iOS Detection of Screenshot?

Question

The app Snapchat, on the App Store, is an app that lets you share pictures with a self-destruct on them. You can only view the pics for X seconds. If you attempt to take a screenshot while the picture is showing using the home-power key combo, it will tell the sender you tried to take a screenshot.

What part of the SDK lets you detect that the user is taking a screenshot? I did not know this was possible.

Answer 1

As of iOS 7 the other answers are no longer true. Apple has made it so touchesCancelled:withEvent: is no longer called when the user takes a screenshot.

This would effectively break Snapchat entirely, so a couple betas in a new solution was added. Now, the solution is as simple as using NSNotificationCenter to add an observer to UIApplicationUserDidTakeScreenshotNotification.

Here's an example:

Objective C

NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
[[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationUserDidTakeScreenshotNotification
                                                  object:nil
                                                   queue:mainQueue
                                              usingBlock:^(NSNotification *note) {
                                                 // executes after screenshot
                                              }];

Swift

NotificationCenter.default.addObserver(
    forName: UIApplication.userDidTakeScreenshotNotification,
    object: nil,
    queue: .main) { notification in
        //executes after screenshot
}