method:
[NSURLSession sessionWithConfiguration:configuration
delegate:delegate
delegateQueue:[NSOperationQueue mainQueue]];
Related
As
NSURLConnection sendSynchronousRequest:returningResponse:error:&connectionError
is set deprecated I will have to replace an Importer I wrote a long time ago.
The Importer does the following:
It fetches data from API-A. The data there can be on multiple pages.
It uses the data from the first fetch (also multipage) to query data from API-B and merges
Results from the API-B query will be merged with data from API-A
I implemented this with a background-operation where I use methods for each API which called recursively if there are mulitple pages for the request.
But as NSURLSession does not support synchronous request I currently only see the option to have lot of overhead (e.g. iVars) control what's called in the completion block (e.g. next Page or start to query API-B).
So, what would be an elegant solution to bring this to NSURLSession.
NB: Just to make sure, my previous solution does not block the main thread at all. But back then it was the easiest way to control the merge of two sources.
This answer is not supposed to be best practice. It was just practical for me.
Faced with the situation when a bunch of synchronous requests is executed in background and the order of execution matters I've ended up using the following:
SyncRequestSender.h
#import <Foundation/Foundation.h>
#interface SyncRequestSender : NSObject
+ (NSData *)sendSynchronousRequest:(NSURLRequest *)request
returningResponse:(NSURLResponse **)response
error:(NSError **)error;
#end
SyncRequestSender.m
#import "SyncRequestSender.h"
#implementation SyncRequestSender
+ (NSData *)sendSynchronousRequest:(NSURLRequest *)request
returningResponse:(NSURLResponse **)response
error:(NSError **)error
{
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
NSError __block *err = NULL;
NSData __block *data;
NSURLResponse __block *resp;
[[[NSURLSession sharedSession] dataTaskWithRequest:request
completionHandler:^(NSData* _data, NSURLResponse* _response, NSError* _error) {
resp = _response;
err = _error;
data = _data;
dispatch_group_leave(group);
}] resume];
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
if (response)
{
*response = resp;
}
if (error)
{
*error = err;
}
return data;
}
#end
This's an example in AFNetworking, it shows how to wait an asynchronous task.
- (NSArray *)tasksForKeyPath:(NSString *)keyPath {
__block NSArray *tasks = nil;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([keyPath isEqualToString:NSStringFromSelector(#selector(dataTasks))]) {
tasks = dataTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(#selector(uploadTasks))]) {
tasks = uploadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(#selector(downloadTasks))]) {
tasks = downloadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(#selector(tasks))]) {
tasks = [#[dataTasks, uploadTasks, downloadTasks] valueForKeyPath:#"#unionOfArrays.self"];
}
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
return tasks;
}
I'm new to queues and threading. Below I have the following code that seeks to run various methods within an asynchronous queue. Each item in the queue will update a count when finished, and when all the items are done in the queue, the update count will be complete and ready for return.
The problem is that when this code runs the last line is called and then, it hangs with no errors and I cannot continue to step through the trace.
__block int masterUpdateCount = 0;
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self updateStuffWithCompletionCount:^(int updatedItemsCount) {
masterUpdateCount += updatedItemsCount;
}];
[self updateMoreStuffWithCompletionCount:^(int updatedItemsCount) {
masterUpdateCount += updatedItemsCount; // Never gets called
}];
}); // <------ APPLICATION HANGS HERE after calling dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
return masterUpdateCount;
Something I noticed is that the completion blocks never get called, which could very well be why it hangs forever, but my question is why? If it helps, inside the updateStuffWithCompletionCount type methods I'm actually initializing an NSURLSession with an NSURLSessionDataTask and I am in fact running the task by calling [task resume];, so I don't see why the completion wouldn't be called.
Here is what it looks like inside the updateStuffWithCompletionCount method:
- (void) updateStuffWithCompletionCount: (void (^)(int)) completionResult
{
__block int updateCount = 0;
NSString *someURL = #"www.someplace.com";
NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];
NSURLSessionDataTask * getDataTask = [defaultSession dataTaskWithURL:[NSURL someURL] completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
{
if(error == nil)
{
// do stuff, if updated, add to count
updateCount ++;
if(updateCount > 0) {
completionResult(updateCount); // Invoke the completion handler
} else {
completionResult(0); // Invoke the completion handler
}
}
else {
NSLog(#"Error: %#", error);
completionResult(-1);
}
}];
[getDataTask resume];
}
Hopefully an experienced eye can point it out. Appreciated.
