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;
}
Related
I have an app that currently uses NSURLConnection for the vast majority of its networking. I would like to move to NSURLSession because Apple tells me that is the way to go.
My app just uses the synchronous version of NSURLConnection by way of the + (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response error:(NSError **)error class method. I do this within a NSBlockOperation running on an NSOperationQueue so I am not needlessly blocking the main queue. The big advantage to doing things this way is that I can make the operations dependent on one another. For example, I can have the task that is requesting data be dependent on the login task finishing.
I have not seen any support for synchronous operations within NSURLSession. All I can find are articles deriding me for even thinking of using it synchronously and that I am a horrible person for blocking the threads. Fine. But I see no way to make NSURLSessionTasks dependent on each other. Is there a way to do that?
Or is there a description of how I would do such a thing in a different way?
The harshest criticisms of synchronous network requests are reserved for those who do it from the main queue (as we know that one should never block the main queue). But you're doing it on your own background queue, which addresses the most egregious problem with synchronous requests. But you're losing some wonderful features that asynchronous techniques provide (e.g. cancelation of requests, if needed).
I'll answer your question (how to make NSURLSessionDataTask behave synchronously) below, but I'd really encourage you to embrace the asynchronous patterns rather than fighting them. I'd suggest refactoring your code to use asynchronous patterns. Specifically, if one task is dependent upon another, simply put the initiation of the dependent task in the completion handler of the prior task.
If you have problems in that conversion, then post another Stack Overflow question, showing us what you tried, and we can try to help you out.
If you want to make an asynchronous operation synchronous, a common pattern is to use a dispatch semaphore so your thread that initiated the asynchronous process can wait for a signal from the completion block of the asynchronous operation before continuing. Never do this from the main queue, but if you're doing this from some background queue, it can be a useful pattern.
You can create a semaphore with:
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
You can then have the completion block of the asynchronous process signal the semaphore with:
dispatch_semaphore_signal(semaphore);
And you can then have the code outside of the completion block (but still on the background queue, not the main queue) wait for that signal:
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
So, with NSURLSessionDataTask, putting that all together, that might look like:
[queue addOperationWithBlock:^{
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
NSURLSession *session = [NSURLSession sharedSession]; // or create your own session with your own NSURLSessionConfiguration
NSURLSessionTask *task = [session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if (data) {
// do whatever you want with the data here
} else {
NSLog(#"error = %#", error);
}
dispatch_semaphore_signal(semaphore);
}];
[task resume];
// but have the thread wait until the task is done
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
// now carry on with other stuff contingent upon what you did above
]);
With NSURLConnection (now deprecated), you have to jump through some hoops to initiate requests from a background queue, but NSURLSession handles it gracefully.
Having said that, using block operations like this means that the operations won't respond to cancellation events (while they're running, at least). So I generally eschew this semaphore technique with block operations and just wrap the data tasks in asynchronous NSOperation subclass. Then you enjoy the benefits of operations, but you can make them cancelable, too. It's more work, but a much better pattern.
For example:
//
// DataTaskOperation.h
//
// Created by Robert Ryan on 12/12/15.
// Copyright © 2015 Robert Ryan. All rights reserved.
//
#import Foundation;
#import "AsynchronousOperation.h"
NS_ASSUME_NONNULL_BEGIN
#interface DataTaskOperation : AsynchronousOperation
/// Creates a operation that retrieves the contents of a URL based on the specified URL request object, and calls a handler upon completion.
///
/// #param request A NSURLRequest object that provides the URL, cache policy, request type, body data or body stream, and so on.
/// #param dataTaskCompletionHandler The completion handler to call when the load request is complete. This handler is executed on the delegate queue. This completion handler takes the following parameters:
///
/// #returns The new session data operation.
- (instancetype)initWithRequest:(NSURLRequest *)request dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler;
/// Creates a operation that retrieves the contents of a URL based on the specified URL request object, and calls a handler upon completion.
///
/// #param url A NSURL object that provides the URL, cache policy, request type, body data or body stream, and so on.
/// #param dataTaskCompletionHandler The completion handler to call when the load request is complete. This handler is executed on the delegate queue. This completion handler takes the following parameters:
///
/// #returns The new session data operation.
