Is there an easy way to display a sorted dropdown list on the iPhone by "closest to current location"?
I.e. if I have a list of cities with their GPS locations, can I quickly sort a dropdown to show you city selections in order of closest to you?
Will this be fast? Or will I have to presort (before you select the dropdown) in the background?
Seems like you'd want to do this in the background, if you can. After all, the majority of the time, it's going to be in the same order (unless the cities are really close together, and you travel a lot, I suppose).
If you do need to sort them at display time, you can probably sort through a few thousand points in no time at all. For many applications, treating the lattitude & longitude as rectangular coordinates is probably good enough. Calculating the actual distance isn't all that hard, but it'll be many times more computation than the sorting.
Oh, and if you've got your data points in an NSArray, look at sortedArrayUsingSelector: and sortedArrayUsingFunction:context:
Depends on the number of cities you have in your database. If it's a lot then you may want to use pre-pruning techniques to limit the search scope, like using pre-calculated 'zones' or limit search to 'cities of size X or larger,' etc. Since the cities don't move and populations and sizes don't change suddenly, think about how you can do as much pre-calculation as possible.
Another common technique is to have a separate 'nearby' table with distances from one city to those nearby. So all you have to do is find a nearby city to your current position and just show the cities that are nearest to that city.
Another trick worth emulating is that of UITableView, where it pulls info on-demand so at any given time, there are only the number of visible cells that need to be calculated. So instead of getting data on every city that is nearby, just try to limit the view to the 5-6 that are visible on the first screen and if you're allowing scrolling, fetch the rest as the user navigates up and down.
On a related note, here's a list of formulae for calculating distances between geopoints: http://www.movable-type.co.uk/scripts/latlong.html
You can sort an array using one of the sortedArrayUsing... messages to your array of locations.
Assuming each of your locations is represented using a CLLocation, you can use CLLocation's 'getDistanceFrom:' with the current location as the argument as part of the sort.