Instance Method

downloadTask(with:completionHandler:)

Creates a download task that retrieves the contents of a URL based on the specified URL request object, saves the results to a file, and calls a handler upon completion. The task bypasses calls to delegate methods for response and data delivery, and instead provides any resulting NSURL, URLResponse, and NSError objects inside the completion handler. Delegate methods for handling authentication challenges, however, are still called.

Declaration

func downloadTask(with request: URLRequest, completionHandler: @escaping (URL?, URLResponse?, Error?) -> Void) -> URLSessionDownloadTask

Parameters

request

An NSURLRequest object that provides the URL, cache policy, request type, body data or body stream, and so on.

completionHandler

The completion handler to call when the load request is complete. This handler is executed on the delegate queue.

If you pass nil, only the session delegate methods are called when the task completes, making this method equivalent to the downloadTask(with:) method.

location

The location of a temporary file where the server’s response is stored. You must move this file or open it for reading before your completion handler returns. Otherwise, the file is deleted, and the data is lost.

response

An object that provides response metadata, such as HTTP headers and status code. If you are making an HTTP or HTTPS request, the returned object is actually an HTTPURLResponse object.

error

An error object that indicates why the request failed, or nil if the request was successful.

Return Value

The new session download task.

Discussion

By creating a task based on a request object, you can tune various aspects of the task’s behavior, including the cache policy and timeout interval.

After you create the task, you must start it by calling its resume() method.

You should pass a nil completion handler only when creating tasks in sessions whose delegates include a urlSession(_:downloadTask:didFinishDownloadingTo:) method.

If the request completes successfully, the location parameter of the completion handler block contains the location of the temporary file, and the error parameter is nil. If the request fails, the location parameter is nil and the error parameter contain information about the failure. If a response from the server is received, regardless of whether the request completes successfully or fails, the response parameter contains that information.