- (instancetype)initWithURL:(NSURL *)url dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler;
#end
NS_ASSUME_NONNULL_END
and
//
// DataTaskOperation.m
//
// Created by Robert Ryan on 12/12/15.
// Copyright © 2015 Robert Ryan. All rights reserved.
//
#import "DataTaskOperation.h"
#interface DataTaskOperation ()
#property (nonatomic, strong) NSURLRequest *request;
#property (nonatomic, weak) NSURLSessionTask *task;
#property (nonatomic, copy) void (^dataTaskCompletionHandler)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error);
#end
#implementation DataTaskOperation
- (instancetype)initWithRequest:(NSURLRequest *)request dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler {
self = [super init];
if (self) {
self.request = request;
self.dataTaskCompletionHandler = dataTaskCompletionHandler;
}
return self;
}
- (instancetype)initWithURL:(NSURL *)url dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler {
NSURLRequest *request = [NSURLRequest requestWithURL:url];
return [self initWithRequest:request dataTaskCompletionHandler:dataTaskCompletionHandler];
}
- (void)main {
NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithRequest:self.request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
self.dataTaskCompletionHandler(data, response, error);
[self completeOperation];
}];
[task resume];
self.task = task;
}
- (void)completeOperation {
self.dataTaskCompletionHandler = nil;
[super completeOperation];
}
- (void)cancel {
[self.task cancel];
[super cancel];
}
#end
Where:
//
// AsynchronousOperation.h
//
#import Foundation;
#interface AsynchronousOperation : NSOperation
/// Complete the asynchronous operation.
///
/// This also triggers the necessary KVO to support asynchronous operations.
- (void)completeOperation;
#end
And
//
// AsynchronousOperation.m
//
#import "AsynchronousOperation.h"
#interface AsynchronousOperation ()
#property (nonatomic, getter = isFinished, readwrite) BOOL finished;
#property (nonatomic, getter = isExecuting, readwrite) BOOL executing;
#end
#implementation AsynchronousOperation
#synthesize finished = _finished;
#synthesize executing = _executing;
- (instancetype)init {
self = [super init];
if (self) {
_finished = NO;
_executing = NO;
}
return self;
}
- (void)start {
if ([self isCancelled]) {
self.finished = YES;
return;
}
self.executing = YES;
[self main];
}
- (void)completeOperation {
self.executing = NO;
self.finished = YES;
}
#pragma mark - NSOperation methods
- (BOOL)isAsynchronous {
return YES;
}
- (BOOL)isExecuting {
#synchronized(self) {
return _executing;
}
}
- (BOOL)isFinished {
#synchronized(self) {
return _finished;
}
}
- (void)setExecuting:(BOOL)executing {
#synchronized(self) {
if (_executing != executing) {
[self willChangeValueForKey:#"isExecuting"];
_executing = executing;
[self didChangeValueForKey:#"isExecuting"];
}
}
}
- (void)setFinished:(BOOL)finished {
#synchronized(self) {
if (_finished != finished) {
[self willChangeValueForKey:#"isFinished"];
_finished = finished;
[self didChangeValueForKey:#"isFinished"];
}
}
}
#end
#Rob I would encourage you to post your reply as a solution, in view of the following documentation note from NSURLSession.dataTaskWithURL(_:completionHandler:):
This method is intended as an alternative to the
sendAsynchronousRequest:queue:completionHandler: method of
NSURLConnection, with the added ability to support custom
authentication and cancellation.
If semaphore based approach doesn't work, try polling based approach.
var reply = Data()
/// We need to make a session object.
/// This is key to make this work. This won't work with shared session.
let conf = URLSessionConfiguration.ephemeral
let sess = URLSession(configuration: conf)
let task = sess.dataTask(with: u) { data, _, _ in
reply = data ?? Data()
}
task.resume()
while task.state != .completed {
Thread.sleep(forTimeInterval: 0.1)
}
FileHandle.standardOutput.write(reply)
Polling based approach works very reliably, but effectively limits maximum throughput to polling interval. In this example, it's been limited to 10 times/sec.
I made a Swift package for this.
Semaphore based approach has been worked well so far, but since Xcode 11 era, it's getting broken. (maybe only for me?)
A data task does not finish if I wait for semaphores. If I wait for semaphore on different thread, it task fails with an error.
nw_connection_copy_protocol_metadata [C2] Client called nw_connection_copy_protocol_metadata on unconnected nw_connection error.
