Retain/release with ARC is slowing me down

Question

I'm using ARC in my iOS app. I have profiled the method below and even though the algorithm is terribly naive and wasteful 77% of the time is spent in objc_retain and objc_release. I think it must be down to the line where i fetch a Unit from the NSArray and ARC is careful to retain and then release the object each time.

I'm looking for an informed advice: how do I fix it elegantly?

-(CGFloat)getUncertaintyForUnits:(NSArray*)units Position:(MKMapPoint)position Zoom:(MKZoomScale)zoomScale {

    CGFloat closest = MAXFLOAT; 

    for (int i = 0; i < [units count]; i++) {
        Unit *units = (Unit*)[units objectAtIndex:i];

        CGFloat distance = [self distanceBetweenMapPoints:unit.mapPoint And:position];

        if (distance < closest) {
            closest = distance;
        }
    }

    CGFloat max = 100 / zoomScale;
    return  (1. - closest / max) * 0.9;
}

Answer 1

you could try fast enumeration:

for (Unit *unit in units) {
    CGFloat distance = [self distanceBetweenMapPoints:unit.mapPoint And:position];
    if (distance < closest) {
        closest = distance;
    }
}

this should avoid extra retains/releases, as in fast enumeration the whole array is blocked.

There are several advantages to using fast enumeration:

• The enumeration is considerably more efficient than, for example, using NSEnumerator directly.
• The syntax is concise.
• Enumeration is “safe”—the enumerator has a mutation guard so that if you attempt to modify the collection during enumeration, an exception is raised.