Using NSURLSession

The NSURLSession class and related classes provide an API for downloading content via HTTP. This API provides a rich set of delegate methods for supporting authentication and gives your app the ability to perform background downloads when your app is not running or, in iOS, while your app is suspended.

To use the NSURLSession API, your app creates a series of sessions, each of which coordinates a group of related data transfer tasks. For example, if you are writing a web browser, your app might create one session per tab or window. Within each session, your app adds a series of tasks, each of which represents a request for a specific URL (and for any follow-on URLs if the original URL returned an HTTP redirect).

Like most networking APIs, the NSURLSession API is highly asynchronous. If you use the default, system-provided delegate, you must provide a completion handler block that returns data to your app when a transfer finishes successfully or with an error. Alternatively, if you provide your own custom delegate objects, the task objects call those delegates’ methods with data as it is received from the server (or, for file downloads, when the transfer is complete).

The NSURLSession API provides status and progress properties, in addition to delivering this information to delegates. It supports canceling, restarting (resuming), and suspending tasks, and it provides the ability to resume suspended, canceled, or failed downloads where they left off.

Understanding URL Session Concepts

The behavior of the tasks in a session depends on three things: the type of session (determined by the type of configuration object used to create it), the type of task, and whether the app was in the foreground when the task was created.

Types of Sessions

The NSURLSession API supports three types of sessions, as determined by the type of configuration object used to create the session:

  • Default sessions behave similarly to other Foundation methods for downloading URLs. They use a persistent disk-based cache and store credentials in the user’s keychain.

  • Ephemeral sessions do not store any data to disk; all caches, credential stores, and so on are kept in RAM and tied to the session. Thus, when your app invalidates the session, they are purged automatically.

  • Background sessions are similar to default sessions, except that a separate process handles all data transfers. Background sessions have some additional limitations, described in Background Transfer Considerations.

Types of Tasks

Within a session, the NSURLSession class supports three types of tasks: data tasks, download tasks, and upload tasks.

  • Data tasks send and receive data using NSData objects. Data tasks are intended for short, often interactive requests from your app to a server. Data tasks can return data to your app one piece at a time after each piece of data is received, or all at once through a completion handler.

  • Download tasks retrieve data in the form of a file, and support background downloads while the app is not running.

  • Upload tasks send data in the form of a file, and support background uploads while the app is not running.

Background Transfer Considerations

The NSURLSession class supports background transfers while your app is suspended. Background transfers are provided only by sessions created using a background session configuration object (as returned by a call to backgroundSessionConfiguration:).

With background sessions, because the actual transfer is performed by a separate process and because restarting your app’s process is relatively expensive, a few features are unavailable, resulting in the following limitations:

  • The session must provide a delegate for event delivery. (For uploads and downloads, the delegates behave the same as for in-process transfers.)

  • Only HTTP and HTTPS protocols are supported (no custom protocols).

  • Redirects are always followed.

  • Only upload tasks from a file are supported (uploading from data objects or a stream will fail after the program exits).

  • If the background transfer is initiated while the app is in the background, the configuration object’s discretionary property is treated as being true.

The way your app behaves when it is relaunched differs slightly between iOS and OS X.

In iOS, when a background transfer completes or requires credentials, if your app is no longer running, iOS automatically relaunches your app in the background and calls the application:handleEventsForBackgroundURLSession:completionHandler: method on your app’s UIApplicationDelegate object. This call provides the identifier of the session that caused your app to be launched. Your app should store that completion handler, create a background configuration object with the same identifier, and create a session with that configuration object. The new session is automatically reassociated with ongoing background activity. Later, when the session finishes the last background download task, it sends the session delegate a URLSessionDidFinishEventsForBackgroundURLSession: message. In that delegate method, call the previously stored completion handler on the main thread so that the operating system knows that it is safe to suspend your app again.

In both iOS and OS X, when the user relaunches your app, your app should immediately create background configuration objects with the same identifiers as any sessions that had outstanding tasks when your app was last running, then create a session for each of those configuration objects. These new sessions are similarly automatically reassociated with ongoing background activity.

If any task completed while your app was suspended, the delegate’s URLSession:downloadTask:didFinishDownloadingToURL: method is then called with the task and the URL for the newly downloaded file associated with it.

Similarly, if any task requires credentials, the NSURLSession object calls the delegate’s URLSession:task:didReceiveChallenge:completionHandler: method or URLSession:didReceiveChallenge:completionHandler: method as appropriate.