It seems something has been changed in the implementation as Apple is moving Network.framework.
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");
}
}
downloadImages is a button and whenever I press on it, a spinner should start rolling, an async request should ping Google (to make sure there is a connection) and after a response is received, I start to synchronically downloading images.
Somehow the spinner won't go and it seems as if the request is sync and not async.
- (IBAction)downloadImages:(id)sender {
NSString *ping=#"http://www.google.com/";
GlobalVars *globals = [GlobalVars sharedInstance];
[self startSpinner:#"Please Wait."];
NSURL *url = [[NSURL alloc] initWithString:ping];
NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:5.0];
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
if (data) {
for(int i=globals.farmerList.count-1; i>=0;i--)
{
//Definitions
NSString * documentsDirectoryPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
//Get Image From URL
NSString *urlString = [NSString stringWithFormat:#"https://myurl.com/%#",[[globals.farmerList objectAtIndex:i] objectForKey:#"Image"]];
UIImage * imageFromURL = [self getImageFromURL:urlString];
//Save Image to Directory
[self saveImage:imageFromURL withFileName:[[globals.farmerList objectAtIndex:i] objectForKey:#"Image"] ofType:#"jpg" inDirectory:documentsDirectoryPath];
}
[self stopSpinner];
}
}];
}
The spinner code:
//show loading activity.
- (void)startSpinner:(NSString *)message {
// Purchasing Spinner.
if (!connectingAlerts) {
connectingAlerts = [[UIAlertView alloc] initWithTitle:NSLocalizedString(message,#"")
message:nil
delegate:self
cancelButtonTitle:nil
otherButtonTitles:nil];
connectingAlerts.tag = (NSUInteger)300;
[connectingAlerts show];
UIActivityIndicatorView *connectingIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
connectingIndicator.frame = CGRectMake(139.0f-18.0f,50.0f,37.0f,37.0f);
[connectingAlerts addSubview:connectingIndicator];
[connectingIndicator startAnimating];
}
}
//hide loading activity.
- (void)stopSpinner {
if (connectingAlerts) {
[connectingAlerts dismissWithClickedButtonIndex:0 animated:YES];
connectingAlerts = nil;
}
// [self performSelector:#selector(showBadNews:) withObject:error afterDelay:0.1];
}
As asked: the getImageFromURL code
-(UIImage *) getImageFromURL:(NSString *)fileURL {
UIImage * result;
NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fileURL]];
result = [UIImage imageWithData:data];
return result;
}
-(void) saveImage:(UIImage *)image withFileName:(NSString *)imageName ofType:(NSString *)extension inDirectory:(NSString *)directoryPath {
if ([[extension lowercaseString] isEqualToString:#"png"]) {
[UIImagePNGRepresentation(image) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:#"%#.%#", imageName, #"png"]] options:NSAtomicWrite error:nil];
} else if ([[extension lowercaseString] isEqualToString:#"jpg"] || [[extension lowercaseString] isEqualToString:#"jpeg"]) {
[UIImageJPEGRepresentation(image, 1.0) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:#"%#.%#", imageName, #"jpg"]] options:NSAtomicWrite error:nil];
} else {
NSLog(#"Image Save Failed\nExtension: (%#) is not recognized, use (PNG/JPG)", extension);
}
}
That's because you're creating an asynchronous operation and then telling it to execute on the main thread by using [NSOperationQueue mainQueue];.
Instead, create a new instance of NSOpeartionQueue and pass that as the parameter.
NSOperationQueue *myQueue = [[NSOperationQueue alloc] init];
This is an asynchronous problem. Asynchronism is infectious. That means, if any small part of the problem is asynchronous, the whole problem becomes asynchronous.
That is, your button action would invoke an asynchronous method like this (and itself becomes "asynchronous" as well):
- (IBAction)downloadImages:(id)sender
{
self.downloadImagesButton.enabled = NO;
[self asyncLoadAndSaveImagesWithURLs:self.urls completion:^(id result, NSError* error){
if (error != nil) {
NSLog(#"Error: %#", error);
}
dispatch_async(dispatch_get_main_queue(), ^{
self.downloadImagesButton.enabled = YES;
};
}];
}
So, your asynchronous problem can be described as this:
Given a list of URLs, asynchronously load each URL and asynchronously save them to disk. When all URLs are loaded and saved, asynchronously notify the call-site by calling a completion handler passing it an array of results (for each download and save operation).
