inset.swift

import Cocoa

public struct UIEdgeInsets {
    
    public var top: CGFloat // specify amount to inset (positive) for each of the edges. values can be negative to 'outset'
    
    public var left: CGFloat
    
    public var bottom: CGFloat
    
    public var right: CGFloat
    
    public init() {
        (top, left, bottom, right) = (0, 0, 0, 0)
    }
    
    public init(top: CGFloat, left: CGFloat, bottom: CGFloat, right: CGFloat) {
        (self.top, self.left, self.right, self.bottom) = (top, left, right, bottom)
    }
}

extension UIEdgeInsets: ExpressibleByDictionaryLiteral {
    public typealias Key = WritableKeyPath<UIEdgeInsets, CGFloat>
    public typealias Value = CGFloat
    
    public init(dictionaryLiteral elements: (WritableKeyPath<UIEdgeInsets, CGFloat>, CGFloat)...) {
        self = UIEdgeInsets()
        for (inset, value) in elements {
            self[keyPath: inset] = value
        }
    }
    
    public var vertical: CGFloat {
        get { return 0 } // meaningless but not fatal
        set { (top, bottom) = (newValue, newValue) }
    }
    
    public var horizontal: CGFloat {
        get { return 0 } // meaningless but not fatal
        set { (left, right) = (newValue, newValue) }
    }
    
    public var all: CGFloat {
        get { return 0 } // meaningless but not fatal
        set { (vertical, horizontal) = (newValue, newValue) }
    }
}

let insets: UIEdgeInsets = [\.vertical: 8, \.horizontal: 20]
print(insets) // (l: 8.0 , r: 20.0, t: 8.0, b: 20.0)

let insets2: UIEdgeInsets = [\.all: 8]
print(insets2) // (l: 8.0 , r: 8.0, t: 8.0, b: 8.0)

let insets3: UIEdgeInsets = [\.left: 8]
print(insets3) // (l: 8.0, r: 0.0, t: 0.0, b: 0.0)