use UIAlertView when camera is running in iPhone

I'm trying to trigger an AlertView when开发者_Python百科 my camera detect a face using OpenCV. I manage to do the face detection and can output an NSLog successfully. But when I tried to trigger the alert view with

NSLog(@"Face Detected");
UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Face Detected" message:@"Do you really  want to try again?" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:nil] autorelease];

[alert addButtonWithTitle:@"Yes"];

[alert show];
[alert release];

I can see the Alert View is kind of triggered as the screen is dimmed but I could never see the alert view came out...

Thanks for helping!

---

Remove [alert release]. You already called autorelease on it.

Also, you can integrate [alert addButtonWithTitle:@"Yes"]; in the initializer:

UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Face Detected" 
                                                message:@"Do you really  want to try again?" 
                                               delegate:self 
                                      cancelButtonTitle:@"Cancel" 
                                      otherButtonTitles:@"OK", nil] autorelease];

---

where are you calling this from ? Main thread or secondary ? Because UIKit stuff should always been done on main thread.

Code example:

- (void)opencvFaceDetect
{
  // stuff before
  [self performSelectorOnMainThread: @selector(openAlertView) withObject:nil waitUntilDone:false];
  // stuff after
}

and then

- (void)openAlertView
{
  UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Face Detected" 
                                                message:@"Do you really  want to try again?" 
                                               delegate:self 
                                      cancelButtonTitle:@"Cancel" 
                                      otherButtonTitles:@"OK", nil] autorelease];
}