Instance Method

dataTask(with:completionHandler:)

Creates a task that retrieves the contents of a URL based on the specified URL request object, and calls a handler upon completion.

Declaration

func dataTask(with request: URLRequest, completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask

Parameters

request

A URL request 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 dataTask(with:) method.

This completion handler takes the following parameters:

data

The data returned by the server.

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 data 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.

By using the completion handler, the task bypasses calls to delegate methods for response and data delivery, and instead provides any resulting NSData, URLResponse, and NSError objects 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(_:dataTask:didReceive:) method.

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

If the request completes successfully, the data parameter of the completion handler block contains the resource data, and the error parameter is nil. If the request fails, the data 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 Data Tasks to a Session

func dataTask(with: URL) -> URLSessionDataTask

Creates a task that retrieves the contents of the specified URL.

func dataTask(with: URL, completionHandler: (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask

Creates a task that retrieves the contents of the specified URL, then calls a handler upon completion.

func dataTask(with: URLRequest) -> URLSessionDataTask

Creates a task that retrieves the contents of a URL based on the specified URL request object.

class URLSessionDataTask

A URL session task that returns downloaded data directly to the app in memory.

protocol URLSessionDataDelegate

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