How to store CLLocation using Core Data (iPhone)?

Question

Hi,

I'm trying to save a location and retrieve the location on a map afterward using Core Location, MapKit and Core Data frameworks.

What I've done is I just made entity named POI and added properties such as latitude (double type), longitude (double type) with few others.

Simply put, my app saves POI with two NSNumbers. (lat and long) but I feel like there must be a smarter way to store CLLocation than that.

cheers.

Answer 1

What you're doing is fine. You should save the latitude and longitude as doubles in Core Data. When you need to get the information again, get the floats back from Core Data and construct a CLLocationCoordinate2D struct with a function like CLLocationCoordinate2DMake. There's no built in way to store a location, so storing the latitude and longitude components is fine.

If you're not doing any math operations on the latitude or longitude (finding bounding boxes etc) you could store them as strings. Floats and doubles can change their values slightly, making comparison operations fail.

Answer 2

CLLocation implements the NSCoding protocol so you can store them in Core Data as transformable attributes. You can use the default NSKeyedUnarchiveFromData value transformer.

You would just pass the CCLocation object to the managed object attribute and it would serialize it to data and store it as a blob in the SQL store. When you need the location object back it would automatically reverse the process and return to you a fully populated CCLocation object.

That might be the easiest way to get what you want.

Answer 3

The "best" way to store it depends on what you want to do with it:

• If you want the "same" CLLocation, serialize it. NSKeyedUnarchiveFromData is fine.
• If you just want to search on latitude and longitude, then store those as doubles (and check the "indexed" checkbox).

There's a bunch of additional properties you can save/restore if you do it manually (altitude, horizontalAccuracy, verticalAccuracy, timestamp). There are some more that you can't (speed, heading); CLLocation doesn't provide a suitable init-method and the properties are readonly.

All of the extra properties are useful if you're recording a track. Altitude is useful if you're recording a POI on mountainous terrain ("we still have to climb 100 m"). horizontal/vertical accuracy might be used to represent how big the POI is (e.g. a city might have a "horizontal accuracy" of several km and be displayed as a big circle).