This is your asynchronous method:
typedef void (^completion_t)(id result, NSError* error);
- (void) asyncLoadAndSaveImagesWithURLs:(NSArray*)urls
completion:(completion_t) completionHandler;
Asynchronous problems can be solved properly only by finding a suitable asynchronous pattern. This involves to asynchronize every part of the problem.
Lets start with your getImageFromURL method. Loading a remote resource is inherently asynchronous, so your wrapper method ultimately will be asynchronous as well:
typedef void (^completion_t)(id result, NSError* error);
- (void) loadImageWithURL:(NSURL*)url completion:(completion_t)completionHandler;
I leave it undefined how that method will be eventually implemented. You may use NSURLConnection's asynchronous convenient class method, a third party helper tool or your own HTTPRequestOperation class. It doesn't matter but it MUST be asynchronous for achieving a sane approach.
Purposefully, you can and should make your saveImage method asynchronous as well. The reason for making this asynchronous is, that this method possibly will get invoked concurrently, and we should *serialize* disk bound (I/O bound) tasks. This improves utilization of system resources and also makes your approach a friendly system citizen.
Here is the asynchronized version:
typedef void (^completion_t)(id result, NSError* error);
-(void) saveImage:(UIImage *)image fileName:(NSString *)fileName ofType:(NSString *)extension
inDirectory:(NSString *)directoryPath
completion:(completion_t)completionHandler;
In order to serialize disk access, we can use a dedicated queue disk_queue where we assume it has been properly initialized as a serial queue by self:
-(void) saveImage:(UIImage *)image fileName:(NSString *)fileName ofType:(NSString *)extension
inDirectory:(NSString *)directoryPath
completion:(completion_t)completionHandler
{
dispatch_async(self.disk_queue, ^{
// save the image
...
if (completionHandler) {
completionHandler(result, nil);
}
});
}
Now, we can define an asynchronous wrapper which loads and saves the image:
typedef void (^completion_t)(id result, NSError* error);
- (void) loadAndSaveImageWithURL:(NSURL*)url completion:(completion_t)completionHandler
{
[self loadImageWithURL:url completion:^(id image, NSError*error) {
if (image) {
[self saveImage:image fileName:fileName ofType:type inDirectory:directory completion:^(id result, NSError* error){
if (result) {
if (completionHandler) {
completionHandler(result, nil);
}
}
else {
DebugLog(#"Error: %#", error);
if (completionHandler) {
completionHandler(nil, error);
}
}
}];
}
else {
if (completionHandler) {
completionHandler(nil, error);
}
}
}];
}
This loadAndSaveImageWithURL method actually performs a "continuation" of two asynchronous tasks:
First, asynchronously load the image.
THEN, if that was successful, asynchronously save the image.
It's important to notice that these two asynchronous tasks are sequentially processed.
Up until here, this all should be quite comprehensive and be straight forward. The tricky part follows now where we try to invoke a number of asynchronous tasks in an asynchronous manner.
Asynchronous Loop
Suppose, we have a list of URLs. Each URL shall be loaded asynchronously, and when all URLs are loaded we want the call-site to be notified.
The traditional for loop is not that appropriate for accomplishing this. But imagine we would have a Category for a NSArray with a method like this:
Category for NSArray
- (void) forEachApplyTask:(task_t)transform completion:(completion_t)completionHandler;
This basically reads: for each object in the array, apply the asynchronous task transform and when all objects have been "transformed" return a list of the transformed objects.
Note: this method is asynchronous!
With the appropriate "transform" function, we can "translate" this to your specific problem:
For each URL in the array, apply the asynchronous task loadAndSaveImageWithURL and when all URLS have been loaded and saved return a list of the results.
The actual implementation of the forEachApplyTask:completion: may appear a bit tricky and for brevity I don't want to post the complete source here. A viable approach requires about 40 lines of code.
I'll provide an example implementation later (on Gist), but lets explain how this method can be used:
The task_t is a "block" which takes one input parameter (the URL) and returns a result.