Include code for your dataTask please. This line masterUpdateCount += updatedItemsCount; // Never gets called , because its in the completion block which means it doesn't get ran until your datatask is complete, Did you set a breakpoint within your datatask and make sure something its jumping in there? Do you have a NSLog(%#, error.description) line in there in case your getting one? Looks like you may need to do more debugging there.
In my App I get JSON from a Web service. This JSON contains the urls of several files that I would like to download.
I want to download each file one by one (wait for the first download to finished until the second one starts and so an and so forth) using NSURLSessionDownloadTask. I would also like to keep track of the total bytes written so I can update UI.
Thanks a lot in advance !
NSURLSessionDownloadTask's as you well know do not play very nicely with NSOperationQueues unlike their counterpart the NSURLConnection (where it could be encapsulated inside an NSOperation).
One option would be to add all your urls to an array, and then inside the completionHandler of the task, simply queue the next item.
So you might create your tasks in a loop, call a progressBlock inside each tasks completion handler, store the tasks in an array, and queue the next task inside each tasks completion handler:
- (void)addRequestsWithURLs:(NSArray *)urls
progressBlock:(void (^)(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations, NSURLSessionDownloadTask *task,NSURL *location, NSURLResponse *response, NSError *error))progressBlock {
__block NSUInteger numberOfFinishedOperations = 0;
NSUInteger totalNumberOfOperations = [urls count];
for (NSString *url in urls) {
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url]];
__block NSURLSessionDownloadTask *task = [self.session downloadTaskWithRequest:request
completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
//downloadFileSomewhere
++numberOfFinishedOperations;
if (progressBlock) {
progressBlock(numberOfFinishedOperations, totalNumberOfOperations,task,destination != nil ? [NSURL fileURLWithPath:destination] : nil,response,error);
}
//queueNext
[self processCompletedTask:task];
}];
//stores an array of NSURLSessionTasks
[self.tasksWaitingToBeQueued addObject:task];
}
}
- (void)processCompletedTask:(NSURLSessionTask *)completedTask {
//clean up and queue next one
[self.tasksWaitingToBeQueued removeObject:completedTask];
nextTask = [self.tasksWaitingToBeQueued firstObject];
if (nextTask) {
[nextTask resume];
}
}
NOTE
In this example I show progress as the number of tasks completed and not the number of bytes, this is the recommended approach (its also simpler). To indicate progress using bytes you would need to know the total number of bytes to download beforehand (since you want to show a progress bar) and also implement the NSURLSession delegate and monitor the progress of each task, capture the bytes downloaded and update your block. If your server doesn't tell you the total number of bytes then you would probably need to do a HEAD request for every resource and aggregate the sizes. Personally this solution is way to complicated for what could simply be resolved by indicating progress as the number of files downloaded.
To achieve this might look something like this:
- (void)URLSession:(NSURLSession *)session
downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
self.totalBytesWritten += totalBytesWritten;
NSUInteger totalProgressSoFar = self.totalBytesWritten;
NSUInteger totalExpectedBytes = self.totalExpectedBytes;
//you would need to capture some progress block locally - beware of retain cycles
self.progressBlock(totalProgressSoFar/totalExpectedBytes)
}
when you finish you should set the progressBlock to nil to prevent any retain cycles.
I'm a bit confuse of how to take advantage of the new iOS 7 NSURLSession background transfers features and AFNetworking (versions 2 and 3).
I saw the WWDC 705 - What’s New in Foundation Networking session, and they demonstrated background download that continues after the app terminated or even crashes.
This is done using the new API application:handleEventsForBackgroundURLSession:completionHandler: and the fact that the session's delegate will eventually get the callbacks and can complete its task.
So I'm wondering how to use it with AFNetworking (if possible) to continue downloading in background.
The problem is, AFNetworking conveniently uses block based API to do all the requests, but if the app terminated or crashes those block are also gone. So how can I complete the task?
Or maybe I'm missing something here...
Let me explain what I mean:
For example my app is a photo messaging app, lets say that I have a PhotoMessage object that represent one message and this object has properties like
state - describe the state of the photo download.
resourcePath - the path to the final downloaded photo file.
