问按钮不能随意移动

EN

    -(IBAction)randomRed:(id)sender
{

    [self redDot];

    CGRect senderFrame = [sender frame];
    CGRect superBounds = [[sender superview ] bounds];
    senderFrame.origin.x = (superBounds.size.width - senderFrame.size.width) * drand48();
    senderFrame.origin.y = (superBounds.size.height - senderFrame.size.height) * drand48();
    [sender setFrame:senderFrame];

    counter = counter-1;

    [self performSelector:@selector(showRedDot) withObject:nil afterDelay:1.0];   
}

-(void)startRedDot
{

    if (counter ==0)
        redDot = [NSTimer scheduledTimerWithTimeInterval:4.0
                                                  target:self
                                                selector:@selector(showRedDot)
                                                userInfo:nil
                                                 repeats:YES];

}

-(void)showRedDot
{
    {
        [UIView beginAnimations:nil context:NULL];
        [UIView setAnimationDuration:1.0];
        [redButton setAlpha:1];
        [UIView commitAnimations];

        [self performSelector:@selector(redDot) withObject:nil afterDelay:1.0];
    }
}
-(void)redDot
{
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.5];
    [redButton setAlpha:0];
    [UIView commitAnimations];
}