Upload and download tasks in background sessions are automatically retried by the URL loading system after network errors. It is unnecessary to use reachability APIs to determine when to retry a failed task.

For an example of how to use NSURLSession for background transfers, see Simple Background Transfer.

Life Cycle and Delegate Interaction

Depending on what you are doing with the NSURLSession class, it may be helpful to fully understand the session life cycle, including how a session interacts with its delegate, the order in which delegate calls are made, what happens when the server returns a redirect, what happens when your app resumes a failed download, and so on.

For a complete description of the life cycle of a URL session, read Life Cycle of a URL Session.

NSCopying Behavior

Session and task objects conform to the NSCopying protocol as follows:

  • When your app copies a session or task object, you get the same object back.

  • When your app copies a configuration object, you get a new copy that you can independently modify.

Sample Delegate Class Interface

The code snippets in the following task sections are based on the class interface shown in Listing 1-1.

Listing 1-1  Sample delegate class interface

#import <Foundation/Foundation.h>
typedef void (^CompletionHandlerType)();
@interface MySessionDelegate : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate>
@property NSURLSession *backgroundSession;
@property NSURLSession *defaultSession;
@property NSURLSession *ephemeralSession;
@property NSMutableDictionary *completionHandlerDictionary;
- (void) addCompletionHandler: (CompletionHandlerType) handler forSession: (NSString *)identifier;
- (void) callCompletionHandlerForSession: (NSString *)identifier;

Creating and Configuring a Session

The NSURLSession API provides a wide range of configuration options:

Because most settings are contained in a separate configuration object, you can reuse commonly used settings. When you instantiate a session object, you specify the following:

After you instantiate the session object, you cannot change the configuration or the delegate without creating a new session.

Listing 1-2 shows examples of how to create normal, ephemeral, and background sessions.

Listing 1-2  Creating and configuring sessions

    self.completionHandlerDictionary = [NSMutableDictionary dictionaryWithCapacity:0];
    /* Create some configuration objects. */
    NSURLSessionConfiguration *backgroundConfigObject = [NSURLSessionConfiguration backgroundSessionConfiguration: @"myBackgroundSessionIdentifier"];
    NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSessionConfiguration *ephemeralConfigObject = [NSURLSessionConfiguration ephemeralSessionConfiguration];
    /* Configure caching behavior for the default session.
       Note that iOS requires the cache path to be a path relative
       to the ~/Library/Caches directory, but OS X expects an
       absolute path.
    NSString *cachePath = @"/MyCacheDirectory";
    NSArray *myPathList = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *myPath    = [myPathList  objectAtIndex:0];
    NSString *bundleIdentifier = [[NSBundle mainBundle] bundleIdentifier];
    NSString *fullCachePath = [[myPath stringByAppendingPathComponent:bundleIdentifier] stringByAppendingPathComponent:cachePath];
    NSLog(@"Cache path: %@\n", fullCachePath);
    NSString *cachePath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"/nsurlsessiondemo.cache"];
    NSLog(@"Cache path: %@\n", cachePath);
    NSURLCache *myCache = [[NSURLCache alloc] initWithMemoryCapacity: 16384 diskCapacity: 268435456 diskPath: cachePath];
    defaultConfigObject.URLCache = myCache;
    defaultConfigObject.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
    /* Create a session for each configurations. */
    self.defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];
    self.backgroundSession = [NSURLSession sessionWithConfiguration: backgroundConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];
    self.ephemeralSession = [NSURLSession sessionWithConfiguration: ephemeralConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];

With the exception of background configurations, you can reuse session configuration objects to create additional sessions. (You cannot reuse background session configurations because the behavior of two background session objects sharing the same identifier is undefined.)

You can also safely modify the configuration objects at any time. When you create a session, the session performs a deep copy on the configuration object, so modifications affect only new sessions, not existing sessions. For example, you might create a second session for content that should be retrieved only if you are on a Wi-Fi connection as shown in Listing 1-3.

Listing 1-3  Creating a second session with the same configuration object

    ephemeralConfigObject.allowsCellularAccess = NO;
    // ...
    NSURLSession *ephemeralSessionWiFiOnly = [NSURLSession sessionWithConfiguration: ephemeralConfigObject delegate: self delegateQueue: [NSOperationQueue mainQueue]];

Fetching Resources Using System-Provided Delegates

The most straightforward way to use the NSURLSession is as a drop-in replacement for the sendAsynchronousRequest:queue:completionHandler: method on NSURLConnection. Using this approach, you need to provide only two pieces of code in your app:

Using system-provided delegates, you can fetch a specific URL with just a single line of code per request. Listing 1-4 shows an example of this simplified form.