Since everything must be treated asynchronously, this block is asynchronous as well, and the eventual result will be provided via a completion block:
typedef void (^completion_t)(id result, NSError* error);
typedef void (^task_t)(id input, completion_t completionHandler);
The completion handler may be defined as follows:
If the tasks succeeds, parameter error equals nil. Otherwise, parameter error is an NSError object. That is, a valid result may also be nil.
We can quite easily wrap our method loadAndSaveImageWithURL:completion: and create a block:
task_t task = ^(id input, completion_t completionHandler) {
[self loadAndSaveImageWithURL:input completion:completionHandler];
};
Given an array of URLs:
self.urls = ...;
your button action can be implemented as follows:
- (IBAction)downloadImages:(id)sender
{
self.downloadImagesButton.enabled = NO;
task_t task = ^(id input, completion_t completionHandler) {
[self loadAndSaveImageWithURL:input completion:completionHandler];
};
[self.urls forEachApplyTask:task ^(id results, NSError*error){
self.downloadImagesButton.enabled = YES;
if (error == nil) {
... // do something
}
else {
// handle error
}
}];
}
Again, notice that method forEachApplyTask:completion: is an asynchronous method, which returns immediately. The call-site will be notified via the completion handler.
The downloadImages method is asynchronous as well, there is no completion handler though. This method disables the button when it starts and enables it again when the asynchronous operation has been completed.
The implementation of this forEachApplyTask method can be found here: (https://gist.github.com/couchdeveloper/6155227).
From your code what I can understand is its not due to assyncronous call to load url. but the following code may heavy.
For assynchronous image loading try https://github.com/rs/SDWebImage
//Get Image From URL
NSString *urlString = [NSString stringWithFormat:#"https://myurl.com/%#",[[globals.farmerList objectAtIndex:i] objectForKey:#"Image"]];
UIImage * imageFromURL = [self getImageFromURL:urlString];
//Save Image to Directory
[self saveImage:imageFromURL withFileName:[[globals.farmerList objectAtIndex:i] objectForKey:#"Image"] ofType:#"jpg" inDirectory:documentsDirectoryPath];
Happy coding :)
I have some networking code with heavy JSON parsing going on. It needs to be done in the background to not block the main thread. The code looks like this :
-(void) getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
// sometimes I have more requests
// startOperations is a wrapper on AFHTTPClient enqueueBatchOfHTTPRequestOperations:progressBlock:completionBlock:
// that handles errors and loading views
[self startOperations:#[op] completionBlock:^(NSArray *operations) {
// getBgQueue = return dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(getBgQueue(), ^{
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
[self.localContext MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
// this is executed on main thread
if(completion) completion(...);
}];
});
}];
}
(AFNetworking 1.x)
The above code works very fine, but it's a pain to setup and write. And often the whole method content is wrapped inside another block to fetch some required data first... basically the blocks just pile up and makes ugly code
I'm using enqueueBatchOfHTTPRequestOperations and not individual completion blocks on AFJSONRequestOperation because batch completion block would sometimes fire before all individual operations completion blocks... (I also read somewhere that Mattt discouraged doing this)
Any pointers on how to do better than this?
I'm not sure what you want here, but just like "longcat is long", it's somewhat inherent in the pattern: 'continuation-passing style is continuation-passing style'. If you want to flatten things out a bit, you could make local block variables, but to a certain degree, you're stuck because you need the completion for -MR_saveToPersistentStoreWithCompletion to close over data in order to pass it to the -getSomeDataWithParameters... completion, but data won't exist until the -startOperations completion is executed.
You could probably achieve a less-nested appearance by using a bunch of __block variables, and splitting the code into several local blocks, but to me that feels kind of like cutting off your nose to spite your face. This code is readily understandable the way it is.
By the way... I notice that you're closing over op in the -startOperations completion block. This is fine because you're enqueuing op by doing -startOperations: #[op] ... but it would arguably be cleaner to get op from the operations parameter to the completion. I tightened this up as much as seemed reasonable:
- (void)getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...;
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
[self startOperations:#[op] completionBlock:^(NSArray *operations) {
for (AFJSONRequestOperation *op in operations) {
dispatch_async(getBgQueue(), ^{
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
void (^mrSaveCompletion)(BOOL, NSError*) = completion ? ^(BOOL success, NSError *error) { completion(data); } : nil;
[self.localContext MR_saveToPersistentStoreWithCompletion: mrSaveCompletion];
});
}
}];
}
This will fan out each response potentially to a different thread. If you want all responses to execute on a single background thread, just swap the nesting of the for loop and the dispatch_async.
