Instance Method

downloadTask(withResumeData:completionHandler:)

Creates a download task to resume a previously canceled or failed download and calls a handler upon completion.

Declaration

func downloadTask(withResumeData resumeData: Data, completionHandler: @escaping (URL?, URLResponse?, Error?) -> Void) -> URLSessionDownloadTask

Parameters

resumeData

A data object that provides the data necessary to resume the download.

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(withResumeData:) 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 using a completion handler, the task bypasses calls to delegate methods for response and data delivery, and instead provides any resulting data, response, or error inside the completion handler. Delegate methods for handling authentication challenges, however, are still called.

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

Your app can obtain a resumeData object in two ways:

  • If your app cancels an existing transfer by calling cancel(byProducingResumeData:), the session object passes a resumeData object to the completion handler that you provided in that call.

  • If a transfer fails, the session object provides an NSError object either to its delegate or to the task’s completion handler. In that object, the NSURLSessionDownloadTaskResumeData key in the userInfo dictionary contains a resumeData object.

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

See Also

Adding Download Tasks to a Session

func downloadTask(with: URL) -> URLSessionDownloadTask

Creates a download task that retrieves the contents of the specified URL and saves the results to a file.

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

Creates a download task that retrieves the contents of the specified URL, saves the results to a file, and calls a handler upon completion.

func downloadTask(with: URLRequest) -> URLSessionDownloadTask

Creates a download task that retrieves the contents of a URL based on the specified URL request object and saves the results to a file.

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

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.

func downloadTask(withResumeData: Data) -> URLSessionDownloadTask

Creates a download task to resume a previously canceled or failed download.

class URLSessionDownloadTask

A URL session task that stores downloaded data to file.

protocol URLSessionDownloadDelegate

A protocol defining methods that URLSession instances call on their delegates to handle task-level events specific to download tasks.