Question

I’m trying to create an UIView and change its alpha property to simulate backlight change.
Here is my code

    TransparentView = [[UIView alloc] initWithFrame:self.view.bounds];
    TransparentView.backgroundColor = [UIColor whiteColor];
    self.view = TransparentView;

    TransparentView.alpha = 0.2; 
    float step = 1.0 / ( appDelegate.OnOffTime * 100);
    for (float f = 0; f < 1; f=f+step) {
        TransparentView.alpha = (CGFloat)(1 - f);
        [NSThread sleepForTimeInterval:0.01];
}

Both TransparentView.alpha = 0.2 and TransparentView.alpha = (CGFloat)(1 - f) do change TransparentView.alpha, but only TransparentView.alpha = 0.2 changes real device “brightness’.
What am I doing wrong?

Answer 1

I think the issue is that you are sleeping the thread, and not actually setting up an asynchronous timer. What this means is, you actually are not letting the run loop get around to making your changes on screen, because you are not exiting this loop until it’s back up to 1 🙂

EDIT: to clarify:

Any change you make to your UI will not actually get drawn until the run loop “comes back around”, which requires your method to return. You likely want an NSTimer instance, configured to call a method on this object which increments the alpha (and checks the alpha against your destination value; invalidating the timer when it reaches it.