From there, the only really "superfluous" code is the dispatch_async. You could eliminate that by making -startOperations:... take a queue parameter where you would pass in the queue you wanted the completion to be called. Maybe like this:
- (void)startOperations: (NSArray*)ops completionQueue: (dispatch_queue_t)queue completionBlock: (void (^)(NSArray*))completion
{
void (^completionWrapper)(NSArray*) = !completion ? nil : ^(NSArray* ops) {
if (queue)
dispatch_async(queue, ^{ completion(ops); });
else
completion(ops);
};
[self startOperations: ops completionBlock: completionWrapper];
}
- (void)getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...;
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
[self startOperations:#[op] completionQueue: getBgQueue() completionBlock:^(NSArray *operations) {
for (AFJSONRequestOperation *op in operations) {
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
void (^mrSaveCompletion)(BOOL, NSError*) = !completion ? nil : ^(BOOL success, NSError *error) { completion(data); };
[self.localContext MR_saveToPersistentStoreWithCompletion: mrSaveCompletion];
});
}];
}
I have a use case that should be rather common but I can't find an easy way to handle it with AFNetworking:
Whenever the server returns a specific status code for any request, I want to:
remove a cached authentication token
re-authenticate (which is a separate request)
repeat the failed request.
I thought that this could be done via some global completion/error handler in AFHTTPClient, but I didn't find anything useful. So, what's the "right" way to do what I want? Override enqueueHTTPRequestOperation: in my AFHTTPClient subclass, copy the operation and wrap the original completion handler with a block that does what I want (re-authenticate, enqueue copied operation)? Or am I on the wrong track altogether?
Thanks!
EDIT: Removed reference to 401 status code, since that's probably reserved for HTTP basic while I'm using token auth.
I use an alternative means for doing this with AFNetworking 2.0.
You can subclass dataTaskWithRequest:success:failure: and wrap the passed completion block with some error checking. For example, if you're working with OAuth, you could watch for a 401 error (expiry) and refresh your access token.
- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)urlRequest completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))originalCompletionHandler{
//create a completion block that wraps the original
void (^authFailBlock)(NSURLResponse *response, id responseObject, NSError *error) = ^(NSURLResponse *response, id responseObject, NSError *error)
{
NSHTTPURLResponse* httpResponse = (NSHTTPURLResponse*)response;
if([httpResponse statusCode] == 401){
NSLog(#"401 auth error!");
//since there was an error, call you refresh method and then redo the original task
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
//call your method for refreshing OAuth tokens. This is an example:
[self refreshAccessToken:^(id responseObject) {
NSLog(#"response was %#", responseObject);
//store your new token
//now, queue up and execute the original task
NSURLSessionDataTask *originalTask = [super dataTaskWithRequest:urlRequest completionHandler:originalCompletionHandler];
[originalTask resume];
}];
});
}else{
NSLog(#"no auth error");
originalCompletionHandler(response, responseObject, error);
}
};
NSURLSessionDataTask *task = [super dataTaskWithRequest:urlRequest completionHandler:authFailBlock];
return task;
}
In the AFHTTPClient's init method register for the AFNetworkingOperationDidFinishNotification which will be posted after a request finishes.
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(HTTPOperationDidFinish:) name:AFNetworkingOperationDidFinishNotification object:nil];
In the notification handler check the status code and copy the AFHTTPRequestOperation or create a new one.
