Question

I’m making a simple compass, and it works just fine but the animation could be smoother. At times it can be a bit jumpy, similar to a game with sub-par frame rate. Is there a way to make it animate more smoothly?

Here is my code:

- (void)viewDidLoad
{
[super viewDidLoad];

CLLocationManager* lm = [[CLLocationManager alloc] init];
self.locman = lm;
self.locman.delegate = self;
self.locman.headingFilter = 2;
[self.locman startUpdatingHeading];
}

- (void) locationManager:(CLLocationManager *)manager 
                didUpdateHeading:(CLHeading *)newHeading {
CGFloat h = newHeading.magneticHeading;
NSString* dir = @"N";
NSArray* cards = [NSArray arrayWithObjects: 
@"N", @"NE", @"E", @"SE", @"S", @"SW", @"W", @"NW", nil];
for (int i = 0; i < 8; i++)
    if (h < 45.0/2.0 + 45*i) {
        dir = [cards objectAtIndex: i]; break;
    }
needle.transform = CGAffineTransformMakeRotation(radianConst*-h);
direction = dir;
}

Answer 1

Your question is confusing because you mention games with poor frame rates, which leads me to wonder which of three problems you’re having:

• You’re getting so many didUpdateHeading: messages that you actually are having a performance problem (in which case some caching of the last value and calculation to determine if you really do need to update the view might be in order)

or

• CoreLocation is actually sending you data that “jumps” around and makes your display look jumpy

or

• You haven’t properly setup the implicit animation parameters for the implicit animation you want to have on the transform property

I’m guessing your problem is actually the second one I listed, and that you’re getting heading updates from core location which “jump” from location to location.

There are two approaches I’d take to this problem – one before the other (because one is easier and the other is harder).

First, I’d try to increase the transition duration slightly, or, at least set it manually so you know what it is… maybe 2 seconds?

[CATransaction begin];
[CATransaction setValue:[NSNumber numberWithFloat:2.0f]
                 forKey:kCATransactionAnimationDuration];

needle.transform = CGAffineTransformMakeRotation(radianConst*-h);

[CATransaction commit];

Failing that, you’ll have to some up with and apply some sort of smoothing algorithm… however, I believe I heard an apple developer mention they did this already in one of their Core Location videos.