So when I get a new message from the server, I create a new PhotoMessage object, and start downloading its photo resource.
PhotoMessage *newPhotoMsg = [[PhotoMessage alloc] initWithInfoFromServer:info];
newPhotoMsg.state = kStateDownloading;
self.photoDownloadTask = [[BGSessionManager sharedManager] downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
NSURL *filePath = // some file url
return filePath;
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
if (!error) {
// update the PhotoMessage Object
newPhotoMsg.state = kStateDownloadFinished;
newPhotoMsg.resourcePath = filePath;
}
}];
[self.photoDownloadTask resume];
As you can see, I use the completion block to update that PhotoMessage object according to the response I get.
How can I accomplish that with a background transfer? This completion block won't be called and as a result, I can't update the newPhotoMsg.
A couple of thoughts:
You have to make sure you do the necessary coding outlined in the Handling iOS Background Activity section of the URL Loading System Programming Guide says:
If you are using NSURLSession in iOS, your app is automatically relaunched when a download completes. Your app’s application:handleEventsForBackgroundURLSession:completionHandler: app delegate method is responsible for recreating the appropriate session, storing a completion handler, and calling that handler when the session calls your session delegate’s URLSessionDidFinishEventsForBackgroundURLSession: method.
That guide shows some examples of what you can do. Frankly, I think the code samples discussed in the latter part of the WWDC 2013 video What’s New in Foundation Networking are even more clear.
The basic implementation of AFURLSessionManager will work in conjunction with background sessions if the app is merely suspended (you'll see your blocks called when the network tasks are done, assuming you've done the above). But as you guessed, any task-specific block parameters that are passed to the AFURLSessionManager method where you create the NSURLSessionTask for uploads and downloads are lost "if the app terminated or crashes."
For background uploads, this is an annoyance (as your task-level informational progress and completion blocks you specified when creating the task will not get called). But if you employ the session-level renditions (e.g. setTaskDidCompleteBlock and setTaskDidSendBodyDataBlock), that will get called properly (assuming you always set these blocks when you re-instantiate the session manager).
As it turns out, this issue of losing the blocks is actually more problematic for background downloads, but the solution there is very similar (do not use task-based block parameters, but rather use session-based blocks, such as setDownloadTaskDidFinishDownloadingBlock).
An alternative, you could stick with default (non-background) NSURLSession, but make sure your app requests a little time to finish the upload if the user leaves the app while the task is in progress. For example, before you create your NSURLSessionTask, you can create a UIBackgroundTaskIdentifier:
UIBackgroundTaskIdentifier __block taskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^(void) {
// handle timeout gracefully if you can
[[UIApplication sharedApplication] endBackgroundTask:taskId];
taskId = UIBackgroundTaskInvalid;
}];
But make sure that the completion block of the network task correctly informs iOS that it is complete:
if (taskId != UIBackgroundTaskInvalid) {
[[UIApplication sharedApplication] endBackgroundTask:taskId];
taskId = UIBackgroundTaskInvalid;
}
This is not as powerful as a background NSURLSession (e.g., you have a limited amount of time available), but in some cases this can be useful.
Update:
I thought I'd add a practical example of how to do background downloads using AFNetworking.
First define your background manager.
//
// BackgroundSessionManager.h
//
// Created by Robert Ryan on 10/11/14.
// Copyright (c) 2014 Robert Ryan. All rights reserved.
//
#import "AFHTTPSessionManager.h"
#interface BackgroundSessionManager : AFHTTPSessionManager
+ (instancetype)sharedManager;
#property (nonatomic, copy) void (^savedCompletionHandler)(void);
#end
and
//
// BackgroundSessionManager.m
//
// Created by Robert Ryan on 10/11/14.
// Copyright (c) 2014 Robert Ryan. All rights reserved.
