CALCULATE ONCE

Let's say it's sunrise/sunset.  The easiest way is keep track of the last time it was calculated, and only do it again if the day has chaned.  you can get the day with Gregorian.info.

var riseSetDay=99;

var sunrise,sunset;



function onUpdate(dc) {
    //get today from Gregorain.info()
    if(riseSetDay!=today) {
        //calcuate sunrise and sunset
        riseSetDay=today;
    }
    //other stuff
}

thanks, 

my problem is this: i got data from 

x = App.getApp().getProperty(y);

and then i want to make a operation like this

var z = (y*4); wherever

but i want to optimize memory and do this only once so, where do i should do that operation? or should the operation be hidden const or something like that

on function initialize() also calculates once per second???

Let's start with you shouldn't be doing the getProperty in onUpdate.  Put it in your view's initialize and store it in a class variable, so you can just use it in onUpdate.  initialize() for the view is only run once (when your app starts).

Also (Garmin plug here!), in the 4.1.0-beta1 SDK, there is a brand new profiler that you can use in the sim for any device to get a good handle on your code, what may be called too often, what takes a long time to run, etc..

i´m using getProperty on initialize, so if i do maths on initialize only runs once?

If you do it in initialize, yes.

For things like this, adding a System.println() call can help you see what's called and when.