Objective C: get location without delegate - ios

Question

I have a problem in Objective C (writing an app for the iPhone). I know how to use the locationManager and its delegates. Once I called startUpdatingLocation on the locationManager, it will call the delegate method didUpdateToLocation whenever the location is updated. But the problem is, that this way is now suitable for what I want to do. I have a method (in a class) which looks as follows:
#implementation SomeClass
- (someType)getDbContentsOrderByDistance
{
... //here I need the current location
return dbContents;
}
#end
Now this function selects entries from a sqlite database ordered by their distance to the users current location. That is why I need the location in this function. What I want to do in the above method is: get the location, then select the stuff from the database and return it. I know that I could just try to access locationManager.location.coordinate.longitude, but I know that there won't be anything useful inside until the first location update has arrived. And I don't want to start the location updates before (e.g. when starting the app) because that would not be very efficient if I only need the location once.
I know how to do it the way that the delegate is called as soon as a location update arrives. The problem is that I don't know how to get that updated location from there into my method above. Basically I would need to let the method 'wait' until there is the first location update and then continue execution with that location.
Thank you for your help!

Answer 1

The short answer is, you can't. Getting the users location is an asynchronous process, and MUST be an asynchronous process. The device has to fire up various hardware like the GPS, cell tower locator, and WiFi triangulation system, get input from those different devices, and synthesize that into a location. That takes multiple seconds to do.
Putz's suggestion of starting a timer is a good one. I would add a few things however.
Typically the first readings you get from the location manager are really bad and should be discarded. The first reading you get is usually the last location reading when the GPS was active, and will have an out-of-date timestamp. The accuracy reading in that location might appear quite good, but it's a lie. I have seen the first location reading be off by several kilometers. You need to check the timestamp on your location updates and discard any reading that is more than 1 second old.
Once you've discarded the stale readings, the first several location updates are often really bad because the GPS hasn't settled down yet. The horizontal accuracy reading (which is really a "circle of confusion", or radius of possible positions) is an absurdly large value, sometimes a kilometer or more. Again, you need to write your location manager delegate method to discard readings that are too inaccurate. I suggest discarding values with a horizontal accuracy reading of >= 100 meters. How much inaccuracy you can tolerate depends on the specific appellation, but beware of making the accuracy requirement too accurate. Sometimes the GPS refuses to settle down. If you require a 5 meter accuracy, you might not get an acceptable accuracy during the entire run of your app. (If you're in an urban environment, a building, or other area where there is a lot of interference/obstruction of GPS signals)
Once you finally do get a value that's accurate enough, save the location and set your locationAvailable flag to YES.
Note that instead of a timer you could use the notification manager to broadcast a "got a good location" message. Any object that needs location information could register for your notification, and get called when the location manager gets an acceptable location reading.

Answer 2

Start a timer that fires every second or so that calls the function you need the location in. check to see if the location has been set. If the location is valid then kill the timer, if not then let the timer continue and check again in one second or so.
The way of knowing if the location is valid is in the delegate method. Set a global variable like locationAvailable = false. Then when the delegate method gets called set that variable to true.

Answer 3

Using location property can solve the purpose
From apple documentation
location The most recently retrieved user location. (read-only)
#property(readonly, nonatomic, copy) CLLocation *location
The value of this property is nil if no location data has ever been retrieved.
In iOS 4.0 and later, this property may contain a more recent location object at launch time. Specifically, if significant location updates are running and your app is terminated, this property is updated with the most recent location data when your app is relaunched (and you create a new location manager object). This location data may be more recent than the last location event processed by your app.
It is always a good idea to check the timestamp of the location stored in this property. If the receiver is currently gathering location data, but the minimum distance filter is large, the returned location might be relatively old. If it is, you can stop the receiver and start it again to force an update.
Whenever I create a new instance of location manager I get the most recent value of location in location property and since my distance filter is not set , it has the current timestamp. This means I dont have to call startUpdatingLocation and my data is accurate. I also need this for calculating distance between user location and a place so I can immediately return true or false if it is within the range. I find this very useful as I get location within 0.006 seconds and handy and got it from apple documentation only but still I dont know if it is a best practice.