Define a static variable in Swift

Question

I want to define a static variable to hold an NSDate object in Swift 2.2. I've tried the following:

static var interval:NSDate
var static interval:NSDate
var interval:static NSDate

None are working. I've written the same thing in Objective-C with the normal:

static NSString* str

But just isn't working in Swift.

Let me be more clear, I want to use a static interval in the didUpdateLocation method for locationManager so that a function will only happen after five minutes, but the user's location will still be seen in real time. This is how I did it in Objective C

-(void) locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray<CLLocation *> *)locations
{
static NSDate *previous; //<--- This in Swift
static int run = 0;

CLLocation *location = locations.lastObject;

if (run == 0 || [location.timestamp timeIntervalSinceDate:previous] > 10)
{
    [_dbObj insert: location.coordinate.latitude : location.coordinate.longitude : [NSDate date] : location.timestamp];

    previous = location.timestamp;

    run++;
}

MKCoordinateRegion viewRegion = MKCoordinateRegionMakeWithDistance(location.coordinate, 2*1609.34, 2*1609.34);

[[self mapView] setRegion:viewRegion animated:YES];

}

From what I gather, I would need to create a class tat contains this static NSDate variable and instantiate it in the didUpdateLocations method. Yes? Thanks!

Answer 1

In C,

static <type> <name>;

at the file level defines a variable name whose scope is restricted to the same compilation unit (roughly speaking: to the same source file), and the same applies to your Objective-C code

static NSString* str;

In Swift, you achieve the same effect with a private property defined on the file level:

private var date = NSDate()

date is visible only in the same source file, compare Access Control in the Swift blog.

---

Function scoped static variables are possible in Swift as well, but you have to put them inside a struct. Example:

func foo() -> Int {
    struct StaticVars {
        static var counter = 0
    }
    StaticVars.counter += 1
    return StaticVars.counter
}

print(foo()) // 1
print(foo()) // 2

Answer 2

There are two problems here:

• static or class members are allowed only inside a class, and
• static members need to be initialized (or be declared optional to be initialized with nil).

Example:

class Foo {
    static var dt : NSDate = NSDate()
    static var optDt : NSDate? = nil
}

Note that the first restriction is not present in Objective-C: you are allowed to have free-standing static variables, in addition to function-scoped static variables.