//
#import "BackgroundSessionManager.h"
static NSString * const kBackgroundSessionIdentifier = #"com.domain.backgroundsession";
#implementation BackgroundSessionManager
+ (instancetype)sharedManager {
static id sharedMyManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedMyManager = [[self alloc] init];
});
return sharedMyManager;
}
- (instancetype)init {
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:kBackgroundSessionIdentifier];
self = [super initWithSessionConfiguration:configuration];
if (self) {
[self configureDownloadFinished]; // when download done, save file
[self configureBackgroundSessionFinished]; // when entire background session done, call completion handler
[self configureAuthentication]; // my server uses authentication, so let's handle that; if you don't use authentication challenges, you can remove this
}
return self;
}
- (void)configureDownloadFinished {
// just save the downloaded file to documents folder using filename from URL
[self setDownloadTaskDidFinishDownloadingBlock:^NSURL *(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, NSURL *location) {
if ([downloadTask.response isKindOfClass:[NSHTTPURLResponse class]]) {
NSInteger statusCode = [(NSHTTPURLResponse *)downloadTask.response statusCode];
if (statusCode != 200) {
// handle error here, e.g.
NSLog(#"%# failed (statusCode = %ld)", [downloadTask.originalRequest.URL lastPathComponent], statusCode);
return nil;
}
}
NSString *filename = [downloadTask.originalRequest.URL lastPathComponent];
NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSString *path = [documentsPath stringByAppendingPathComponent:filename];
return [NSURL fileURLWithPath:path];
}];
[self setTaskDidCompleteBlock:^(NSURLSession *session, NSURLSessionTask *task, NSError *error) {
if (error) {
// handle error here, e.g.,
NSLog(#"%#: %#", [task.originalRequest.URL lastPathComponent], error);
}
}];
}
- (void)configureBackgroundSessionFinished {
typeof(self) __weak weakSelf = self;
[self setDidFinishEventsForBackgroundURLSessionBlock:^(NSURLSession *session) {
if (weakSelf.savedCompletionHandler) {
weakSelf.savedCompletionHandler();
weakSelf.savedCompletionHandler = nil;
}
}];
}
- (void)configureAuthentication {
NSURLCredential *myCredential = [NSURLCredential credentialWithUser:#"userid" password:#"password" persistence:NSURLCredentialPersistenceForSession];
[self setTaskDidReceiveAuthenticationChallengeBlock:^NSURLSessionAuthChallengeDisposition(NSURLSession *session, NSURLSessionTask *task, NSURLAuthenticationChallenge *challenge, NSURLCredential *__autoreleasing *credential) {
if (challenge.previousFailureCount == 0) {
*credential = myCredential;
return NSURLSessionAuthChallengeUseCredential;
} else {
return NSURLSessionAuthChallengePerformDefaultHandling;
}
}];
}
#end
Make sure app delegate saves completion handler (instantiating the background session as necessary):
- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
NSAssert([[BackgroundSessionManager sharedManager].session.configuration.identifier isEqualToString:identifier], #"Identifiers didn't match");
[BackgroundSessionManager sharedManager].savedCompletionHandler = completionHandler;
}
Then start your downloads:
for (NSString *filename in filenames) {
NSURL *url = [baseURL URLByAppendingPathComponent:filename];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
[[[BackgroundSessionManager sharedManager] downloadTaskWithRequest:request progress:nil destination:nil completionHandler:nil] resume];
}
Note, I don't supply any of those task related blocks, because those aren't reliable with background sessions. (Background downloads proceed even after the app is terminated and these blocks have long disappeared.) One must rely upon the session-level, easily recreated setDownloadTaskDidFinishDownloadingBlock only.
Clearly this is a simple example (only one background session object; just saving files to the docs folder using last component of URL as the filename; etc.), but hopefully it illustrates the pattern.
It shouldn't make any difference whether or not the callbacks are blocks or not. When you instantiate an AFURLSessionManager, make sure to instantiate it with NSURLSessionConfiguration backgroundSessionConfiguration:. Also, make sure to call the manager's setDidFinishEventsForBackgroundURLSessionBlock with your callback block - this is where you should write the code typically defined in NSURLSessionDelegate's method:
URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session. This code should invoke your app delegate's background download completion handler.
One word of advice regarding background download tasks - even when running in the foreground, their timeouts are ignored, meaning you could get "stuck" on a download that's not responding. This is not documented anywhere and drove me crazy for some time. The first suspect was AFNetworking but even after calling NSURLSession directly, the behaviour remained the same.
Good luck!
AFURLSessionManager
AFURLSessionManager creates and manages an NSURLSession object based on a specified NSURLSessionConfiguration object, which conforms to <NSURLSessionTaskDelegate>, <NSURLSessionDataDelegate>, <NSURLSessionDownloadDelegate>, and <NSURLSessionDelegate>.
link to documentation here documentation
What are the best practices for making a serial queue of NSURLSessionTasks ?
