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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