CODEWITHSUNDEEP
swiftdatetimensdatedefault-value
Yaron Levi
Yaron Levi

Reputation: 13079

Swift - How to initialize a default value for NSDate property?

I have a simple class definition: 

class Course{

    var date:NSDate

    init(){

    }
}

In c#, for example, when creating this class, the date property will have the default of DateTime.MinValue.

How can I do the same in swift?

Upvotes: 3

Views: 12818

Answers (3)

Rob
Rob

Reputation: 437592

If you always initialize date in the init method (which, if, as you say, it must have a date), then the following is fine:

class Course {    
    var date: Date          // if Swift 2 we’d use `NSDate`, but in Swift 3 and later, we’d use `Date`

    init() {
        date = ... // set it to some reasonable, default value
    }
}

Or, if there is no reasonable, default value, you'd specify an initializer where you supply the value:

class Course {    
    var date: Date

    init(date: Date) {
        self.date = date
    }
}

If, however, there are reasons why you cannot specify the value when the object is instantiated and can only set date at some later point, but it should always have a value from that point forward, you would use an implicitly unwrapped optional. As the documentation for implicitly unwrapped optionals says:

Sometimes it is clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it is useful to remove the need to check and unwrap the optional’s value every time it is accessed, because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.

Thus, you would have

class Course {
    var date: Date!

    init() {
        // the `date` is not necessarily set here, but once it is set, it shouldn't be `nil` thereafter
    }
}

Yes, technically, with implicitly unwrapped optional, date could be nil, but the practice is that if it might initially be nil, but once set, it will never be nil again, you would use implicitly wrapped optional.

Upvotes: 0

Matthias Bauch
Matthias Bauch

Reputation: 90117

If all Courses need valid dates, why do you want to create Courses that don't have a real date? Get rid of init() and create a designated initializer. 

class Course{
    let date: NSDate

    init(date: NSDate) {
        self.date = date
    }
}

If you insist on having these weird "not real dates" you can use distantPast(). 

class Course{
    var date: NSDate

    init() {
        self.date = NSDate.distantPast() as NSDate
    }
}

Upvotes: 10

Erik
Erik

Reputation: 12858

You can create an extension to NSDate like so:

extension NSDate
{
    class func minimumDate() -> NSDate
    {
        let calendar = NSCalendar(calendarIdentifier: NSGregorianCalendar)!
        return calendar.dateWithEra(1, year: 1, month: 1, day: 1, hour: 0, minute: 0, second: 0, nanosecond: 0)!
    }
}

This will allow you to instantiate a new NSDate with the similar 1/1/0001 00:00:00 timestamp:

let myDate = NSDate.minimumDate()

Swift also has a null-coalesce operator (??) which allows you do to something like this in your initializer:

class Widget
{
    var dateCreated : NSDate

    init(dateCreated: NSDate?)
    {
        self.dateCreated = dateCreated ?? NSDate.minimumDate()
    }
}

Upvotes: 1

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