NSURLSession

NSURLSession is a high-level API in Apple’s Foundation framework for network data transfer. Introduced as a replacement for NSURLConnection, it provides a flexible, task-based model for HTTP and other URL requests on iOS, macOS, watchOS and tvOS.

A session is created from an NSURLSessionConfiguration (default, ephemeral, or background) and manages one or more NSURLSessionTask objects. Task types include data tasks for in-memory responses, download tasks that write to disk and support resume data, upload tasks for sending files or data, and stream tasks for bidirectional streams. Sessions may be used with completion-handler callbacks, a delegate implementing NSURLSessionDelegate and related protocols, or a mixture of both. Delegate callbacks supply finer-grained control over authentication, redirection, progress, background events and metrics.

Key features include background transfers that continue when an app suspends (handled by system relaunch and app delegate callbacks), cookie and cache policies, TLS/HTTPS support, HTTP/2 multiplexing, and task-level progress and metrics. Sessions use operation queues for delegate callbacks and are designed for concurrency and reuse; tasks can be suspended, resumed, or cancelled.

Common usage patterns create a shared or custom session, configure caching and timeouts, start tasks, and handle results in completion handlers or delegate methods. NSURLSession remains the recommended networking API for Apple platforms for efficient, secure and extensible URL loading.