Download and Upload Task - Networking with URLSession on iOS 11 / Xcode 9 / Swift 4

Learn about URLSessionDownloadTask and URLSessionUploadTask to save results to a file, or upload a data object or a file.

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From Apple Developer Documentation (https://developer.apple.com/documenta...

URLSession is an object that coordinates a group of related network data transfer tasks.

The URLSession class and related classes provide an API for downloading content. 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 isn’t running or, in iOS, while your app is suspended.

The URLSession class natively supports the data, file, ftp, http, and https URL schemes, with transparent support for proxy servers and SOCKS gateways, as configured in the user’s system preferences.

URLSession supports the HTTP/1.1, SPDY, and HTTP/2 protocols. HTTP/2 support is described by RFC 7540, and requires a server supporting either ALPN or NPN for protocol negotiation.

You can also add support for your own custom networking protocols and URL schemes (for your app’s private use) using URLProtocol.

With the URLSession API, your app creates one or more sessions, each of which coordinates a group of related data transfer tasks. For example, if you’re creating a web browser, your app might create one session per tab or window, or one session for interactive use and another for background downloads. Within each session, your app adds a series of tasks, each of which represents a request for a specific URL (following HTTP redirects, if necessary).

The tasks within a given URL session share a common session configuration object, which defines connection behavior, such as the maximum number of simultaneous connections to make to a single host, whether to allow connections over a cellular network, and so on. The behavior of a session is determined in part by which method you call when creating its configuration object:

The singleton shared session (which has no configuration object) is for basic requests. It’s not as customizable as sessions that you create, but it serves as a good starting point if you have very limited requirements. You access this session by calling the shared class method. See that method’s discussion for more information about its limitations.

Default sessions behave much like the shared session (unless you customize them further), but let you obtain data incrementally using a delegate. You can create a default session configuration by calling the default method on the URLSessionConfiguration class.

Ephemeral sessions are similar to default sessions, but they don’t write caches, cookies, or credentials to disk. You can create an ephemeral session configuration by calling the ephemeral method on the URLSessionConfiguration class.

Background sessions let you perform uploads and downloads of content in the background while your app isn’t running. You can create a background session configuration by calling the backgroundSessionConfiguration(_:) method on the URLSessionConfiguration class.

The session configuration object also contains a reference to URL cache and cookie storage objects that may be used when making requests and handling the responses, depending on the configuration and request type.

The tasks in a session also share a common delegate that lets you provide and obtain information when various events occur—when authentication fails, when data arrives from the server, when data is ready to be cached, and so on. Using a URL Session has a step-by-step list of events that occur when a session is performing a task, and which delegate methods are called as a result.

On the other hand, if you don’t need any of the features provided by a delegate, you can use this API without providing one by passing nil when you create a session.