- (void)HTTPOperationDidFinish:(NSNotification *)notification {
AFHTTPRequestOperation *operation = (AFHTTPRequestOperation *)[notification object];
if (![operation isKindOfClass:[AFHTTPRequestOperation class]]) {
return;
}
if ([operation.response statusCode] == 401) {
// enqueue a new request operation here
}
}
EDIT:
In general you should not need to do that and just handle the authentication with this AFNetworking method:
- (void)setAuthenticationChallengeBlock:(void (^)(NSURLConnection *connection, NSURLAuthenticationChallenge *challenge))block;
Here is the Swift implementation of user #adamup 's answer
class SessionManager:AFHTTPSessionManager{
static let sharedInstance = SessionManager()
override func dataTaskWithRequest(request: NSURLRequest!, completionHandler: ((NSURLResponse!, AnyObject!, NSError!) -> Void)!) -> NSURLSessionDataTask! {
var authFailBlock : (response:NSURLResponse!, responseObject:AnyObject!, error:NSError!) -> Void = {(response:NSURLResponse!, responseObject:AnyObject!, error:NSError!) -> Void in
var httpResponse = response as! NSHTTPURLResponse
if httpResponse.statusCode == 401 {
//println("auth failed")
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), { () -> Void in
self.refreshToken(){ token -> Void in
if let tkn = token{
var mutableRequest = request.mutableCopy() as! NSMutableURLRequest
mutableRequest.setValue(tkn, forHTTPHeaderField: "Authorization")
var newRequest = mutableRequest.copy() as! NSURLRequest
var originalTask = super.dataTaskWithRequest(newRequest, completionHandler: completionHandler)
originalTask.resume()
}else{
completionHandler(response,responseObject,error)
}
}
})
}
else{
//println("no auth error")
completionHandler(response,responseObject,error)
}
}
var task = super.dataTaskWithRequest(request, completionHandler:authFailBlock )
return task
}}
where refreshToken (...) is an extension method I wrote to get a new token from the server.
Took a similar approach, but I couldn't get the status code object with phix23's answer so I needed a different plan of action. AFNetworking 2.0 changed a couple of things.
-(void)networkRequestDidFinish: (NSNotification *) notification
{
NSError *error = [notification.userInfo objectForKey:AFNetworkingTaskDidCompleteErrorKey];
NSHTTPURLResponse *httpResponse = error.userInfo[AFNetworkingOperationFailingURLResponseErrorKey];
if (httpResponse.statusCode == 401){
NSLog(#"Error was 401");
}
}
If you are subclassing AFHTTPSessionManager or using directly an AFURLSessionManager you could use the following method to set a block executed after the completion of a task:
/**
Sets a block to be executed as the last message related to a specific task, as handled by the `NSURLSessionTaskDelegate` method `URLSession:task:didCompleteWithError:`.
#param block A block object to be executed when a session task is completed. The block has no return value, and takes three arguments: the session, the task, and any error that occurred in the process of executing the task.
*/
- (void)setTaskDidCompleteBlock:(void (^)(NSURLSession *session, NSURLSessionTask *task, NSError *error))block;
Just perform whatever you want to do for each tasks of the session in it:
[self setTaskDidCompleteBlock:^(NSURLSession *session, NSURLSessionTask *task, NSError *error) {
if ([task.response isKindOfClass:[NSHTTPURLResponse class]]) {
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)task.response;
if (httpResponse.statusCode == 500) {
}
}
}];
EDIT:
In fact if you need to handle an error returned in the response object the above method won't do the job.
One way if you are subclassing AFHTTPSessionManager could be to subclass and set a custom response serializer with it's responseObjectForResponse:data:error: overloaded like that:
#interface MyJSONResponseSerializer : AFJSONResponseSerializer
#end
#implementation MyJSONResponseSerializer
#pragma mark - AFURLResponseSerialization
- (id)responseObjectForResponse:(NSURLResponse *)response
data:(NSData *)data
error:(NSError *__autoreleasing *)error
{
id responseObject = [super responseObjectForResponse:response data:data error:error];
if ([responseObject isKindOfClass:[NSDictionary class]]
&& /* .. check for status or error fields .. */)
{
// Handle error globally here
}
return responseObject;
}
#end
and set it in your AFHTTPSessionManager subclass:
#interface MyAPIClient : AFHTTPSessionManager
+ (instancetype)sharedClient;
#end
#implementation MyAPIClient
+ (instancetype)sharedClient {
static MyAPIClient *_sharedClient = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_sharedClient = [[MyAPIClient alloc] initWithBaseURL:[NSURL URLWithString:MyAPIBaseURLString]];
_sharedClient.responseSerializer = [MyJSONResponseSerializer serializer];
});
return _sharedClient;
}
#end
To ensure that multiple token refreshes are not issued at around the same time, it is beneficial to either queue your network requests and block the queue when the token is refreshing, or add a mutex lock (#synchronized directive) to your token refresh method.