Listing 1-4  Requesting a resource using system-provided delegates

    NSURLSession *delegateFreeSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: nil delegateQueue: [NSOperationQueue mainQueue]];
    [[delegateFreeSession dataTaskWithURL: [NSURL URLWithString: @""]
                       completionHandler:^(NSData *data, NSURLResponse *response,
                                           NSError *error) {
                           NSLog(@"Got response %@ with error %@.\n", response, error);
                           NSLog(@"DATA:\n%@\nEND DATA\n",
                                 [[NSString alloc] initWithData: data
                                         encoding: NSUTF8StringEncoding]);
                       }] resume];

Fetching Data Using a Custom Delegate

If you are using a custom delegate to retrieve data, the delegate must implement at least the following methods:

If your app needs to use the data after its URLSession:dataTask:didReceiveData: method returns, your code is responsible for storing the data in some way.

For example, a web browser might need to render the data as it arrives along with any data it has previously received. To do this, it might use a dictionary that maps the task object to an NSMutableData object for storing the results, and then use the appendData: method on that object to append the newly received data.

Listing 1-5 shows how you create and start a data task.

Listing 1-5  Data task example

    NSURL *url = [NSURL URLWithString: @""];
    NSURLSessionDataTask *dataTask = [self.defaultSession dataTaskWithURL: url];
    [dataTask resume];

Downloading Files

At a high level, downloading a file is similar to retrieving data. Your app should implement the following delegate methods:

If you schedule the download in a background session, the download continues when your app is not running. If you schedule the download in a standard or ephemeral session, the download must begin anew when your app is relaunched.

During the transfer from the server, if the user tells your app to pause the download, your app can cancel the task by calling the cancelByProducingResumeData: method. Later, your app can pass the returned resume data to either the downloadTaskWithResumeData: or downloadTaskWithResumeData:completionHandler: method to create a new download task that continues the download.

If the transfer fails, your delegate’s URLSession:task:didCompleteWithError: method is called with an NSError object. If the task is resumable, that object’s userInfo dictionary contains a value for the NSURLSessionDownloadTaskResumeData key; your app can pass the returned resume data to either the downloadTaskWithResumeData: or downloadTaskWithResumeData:completionHandler: method to create a new download task that retries the download.

Listing 1-6 provides an example of downloading a moderately large file. Listing 1-7 provides an example of download task delegate methods.

Listing 1-6  Download task example

    NSURL *url = [NSURL URLWithString: @""
    NSURLSessionDownloadTask *downloadTask = [self.backgroundSession downloadTaskWithURL: url];
    [downloadTask resume];

Listing 1-7  Delegate methods for download tasks

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
    NSLog(@"Session %@ download task %@ finished downloading to URL %@\n",
        session, downloadTask, location);
#if 0
    /* Workaround */
    [self callCompletionHandlerForSession:session.configuration.identifier];
#define READ_THE_FILE 0
    /* Open the newly downloaded file for reading. */
    NSError *err = nil;
    NSFileHandle *fh = [NSFileHandle fileHandleForReadingFromURL:location
        error: &err];
    /* Store this file handle somewhere, and read data from it. */
    // ...
    NSError *err = nil;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *cacheDir = [[NSHomeDirectory()
    NSURL *cacheDirURL = [NSURL fileURLWithPath:cacheDir];
    if ([fileManager moveItemAtURL:location
        error: &err]) {
        /* Store some reference to the new URL */
    } else {
        /* Handle the error. */
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
    NSLog(@"Session %@ download task %@ wrote an additional %lld bytes (total %lld bytes) out of an expected %lld bytes.\n",
        session, downloadTask, bytesWritten, totalBytesWritten, totalBytesExpectedToWrite);
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes
    NSLog(@"Session %@ download task %@ resumed at offset %lld bytes out of an expected %lld bytes.\n",
        session, downloadTask, fileOffset, expectedTotalBytes);

Uploading Body Content

Your app can provide the request body content for an HTTP POST request in three ways: as an NSData object, as a file, or as a stream. In general, your app should:

Regardless of which style you choose, if your app provides a custom session delegate, that delegate should implement the URLSession:task:didSendBodyData:totalBytesSent:totalBytesExpectedToSend: delegate method to obtain upload progress information.