In my case, I need to:
Fetch a URL inside a JSON file (NSURLSessionDataTask)
Download the file at that URL (NSURLSessionDownloadTask)
Here’s what I have so far:
session = [NSURLSession sharedSession];
//Download the JSON:
NSURLRequest *dataRequest = [NSURLRequest requestWithURL:url];
NSURLSessionDataTask *task =
[session dataTaskWithRequest:dataRequest
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
//Figure out the URL of the file I want to download:
NSJSONSerialization *json = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:nil];
NSURL *downloadURL = [NSURL urlWithString:[json objectForKey:#"download_url"]];
NSURLSessionDownloadTask *fileDownloadTask =
[session downloadTaskWithRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:playlistURL]]
completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
NSLog(#"completed!");
}];
[fileDownloadTask resume];
}
];
Apart from the fact that writing a completion block within another completion looks messy, I am getting an EXC_BAD_ACCESS error when I call [fileDownloadTask resume]... Even though fileDownloadTask is not nil!
So, what is the best of way of sequencing NSURLSessionTasks?
You need to use this approach which is the most straight forward: https://stackoverflow.com/a/31386206/2308258
Or use an operation queue and make the tasks dependent on each others
=======================================================================
Regarding the HTTPMaximumConnectionsPerHost method
An easy way to implement a first-in first-out serial queue of NSURLSessionTasks is to run all tasks on a NSURLSession that has its HTTPMaximumConnectionsPerHost property set to 1
HTTPMaximumConnectionsPerHost only ensure that one shared connection will be used for the tasks of that session but it does not mean that they will be processed serially.
You can verify that on the network level using http://www.charlesproxy.com/, you wil discover that when setting HTTPMaximumConnectionsPerHost, your tasks will be still be started together at the same time by NSURLSession and not serially as believed.
Expriment 1:
Declaring a NSURLSession with HTTPMaximumConnectionsPerHost to 1
With task1: url = download.thinkbroadband.com/20MB.zip
With task2: url = download.thinkbroadband.com/20MB.zip
calling [task1 resume];
calling [task2 resume];
Result: task1 completionBlock is called then task2 completionBlock is called
The completion blocks might be called in the order you expected in case the tasks take the same amount of time however if you try to download two different thing using the same NSURLSession you will discover that NSURLSession does not have any underlying ordering of your calls but only completes whatever finishes first.
Expriment 2:
Declaring a NSURLSession with HTTPMaximumConnectionsPerHost to 1
task1: url = download.thinkbroadband.com/20MB.zip
task2: url = download.thinkbroadband.com/10MB.zip (smaller file)
calling [task1 resume];
calling [task2 resume];
Result: task2 completionBlock is called then task1 completionBlock is called
In conclusion you need to do the ordering yourself, NSURLSession does not have any logic about ordering requests it will just call the completionBlock of whatever finishes first even when setting the maximum number of connections per host to 1
PS: Sorry for the format of the post I do not have enough reputation to post screenshots.
Edit:
As mataejoon has pointed out, setting HTTPMaximumConnectionsPerHost to 1 will not guarantee that the connections are processed serially. Try a different approach (as in my original answer bellow) if you need a reliable serial queue of NSURLSessionTask.
An easy way to implement a first-in first-out serial queue of NSURLSessionTasks is to run all tasks on a NSURLSession that has its HTTPMaximumConnectionsPerHost property set to 1:
+ (NSURLSession *)session
{
static NSURLSession *session = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
[configuration setHTTPMaximumConnectionsPerHost:1];
session = [NSURLSession sessionWithConfiguration:configuration];
});
return session;
}
then add tasks to it in the order you want.
