CLLocation, getting new locations - iphone

Question

In More iPhone Programming book, the author does:
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
if ([newLocation.timestamp timeIntervalSince1970] < [NSDate timeIntervalSinceReferenceDate] - 60)
locationCoordinate = newLocation.coordinate;
return;
...
To make sure the data was taken in the last minute. Two questions:
1) What is the if statement doing. It seems like on the left hand side, you are getting the time difference in seconds between when this method fires and the 1970 date. Then on the right hand side, you get the difference in seconds between the 2001 date and now minus 60 seconds. So to me, the if statement would never be valid since the data on the left is always going to be a greater amount of seconds. Or am I understanding it wrong?
2) What does return in a void function do? Is that considered good coding here? Thx.

Answer 1

I don't know what is going on here, its confusing statement, I have use something like this
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation{
NSDate* eventDate = newLocation.timestamp;
NSTimeInterval howRecent = [eventDate timeIntervalSinceNow];
if( abs(howRecent) > 1.0 )
return;
////process your event here
}
It is right to put return in void method, as I don't want to execute the statement next for some conditions..the same code can be written as
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation{
NSDate* eventDate = newLocation.timestamp;
NSTimeInterval howRecent = [eventDate timeIntervalSinceNow];
if( abs(howRecent) < 1.0 ){
///process your event
}
}
it just depends upon your need.

Answer 2

for the if block :
the value return by timeIntervalSinceReferenceDate could be negative. see the description .
If the receiver is earlier than the
reference date, the value is negative.
So if condition could be false.
For the return statement.
if you want your function to return the control to the calling function before reaching the function closed bracket on some condition. See the eg.
-(void) SomeFunction
{
if(Condition1)
{
return;
}
}