A potentially dumb questions about swift (and dateFormatters)

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Created Jan ’21

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I seem to recall hearing that DateFormatters are (or were) expensive to instantiate.
With this in mind, I tried a small experiment with the following code:

Code Block class MyClass {
    static let df = DrawingCellView.dateFormatter
    static var dateFormatter: DateFormatter {
        let result = DateFormatter()
        result.dateFormat = "yyyy-MM-dd"
        return result
    }
    func dateText() -> String {
        return MyClass.dateFormatter.string(from: Date())
    }

When I put a breakpoint in the dateFormatter code, it fires each time I use it. However if I instead use df, the breakpoint only fires once.
Does this make sense?
If so, is this the recommended way to construct a runOnce static constant declaration/assignment?
thanks!
Mike

Answered by OOPer in 655350022

Does this make sense?

YES.

In your code, df is a stored property. So, the initial value of the declaration (DrawingCellView.dateFormatter) is called only once at the first time when df is used.

On the other hand, dateFormatter is a computed property. You see there is no = between dateFormatter: DateFormatter and {. The body of a computed property is called at each time the property is used.

If so, is this the recommended way to construct a runOnce static constant declaration/assignment?

I would declare the dateFormatter as follows and use it:

Code Block class MyClass {
    static let dateFormatter: DateFormatter = { //<- There is `=` here
        let result = DateFormatter()
        result.dateFormat = "yyyy-MM-dd"
        result.locale = Locale(identifier: "en_US_POSIX")
        return result
    }() //<- And there is `()`
    
    func dateText() -> String {
        return MyClass.dateFormatter.string(from: Date())
    }
    //...
}

With this declaration, dateFormatter becomes a stored property and the initial value ({ ... }()) is called only once.

(You should better set locale of the DateFormatter when you use it with fixed format in dateFormat.)

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Answer 1

OOPer OP

Jan ’21

Accepted Answer

Does this make sense?

YES.

In your code, df is a stored property. So, the initial value of the declaration (DrawingCellView.dateFormatter) is called only once at the first time when df is used.

On the other hand, dateFormatter is a computed property. You see there is no = between dateFormatter: DateFormatter and {. The body of a computed property is called at each time the property is used.

If so, is this the recommended way to construct a runOnce static constant declaration/assignment?

I would declare the dateFormatter as follows and use it:

Code Block class MyClass {
    static let dateFormatter: DateFormatter = { //<- There is `=` here
        let result = DateFormatter()
        result.dateFormat = "yyyy-MM-dd"
        result.locale = Locale(identifier: "en_US_POSIX")
        return result
    }() //<- And there is `()`
    
    func dateText() -> String {
        return MyClass.dateFormatter.string(from: Date())
    }
    //...
}

With this declaration, dateFormatter becomes a stored property and the initial value ({ ... }()) is called only once.

(You should better set locale of the DateFormatter when you use it with fixed format in dateFormat.)

0

Answer 2

mikeTheDad OP

Jan ’21

Thanks OOPer, that is exactly the answer I was looking for.
(and fair point about the 'lazy' configuration of the DateFormatter :-)

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