UIActivityViewControllerCompletionHandler How to?

typedef void (^UIActivityViewControllerCompletionHandler)
    (NSString *activityType, BOOL completed);

The second parameter of the completion handler tells you whether the user dismissed the controller. If they dismissed the controller, completed will be set to NO.

There's more details in the completion handler's documentation.

With iOS 8 completionHandler is deprecated, so you will want to provide alternative completionWithItemsHandler. It also allows to handle if extension modified data you set (say Photo extension). See Documentation

    UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:@[text, URL, image] applicationActivities:nil];
    activityViewController.excludedActivityTypes = @[UIActivityTypePrint, UIActivityTypeCopyToPasteboard, UIActivityTypeAssignToContact, UIActivityTypeAirDrop, UIActivityTypeAddToReadingList];

    // check if new API supported
    if ([activityViewController respondsToSelector:@selector(completionWithItemsHandler)]) {
        activityViewController.completionWithItemsHandler = ^(NSString *activityType, BOOL completed, NSArray *returnedItems, NSError *activityError) {
            // When completed flag is YES, user performed specific activity
        };
    } else {
        activityViewController.completionHandler = ^(NSString *activityType, BOOL completed) {
            // When completed flag is YES, user performed specific activity
        };
    }

Here's how you show a sharing dialog and set its completion handler

NSArray *imageArray = [NSArray arrayWithObject:shareImage];

UIActivityViewController *sharing = [[UIActivityViewController alloc] initWithActivityItems:imageArray applicationActivities:nil];

[sharing setCompletionHandler:^(NSString *activityType, BOOL completed) {
    NSLog(@"completed dialog - activity: %@ - finished flag: %d", activityType, completed);
}];

[self presentViewController:sharing animated:YES completion:nil];

Swift 5:

func showShareOf(url: URL, title: String) {
    let shareItems: [Any] = ["\(title)\n", url]
    let activityVC = UIActivityViewController(activityItems: shareItems, applicationActivities: nil)
    activityVC.completionWithItemsHandler = doneSharingHandler
    self.present(activityVC, animated: true, completion: nil)
}

@objc func doneSharingHandler(activityType: UIActivity.ActivityType?, completed: Bool, _ returnedItems: [Any]?, error: Error?) {
    if (!completed) { return }
    // If here, log which activity occurred
    print("Shared activity: \(activityType)")
}