CODEWITHSUNDEEP
swift
JoshPeel
JoshPeel

Reputation: 11

What do parentheses around a closure mean?

I am trying to understand how this custom Navigation bar / Paging View works, found here. What is tripping me up when I went through the README was setting up the tinder-like custom behavior:

// Tinder Like
controller?.pagingViewMoving = ({ subviews in
    for v in subviews {
        var lbl = v as UIImageView
        var c = gray

        if(lbl.frame.origin.x > 45 && lbl.frame.origin.x < 145) {
            c = self.gradient(Double(lbl.frame.origin.x), topX: Double(46), bottomX: Double(144), initC: orange, goal: gray)
        }
        else if (lbl.frame.origin.x > 145 && lbl.frame.origin.x < 245) {
            c = self.gradient(Double(lbl.frame.origin.x), topX: Double(146), bottomX: Double(244), initC: gray, goal: orange)
        }
        else if(lbl.frame.origin.x == 145){
            c = orange
        }
        lbl.tintColor = c
    }
})

I don't understand why there are parentheses around the closure that is being set to the controller?.pagingViewMoving property.

When I look in the SLPagingViewSwift.swift file, the .pagingViewMoving property is set to this alias:

public typealias SLPagingViewMoving = ((subviews: [UIView])-> ())

What are the extra set of parentheses doing outside the function type?

Upvotes: 1

Views: 462

Answers (1)

Blake Lockley
Blake Lockley

Reputation: 2961

In this case the parentheses are purely for clarity and are unnecessary. Its just like saying let a = (2 + 2). Although this should not be confused with closures provided as arguments in the following case.

If a function itself takes an argument which is a closure, the closure is simply within the parentheses thats contain the functions parameters.

So thanks to the syntax higher order functions that take a closure as their last (or only) argument that be represented in two ways - inside the argument parentheses or as whats known as a trailing closure. Consider the following function:

func foo(bar: (Int -> Int)) {...}

it can be called in two ways - first with the closure in the parentheses like so:

foo({(i) in i + 2})

or as a trailing closure:

foo {(i) in i + 2}

Upvotes: 5

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