NSURLSessionDataTask *sizeTask =
[[[self class] session] dataTaskWithURL:url
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
#import "SessionTaskQueue.h"
#interface SessionTaskQueue ()
#property (nonatomic, strong) NSMutableArray * sessionTasks;
#property (nonatomic, strong) NSURLSessionTask * currentTask;
#end
#implementation SessionTaskQueue
- (instancetype)init {
self = [super init];
if (self) {
self.sessionTasks = [[NSMutableArray alloc] initWithCapacity:15];
}
return self;
}
- (void)addSessionTask:(NSURLSessionTask *)sessionTask {
[self.sessionTasks addObject:sessionTask];
[self resume];
}
// call in the completion block of the sessionTask
- (void)sessionTaskFinished:(NSURLSessionTask *)sessionTask {
self.currentTask = nil;
[self resume];
}
- (void)resume {
if (self.currentTask) {
return;
}
self.currentTask = [self.sessionTasks firstObject];
if (self.currentTask) {
[self.sessionTasks removeObjectAtIndex:0];
[self.currentTask resume];
}
}
#end
and use like this
__block __weak NSURLSessionTask * wsessionTask;
use_wself();
wsessionTask = [[CommonServices shared] doSomeStuffWithCompletion:^(NSError * _Nullable error) {
use_sself();
[self.sessionTaskQueue sessionTaskFinished:wsessionTask];
...
}];
[self.sessionTaskQueue addSessionTask:wsessionTask];
I use NSOperationQueue (as Owen has suggested). Put the NSURLSessionTasks in NSOperation subclasses and set any dependancies. Dependent tasks will wait until the task they are dependent on is completed before running but will not check the status (success or failure) so add some logic to control the process.
In my case, the first task checks if the user has a valid account and creates one if necessary. In the first task I update a NSUserDefault value to indicate the account is valid (or there is an error). The second task checks the NSUserDefault value and if all OK uses the user credentials to post some data to the server.
(Sticking the NSURLSessionTasks in separate NSOperation subclasses also made my code easier to navigate)
Add the NSOperation subclasses to the NSOperationQueue and set any dependencies:
NSOperationQueue *ftfQueue = [NSOperationQueue new];
FTFCreateAccount *createFTFAccount = [[FTFCreateAccount alloc]init];
[createFTFAccount setUserid:#"********"]; // Userid to be checked / created
[ftfQueue addOperation:createFTFAccount];
FTFPostRoute *postFTFRoute = [[FTFPostRoute alloc]init];
[postFTFRoute addDependency:createFTFAccount];
[ftfQueue addOperation:postFTFRoute];
In the first NSOperation subclass checks if account exists on server:
#implementation FTFCreateAccount
{
NSString *_accountCreationStatus;
}
- (void)main {
NSDate *startDate = [[NSDate alloc] init];
float timeElapsed;
NSString *ftfAccountStatusKey = #"ftfAccountStatus";
NSString *ftfAccountStatus = (NSString *)[[NSUserDefaults standardUserDefaults] objectForKey:ftfAccountStatusKey];
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setValue:#"CHECKING" forKey:ftfAccountStatusKey];
// Setup and Run the NSURLSessionTask
[self createFTFAccount:[self userid]];
// Hold it here until the SessionTask completion handler updates the _accountCreationStatus
// Or the process takes too long (possible connection error)
while ((!_accountCreationStatus) && (timeElapsed < 5.0)) {
NSDate *currentDate = [[NSDate alloc] init];
timeElapsed = [currentDate timeIntervalSinceDate:startDate];
}
if ([_accountCreationStatus isEqualToString:#"CONNECTION PROBLEM"] || !_accountCreationStatus) [self cancel];
if ([self isCancelled]) {
NSLog(#"DEBUG FTFCreateAccount Cancelled" );
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setValue:#"ERROR" forKey:ftfAccountStatusKey];
}
}
In the next NSOperation post data:
#implementation FTFPostRoute
{
NSString *_routePostStatus;
}
- (void)main {
NSDate *startDate = [[NSDate alloc] init];
float timeElapsed;
NSString *ftfAccountStatusKey = #"ftfAccountStatus";
NSString *ftfAccountStatus = (NSString *)[[NSUserDefaults standardUserDefaults] objectForKey:ftfAccountStatusKey];
if ([ftfAccountStatus isEqualToString:#"ERROR"])
{
// There was a ERROR in creating / accessing the user account. Cancel the post
[self cancel];
} else
{
// Call method to setup and run json post
// Hold it here until a reply comes back from the operation
while ((!_routePostStatus) && (timeElapsed < 3)) {
NSDate *currentDate = [[NSDate alloc] init];
timeElapsed = [currentDate timeIntervalSinceDate:startDate];
NSLog(#"FTFPostRoute time elapsed: %f", timeElapsed);
}
}
if ([self isCancelled]) {
NSLog(#"FTFPostRoute operation cancelled");
}
}