Thanks, Graham, I realized the timer would have jitter. When I realized what
was going on and began to think about how to fix it, that is when I had one of
those head-slap moments where I asked, "Why am I not using CoreAnimation for
this?"
-Michael
On Mar 11, 2010, at 4:53 PM, Graham Cox wrote:
>
> On 12/03/2010, at 5:23 AM, Michael A. Crawford wrote:
>
>> - (void)timerFireMethod:(NSTimer*)theTimer
>> {
>> #if 0
>> // Here I was using a periodic timer to animate the scroll. I noticed that
>> // the animation wasn't smooth and then remembered that CA is supposed to
>> do
>> // the animating for me. So, I switched to trying the code below but that
>> // doesn't work either. I'm really just grasping at straws here.
>> static CGPoint origin = {0.0f, 0.0f};
>> origin.x += 5.0f;
>> [scrollLayer scrollToPoint:origin];
>> #else
>
>
> Hi Michael,
>
> This is a classic "naive" mistake. You're incrementing the position by a
> fixed amount each time the timer fires. Problem is, you can't guarantee that
> the timer will fire exactly at the time it should, so your scrolling speed is
> at the mercy of how busy things are, so will speed up and slow down.
>
> Recall that speed is distance/time, so if you want a constant speed, you have
> to work out how much distance the thing should have moved in the actual time
> interval you got.
>
> Roughly (typed into mail):
>
> - (void) timerFireMethod:(NSTimer*) theTimer
> {
> NSTimeInterval elapsedTime = [NSDate timeIntervalSinceReferenceDate] -
> m_startTime; // m_startTime ivar set when the animation began
> CGFloat distance = m_speed * elapsedTime; // m_speed is the scrolling
> speed in points per second
>
> [thing setPosition:distance];
> }
>
> With this approach, the exact timing intervals don't matter - the position
> will be correct. If things get busy what will happen is that the jumps
> between positions will get a bit larger.
>
> That said, Core Animation might do the job better, but I just wanted to point
> out what the problem was with your original approach.
>
> --Graham
>
>
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