UIActivityViewController completion handler still calls action if user presses cancel

Note: the completionHandler property is deprecated in iOS8, so it's not possible anymore to know the result of a share action. https://developer.apple.com/documentation/uikit/uiactivityviewcontroller/1622010-completionhandler

Update: Like adruzh said, on iOS8 there's a new completionHandler that Apple forgot to mention in the documentation:

[activityController setCompletionWithItemsHandler:
    ^(NSString *activityType, BOOL completed, NSArray *returnedItems, NSError *activityError) {
}];

https://developer.apple.com/documentation/uikit/uiactivityviewcontroller/1622022-completionwithitemshandler

That's what the completed argument is for:

[controller setCompletionHandler:^(NSString *activityType, BOOL completed) {
    if (!completed) return;

    CWStatusBarNotification *notification = [CWStatusBarNotification new];
    [notification displayNotificationWithMessage:@"✓ Successfully Shared Centre!"
                                     forDuration:3.0f];

    notification.notificationLabelBackgroundColor = [UIColor colorWithRed:38.0f/255.0f green:81.0f/255.0f blue:123.0f/255.0f alpha:1.0f];
    notification.notificationLabelTextColor = [UIColor whiteColor];
}];

For Swift, this is what worked for us:

    ...

    // Configure UIActivityViewController
    let activityViewController = UIActivityViewController(activityItems: objectsToShare, applicationActivities: nil)
    activityViewController.excludedActivityTypes = [UIActivityTypeAirDrop,
        UIActivityTypeAddToReadingList,
        UIActivityTypeAssignToContact,
        UIActivityTypePrint,
        UIActivityTypeCopyToPasteboard]

    // Show UIActivityViewController
    presentViewController(activityViewController, animated: true, completion: nil)

    // Define completion handler
    activityViewController.completionWithItemsHandler = doneSharingHandler

    ...

func doneSharingHandler(activityType: String!, completed: Bool, returnedItems: [AnyObject]!, error: NSError!) {
    // Return if cancelled
    if (!completed) {
        return
    }

    // If here, log which activity occurred
    println("Shared video activity: \(activityType)")
}