Extensions I use often in my projects. Swift 4.0

Extensions

import Foundation
import UIKit

extension UIColor{
    class var random: UIColor {
        let hue : CGFloat = CGFloat(arc4random() % 256) / 256 // use 256 to get full range from 0.0 to 1.0
        let saturation : CGFloat = CGFloat(arc4random() % 128) / 256 + 0.5 // from 0.5 to 1.0 to stay away from white
        let brightness : CGFloat = CGFloat(arc4random() % 128) / 256 + 0.5 // from 0.5 to 1.0 to stay away from black
        return UIColor(hue: hue, saturation: saturation, brightness: brightness, alpha: 1)
    }
    
    class func hex(_ hex:String, alpha:CGFloat = 1.0) -> UIColor {
        var cString:String = hex.trimmingCharacters(in: .whitespacesAndNewlines).uppercased()
        
        if (cString.hasPrefix("#")) {
            let index = cString.index(cString.startIndex, offsetBy: 1)
            cString = String(cString[index...])
        }
        
        if (cString.characters.count != 6) {
            return UIColor.gray
        }
        
        var rgbValue:UInt32 = 0
        Scanner(string: cString).scanHexInt32(&rgbValue)
        
        return UIColor(
            red: CGFloat((rgbValue & 0xFF0000) >> 16) / 255.0,
            green: CGFloat((rgbValue & 0x00FF00) >> 8) / 255.0,
            blue: CGFloat(rgbValue & 0x0000FF) / 255.0,
            alpha: CGFloat(alpha)
        )
    }
    
    var invert: UIColor {
        var red         :   CGFloat  =   255.0
        var green       :   CGFloat  =   255.0
        var blue        :   CGFloat  =   255.0
        var alpha       :   CGFloat  =   1.0
        
        self.getRed(&red, green: &green, blue: &blue, alpha: &alpha)
        
        red     =   255.0 - (red * 255.0)
        green   =   255.0 - (green * 255.0)
        blue    =   255.0 - (blue * 255.0)
        
        return UIColor(red: red / 255.0, green: green / 255.0, blue: blue / 255.0, alpha: alpha)
    }

}

extension UIFont{
    // FONTELLO
    public class func fontello(withSize fontSize: CGFloat) -> UIFont{ return self.init(name: "fontello", size: fontSize)! }
}

extension UITextField{
    
    func addDoneButtonToKeyboard(withAction action: Selector?){
        let doneToolbar: UIToolbar = UIToolbar(frame: CGRect(x: 0, y: 0, width: 300, height: 40))
        doneToolbar.barStyle = UIBarStyle.default
        
        let flexSpace = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.flexibleSpace, target: nil, action: nil)
        let done: UIBarButtonItem = UIBarButtonItem(title: "Done", style: UIBarButtonItemStyle.done, target: self, action: action)
        
        var items = [UIBarButtonItem]()
        items.append(flexSpace)
        items.append(done)
        
        doneToolbar.items = items
        doneToolbar.sizeToFit()
        
        self.inputAccessoryView = doneToolbar
    }
}

extension String{
    
    var underlined: NSAttributedString {
        get {
            let textRange = NSMakeRange(0, self.characters.count)
            let attributedText = NSMutableAttributedString(string: self)
            attributedText.addAttribute(NSUnderlineStyleAttributeName , value: NSUnderlineStyle.styleSingle.rawValue, range: textRange)
            return attributedText
        }
    }
    func underlined(withColour colour: UIColor) -> NSAttributedString{
        let textRange = NSMakeRange(0, self.characters.count)
        let attributedText = NSMutableAttributedString(string: self)
        attributedText.addAttribute(NSUnderlineStyleAttributeName , value: NSUnderlineStyle.styleSingle.rawValue, range: textRange)
        attributedText.addAttribute(NSForegroundColorAttributeName, value: colour, range: textRange)
        return attributedText
    }
    
    var localised: String { return NSLocalizedString(self, comment: "") }
}

// Get the topmost view controller
extension UIApplication {
    class func topViewController(_ base: UIViewController? = UIApplication.shared.keyWindow?.rootViewController) -> UIViewController? {
        
        if let nav = base as? UINavigationController {
            return topViewController(nav.visibleViewController)
        }
        if let tab = base as? UITabBarController, let selected = tab.selectedViewController {
            return topViewController(selected)
        }
        if let presented = base?.presentedViewController {
            return topViewController(presented)
        }
        
        return base
    }
}

extension NSLayoutConstraint {
    
    public class func useAndActivate(_ constraints: [NSLayoutConstraint]) {
        for constraint in constraints {
            if let view = constraint.firstItem as? UIView {
                view.translatesAutoresizingMaskIntoConstraints = false
            }
        }
        activate(constraints)
    }
}

// Make cell identifiers reusable for collection views
// Source: https://medium.com/@gonzalezreal/ios-cell-registration-reusing-with-swift-protocol-extensions-and-generics-c5ac4fb5b75e#.3ey5f2nvs

protocol ReusableView: class {}

extension ReusableView where Self: UIView {
    
    static var reuseIdentifier: String {
        return String(describing: self)
    }
}

//extension UICollectionViewCell: ReusableView {}
extension UICollectionReusableView: ReusableView {}

extension UICollectionView {
    
    func register<T: UICollectionViewCell>(_: T.Type){
        register(T.self, forCellWithReuseIdentifier: T.reuseIdentifier)
    }
    
    func dequeueReusableCell<T: UICollectionViewCell>(forIndexPath indexPath: IndexPath) -> T{
        guard let cell = dequeueReusableCell(withReuseIdentifier: T.reuseIdentifier, for: indexPath) as? T else{
            fatalError("Could not dequeue cell with identifier: \(T.reuseIdentifier)")
        }
        return cell
    }
    
    func registerHeader<T: UICollectionReusableView>(_: T.Type){
        register(T.self, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: T.reuseIdentifier)
    }
    
    func registerFooter<T: UICollectionReusableView>(_: T.Type){
        register(T.self, forSupplementaryViewOfKind: UICollectionElementKindSectionFooter, withReuseIdentifier: T.reuseIdentifier)
    }
    
    func dequeueReusableHeader<T: UICollectionReusableView>(forIndexPath indexPath: IndexPath) -> T{
        guard let reusableView = dequeueReusableSupplementaryView(ofKind: UICollectionElementKindSectionHeader, withReuseIdentifier: T.reuseIdentifier, for: indexPath) as? T else {
            fatalError("Could not dequeue header with identifier: \(T.reuseIdentifier)")
        }
        return reusableView
    }
    
    func dequeueReusableFooter<T: UICollectionReusableView>(forIndexPath indexPath: IndexPath) -> T{
        guard let reusableView = dequeueReusableSupplementaryView(ofKind: UICollectionElementKindSectionFooter, withReuseIdentifier: T.reuseIdentifier, for: indexPath) as? T else {
            fatalError("Could not dequeue footer with identifier: \(T.reuseIdentifier)")
        }
        return reusableView
    }
}

// This function makes sure the index is in the array by using the "safe" keyword
// Example: if let item = array[safe: index] { .. }
// Source: http://stackoverflow.com/questions/25329186/safe-bounds-checked-array-lookup-in-swift-through-optional-bindings
extension Collection {
    /// Returns the element at the specified index iff it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Iterator.Element? {
        return index >= startIndex && index < endIndex ? self[index] : nil
    }
}