What is the purpose of `returningItems` in `NSExtensionContext.completeRequest`?

Question

I'm working on a standard Action Extension in my iOS app and the Xcode template contains the lines:

@IBAction func done() {
    // Return any edited content to the host app.
    // This template doesn't do anything, so we just echo the passed in items.
    self.extensionContext!.completeRequest(returningItems: self.extensionContext!.inputItems, completionHandler: nil)
}

Here is the documentation for completeRequest.

Questions

• What is the purpose of passing returningItems to completeRequest?
• Do apps actually receive edited content from an action extension?
• If yes, where can I find the API on the receiving end?
• What are the consequences for me to pass an empty array?

Answer 1

I made small research for other question, that may be found helpful here.

Shortly:

There is callback in UIActivityViewController with parameter:

returnedItems - An array of NSExtensionItem objects containing any modified data. Use the items in this array to get any changes made to the original data by an extension