NSURLConnection

An NSURLConnection object lets you load the contents of a URL by providing a URL request object. The interface for NSURLConnection is sparse, providing only the controls to start and cancel asynchronous loads of a URL request. You perform most of your configuration on the URL request object itself.

The NSURLConnection class provides convenience class methods to load URL requests both asynchronously using a callback block and synchronously.

For greater control, you can create a URL connection object with a delegate object that conforms to the NSURLConnectionDelegate and NSURLConnectionDataDelegate protocols. The connection calls methods on that delegate to provide you with progress and status as the URL request is loaded asynchronously. The connection also calls delegate methods to let you override the connection’s default behavior (for example, specifying how a particular redirect should be handled). These delegate methods are called on the thread that initiated the asynchronous load operation.

For more information about errors, see the NSURLError.h header, Foundation Constants, and URL Loading System Error Codes in Error Handling Programming Guide.

The NSURLConnection class works in tandem with three formal protocols: NSURLConnectionDelegate, NSURLConnectionDataDelegate, and NSURLConnectionDownloadDelegate. To use these protocols, you write a class that conforms to them and implement any methods that are appropriate, then provide an instance of that class as the delegate when you create a connection object.

The NSURLConnectionDelegate protocol is primarily used for credential handling, but also handles connection completion. Because it handles connection failure during data transfers, all connection delegates must typically implement this protocol.

In addition, unless you’re using Newsstand Kit, your delegate must also conform to the NSURLConnectionDataDelegate protocol, because this protocol provides methods that the NSURLConnection class calls with progress information during an upload, with fragments of the response data during a download, and to provide a new upload body stream if the server’s response necessitates a second connection attempt—for example, if NSURLConnection must retry the request with different credentials.

Finally, if you’re using Newsstand Kit, your delegate can conform to the NSURLConnectionDownloadDelegate protocol. This protocol provides support for continuing interrupted file downloads and receiving a notification whenever a download finishes. This protocol is solely for use with NSURLConnection objects created using Newsstand Kit’s download(with:) method.