UIView Delta Time - objective-c

Question

I'm animating a UIView by updating its anchorpoint 60 times a second using an NSTimer.
The location of the UIView changes depending on its angle, so it always appears to be down relative to the device...
However, the NSTimer doesn't fire precisely 60 times a second. It's always a little off, causing jerky animation. I've searched this a lot, I know a bit about delta time, but I don't know how to apply it to my situation.
Here's the movement code I'm using:
float rotation = 0;
if (leftSideIsBeingHeldDown) {
rotation += (0.05f/rotationFactor);
} else if (rightSideIsBeingHeldDown) {
rotation -= (0.05f/rotationFactor);
}
movementX += -sinf(rotation);
movementY += -cosf(rotation);
float finalX = 0.0001 * movementX;
float finalY = 0.0001 * movementY;
mapView.layer.anchorPoint = CGPointMake(finalX, finalY);
mapView.transform = CGAffineTransformMakeRotation(rotation);
Does anyone know how to apply delta time to this?

Answer 1

You might want to look into the CADisplayLink class which provides you a timer that is tied to the display refresh rate. It should be a better solution than an NSTimer in this case.
Additionally, you need to remember the time of each "tick" and calculate the rotation or movement that should have been done since the last tick. For example (pseudo-code):
- (void)displayLinkTick:(id)sender
{
NSTimeInterval timespan;
NSDate *now;
now = [NSDate date];
if (myPreviousTick) {
timespan = [now timeintervalSinceDate:myPreviousTick];
} else {
// The very first tick.
timespan = 0;
}
// Calculate the angle according to the timespan. You need a
// value that specifies how many degrees/radians you want to
// revolve per second and simply multiply that with the timespan.
angle += myRadiansPerSecond * timespan;
// You'd do the same with the position. I guess this involves
// minor vector math which I don't remember right now and am
// too lazy to look up. You need to have a distance per second
// which you multiply with the timespan. Together with the
// direction vector you can calculate the new position.
// At the end, remember when this tick ran.
[myPreviousTick release];
myPreviousTick = [now retain];
}

Answer 2

You want to record the time you last rotated, and the difference in time between then and now, and use that to work out a factor, which you can use to adjust the rotation and x/y values.
for example:
NSDate now = [NSDate now];
timeDiff = now - lastRotateTime;
factor = timeDiff / expectedTimeDiff;
x = x + xIncrement * factor;
y = y + yIncrement * factor;
angle = angle + angleIncrement * factor;
There are many better examples on game dev forums, which explain it in more detail.