Additionally, if your app provides the request body using a stream, it must provide a custom session delegate that implements the URLSession:task:needNewBodyStream: method, described in more detail in Uploading Body Content Using a Stream.

Uploading Body Content Using an NSData Object

To upload body content with an NSData object, your app calls either the uploadTaskWithRequest:fromData: or uploadTaskWithRequest:fromData:completionHandler: method to create an upload task, and provides request body data through the fromData parameter.

The session object computes the Content-Length header based on the size of the data object.

Your app must provide any additional header information that the server might require—content type, for example—as part of the URL request object.

Uploading Body Content Using a File

To upload body content from a file, your app calls either the uploadTaskWithRequest:fromFile: or uploadTaskWithRequest:fromFile:completionHandler: method to create an upload task, and provides a file URL from which the task reads the body content.

The session object computes the Content-Length header based on the size of the data object. If your app does not provide a value for the Content-Type header, the session also provides one.

Your app can provide any additional header information that the server might require as part of the URL request object.

Uploading Body Content Using a Stream

To upload body content using a stream, your app calls the uploadTaskWithStreamedRequest: method to create an upload task. Your app provides a request object with an associated stream from which the task reads the body content.

Your app must provide any additional header information that the server might require—content type and length, for example—as part of the URL request object.

In addition, because the session cannot necessarily rewind the provided stream to re-read data, your app is responsible for providing a new stream in the event that the session must retry a request (for example, if authentication fails). To do this, your app provides a URLSession:task:needNewBodyStream: method. When that method is called, your app should perform whatever actions are needed to obtain or create a new body stream, and then call the provided completion handler block with the new stream.

Uploading a File Using a Download Task

To upload body content for a download task, your app must provide either an NSData object or a body stream as part of the NSURLRequest object provided when it creates the download request.

If you provide the data using a stream, your app must provide a URLSession:task:needNewBodyStream: delegate method to provide a new body stream in the event of an authentication failure. This method is described further in Uploading Body Content Using a Stream.

The download task behaves just like a data task except for the way in which the data is returned to your app.

Handling Authentication and Custom TLS Chain Validation

If the remote server returns a status code that indicates authentication is required and if that authentication requires a connection-level challenge (such as an SSL client certificate), NSURLSession calls an authentication challenge delegate method.

When authentication fails for a task that has a stream-based upload body, the task cannot necessarily rewind and reuse that stream safely. Instead, the NSURLSession object calls the delegate’s URLSession:task:needNewBodyStream: delegate method to obtain a new NSInputStream object that provides the body data for the new request. (The session object does not make this call if the task’s upload body is provided from a file or an NSData object.)

For more information about writing an authentication delegate method for NSURLSession, read Authentication Challenges and TLS Chain Validation.

Handling iOS Background Activity

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.

Listing 1-8 and Listing 1-9 show examples of these session and app delegate methods, respectively.

Listing 1-8  Session delegate methods for iOS background downloads

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session
    NSLog(@"Background URL session %@ finished events.\n", session);
    if (session.configuration.identifier)
        [self callCompletionHandlerForSession: session.configuration.identifier];
- (void) addCompletionHandler: (CompletionHandlerType) handler forSession: (NSString *)identifier
    if ([ self.completionHandlerDictionary objectForKey: identifier]) {
        NSLog(@"Error: Got multiple handlers for a single session identifier.  This should not happen.\n");
    [ self.completionHandlerDictionary setObject:handler forKey: identifier];
- (void) callCompletionHandlerForSession: (NSString *)identifier
    CompletionHandlerType handler = [self.completionHandlerDictionary objectForKey: identifier];
    if (handler) {
        [self.completionHandlerDictionary removeObjectForKey: identifier];
        NSLog(@"Calling completion handler.\n");

Listing 1-9  App delegate methods for iOS background downloads

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler
    NSURLSessionConfiguration *backgroundConfigObject = [NSURLSessionConfiguration backgroundSessionConfiguration: identifier];
    NSURLSession *backgroundSession = [NSURLSession sessionWithConfiguration: backgroundConfigObject delegate: self.mySessionDelegate delegateQueue: [NSOperationQueue mainQueue]];
    NSLog(@"Rejoining session %@\n", identifier);
    [ self.mySessionDelegate addCompletionHandler: completionHandler forSession: identifier];