CODEWITHSUNDEEP
swiftvariablesclosuresdeclaration
Yao
Yao

Reputation: 709

Swift variable declaration with closure

I saw a declaration which confuses me. ( the grammar here)

static var dateFormatter: NSDateFormatter = {
    var formatter = NSDateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"
    return formatter
}()

To declare a variable, it looks like it uses a function(initializer) to create one. Since I am not familiar with closure in Swift, I found some useful tutorial. However, the example above seems to not fit in any of them. ref: http://fuckingclosuresyntax.com/ Any suggestions, references, or tutorial will be appreciated.

Upvotes: 4

Views: 2982

Answers (1)

Bell App Lab
Bell App Lab

Reputation: 818

Syntax

As @findall said in the comments, you are basically creating a function and executing it.

To clarify the syntax, I'll try to create an example in JavaScript.

Consider this code snippet:

Example #1

//creates a global variable
var globalString = "Very important global string";

This string will be created and stored in memory as soon as this line of code is interpreted. Now compare it to this other implementation:

Example #2

//also creates a global variable 
var globalString = function() {
    return "Very important global string";
};

The second implementation does not create a string, but creates a function that ultimately produces a string.

In Swift, when you declare a variable with the {...}() syntax, you are actually doing something similar to Example #2.

Use case

When would it be useful to declare a variable in such way? When the declaration would require some extra setup to take place.

In the example posted in the question, an NSDateFormatter may need a few extra steps to be instantiated the way your app expects it to behave. In other words:

class ThisClass {
    //if you do this, you'll then have to configure your number formatter later on
    static var dateFormatter = NSDateFormatter()
    
    func userFormatter() {
        //you probably want this setup to take place only once
        //not every time you use the formatter
        ThisClass.dateFormatter.dateFormat = "yyyy-MM-dd"
    
        //do something with the formatter
    }
}

This example is quite nicely replaced by:

class ThisClass {
    static var dateFormatter: NSDateFormatter = {
        let formatter = NSDateFormatter()
        formatter.dateFormat = "yyyy-MM-dd"
        return formatter
    }()
    
    func userFormatter() {
        //do something with the formatter with no setup needed!
    }
}

Upvotes: 6

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