Question

I’d like to create a flicker effect, similar to a fluorescent light turning on. The sequence will take around 2 seconds. As it’s on a black background (and the image is white) I could adjust the alpha in a sequence like this:

0.5sec   Alpha = 1.0;
0.15sec  Alpha = 0.0;
0.4sec   Alpha = 1.0;
0.35sec  Alpha = 0.0;
0.3sec   Alpha = 1.0;
0.1sec   Alpha = 0.0;
~        Alpha = 1.0;

At this point, the alpha is at 1.0 and id like to leave it there. The problem is that I cant just change the values, as they would just get queued up for drawing and wouldn’t happen immediately, instead they’d all happen at once next time the UI is drawn, leaving me with no flicker effect.

Neither can I use a UIView.animate block, as this will tween/animate the view’s alpha over the times specified, I dont want it smoothed out, I want it to instantly flicker between the alpha levels.

I could possibly use UIImageView image frame animation, but was wondering is there a cleaner, faster, easier method?

Answer 1

Use a NSTimer. This is what I would do (psuedo-code):

timer = new scheduledTimer(0.5 seconds) selector:flicker() repeats:YES;
timeSpent = 0;

    flicker()
    {
        timeSpent+= 5;
        switch(seconds)
             case 5: alpha = 1.0; break;
             case 15: alpha = 0.0; break;
    ...
        case 120: alpha = 1.0; [timer invalidate]; break;

    }

if you have some pattern to the flicker you can make something better than a long switch and maybe just have something like if (timeSpent % 5 == 0) { alpha = 1.0; } else { alpha = 0.0; }

EDIT: How about something like this:

CAKeyframeAnimation* animation = [CAKeyframeAnimation animationWithKeyPath: @"opacity"];
    animation.duration = 0.7f;
    animation.values   = [NSArray arrayWithObjects:
                          [NSNumber numberWithFloat:1.0f],
                          [NSNumber numberWithFloat:1.0f],
                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:1.0f],

                          [NSNumber numberWithFloat:1.0f],
                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:1.0f],
                          [NSNumber numberWithFloat:1.0f],

                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:0.0f],
                          [NSNumber numberWithFloat:1.0f],
                          nil];
    animation.keyTimes = [NSArray arrayWithObjects:
                          [NSNumber numberWithFloat:0.1f],
                          [NSNumber numberWithFloat:0.2f],
                          [NSNumber numberWithFloat:0.2f],
                          [NSNumber numberWithFloat:0.3f],
                          [NSNumber numberWithFloat:0.3f],

                          [NSNumber numberWithFloat:0.4f],
                          [NSNumber numberWithFloat:0.4f],
                          [NSNumber numberWithFloat:0.5f],
                          [NSNumber numberWithFloat:0.5f],
                          [NSNumber numberWithFloat:0.6f],

                          [NSNumber numberWithFloat:0.6f],
                          [NSNumber numberWithFloat:0.7f],
                          [NSNumber numberWithFloat:0.7f],
                          nil];    

    [self.view.layer addAnimation:animation forKey:nil];