Is it possible to respond to a shake gesture in UIAlertView? - ios

Question

I have a UIAlertView which pops up with a message and a single button "Continue". What I would like for is that a Shake Gesture would also call the same code as hitting the continue button. However, I'm not sure if this is possible with the UIAlertView in iOS7 or if I have to create my own custom alert view.
I have tried implementing:
(void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event { ... }
and
canBecomeFirstResponder { return YES; }
But while they work normally with the main view set to becomeFirstResponder they don't respond once the UIAlertView pops up. So then, I tried implementing these in a Category for UIAlertView+MotionResponder.h to try to extend UIAlertView, but the motionEnded is not getting called even though canBecomeFirstResponder is correctly returning YES.
So should this work? Or do I have to implement my own alert view?

Answer 1

According to the documentation
The UIAlertView class is intended to be used as-is and does not support subclassing. The view hierarchy for this class is private and must not be modified.
The internal view hierarchy of UIAlertView is much more complicated than it appears and starting from iOS 7 it is not even added to any UIWindow, so it's no big surprise that it doesn't participate to the responder chain as you expect.
However, you might consider of implementing the motion recognition logic within your controller instead and to have it triggering the UIAlertView dismissal.
You just need to keep a reference to the UIAlertView instance, detect the shake gesture in the controller and send a –dismissWithClickedButtonIndex:animated: message to the alert, in order to dismiss it.
EDIT
The above technique won't work when the UIAlertView is presented on top of the current controller since it will intercept the shake gesture and the controller method will never be called.
To make it work, you have to detect the gesture at a window level. Here's a brief example of how you can do.
First we define a category over UIWindow that intercepts the shake gesture and broadcasts a notification (I'll just post the #implementation, since the #interface is empty)
#implementation UIWindow (Shake)
- (void)motionEnded:(UIEventSubtype)motion withEvent:(UIEvent *)event {
if (event.type == UIEventTypeMotion && event.subtype == UIEventSubtypeMotionShake) {
[[NSNotificationCenter defaultCenter] postNotificationName:#"UIWindowDidShake" object:nil userInfo:nil];
}
}}
#end
then in the controller we register to the notification when showing the alert and we dismiss the alert when the notification arrives. Finally we can remove the controller from the observers as soon as the alert has been dismissed.
#property (nonatomic, strong) UIAlertView *alert;
...
- (IBAction)showAlert:(id)sender {
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(dismissAlert) name:#"UIWindowDidShake" object:nil];
self.alert = [[UIAlertView alloc] initWithTitle:nil message:#"An alert" delegate:self cancelButtonTitle:#"Ok" otherButtonTitles:nil];
[self.alert show];
}
- (void)dismissAlert {
[self.alert dismissWithClickedButtonIndex:0 animated:YES];
}
- (void)alertView:(UIAlertView *)alertView didDismissWithButtonIndex:(NSInteger)buttonIndex {
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"UIWindowDidShake" object:nil];
}