Swift tips and tricks

Here's list of Swift tips & tricks with all additional sources (playgrounds, images) that I would like to share. Also you can find them on Twitter @szubyak and Facebook @szubyakdev, where you can ask questions and respond with feedback. I will really glad to have you there! 😀

Table of contents

#1 Safe way to return element at specified index
#2 Easy way to hide Status Bar
#3 Enumerated iteration
#4 Combinations of pure functions
#5 Profit to compiler
#6 Tips for writing error messages
#7 Testing settings
#8 forEach and map execution order difference
#9 Change type of items in array
#10 Invoke didSet when property’s value is set inside init context
#11 Fake AppDelegate
#12 Semicolons in Swift
#13 Group objects by property
#14 Transparent/Opaque Navigation Bar
#15 Split array by chunks of given size
#16 Get next element of array
#17 Apply gradient to Navigation Bar
#18 Common elements in two arraysr
#19 Left/rigth text offset inside UITextField
#20 How to detect that user stop typing
#21 Comparing tuples
#22 Split String into words
#23 Observe MOC changes

#1 Safe way to return element at specified index

You can extend collections to return the element at the specified index if it is within bounds, otherwise nil.

extension Collection {
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

let cars = ["Lexus", "Ford", "Volvo", "Toyota", "Opel"]
let selectedCar1 = cars[safe: 3] // Toyota
let selectedCar2 = cars[safe: 6] // not crash, but nil

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#2 Easy way to hide Status Bar

Ever faced the problem that u can't hide status bar because of prefersStatusBarHidden is get-only? The simplest solution is to override it 🧐👨‍💻

let vc = UIViewController()
vc.prefersStatusBarHidden = true // error
print("statusBarHidded \(vc.prefersStatusBarHidden)") // false

class TestViewController: UIViewController {
    override var prefersStatusBarHidden: Bool {
        return true
    }
}

let testVC = TestViewController()
print("statusBarHidded \(testVC.prefersStatusBarHidden)") // true

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#3 Enumerated iteration

Use enumerated when you iterate over the collection to return a sequence of pairs (n, c), where n - index for each element and c - its value 👨‍💻💻

for (n, c) in "Swift".enumerated() {
    print("\(n): \(c)")
}

Result:

0: S
1: w
2: i
3: f
4: t

Also be careful with this tricky thing, enumerated on collection will not provide actual indices, but monotonically increasing integer, which happens to be the same as the index for Array but not for anything else, especially slices.

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#4 Combinations of pure functions

flatMap func is effectively the combination of using map and joined in a single call, in that order. It maps items in array A into array B using a func you provide, then joins the results using concatenation.

Functions min and max could be also combinations of sorted.first and sorted.last in single call.

let colors = ["red", "blue", "black", "white"]

let min = colors.min() // black
let first = colors.sorted().first // black

let max = colors.max() // white
let last = colors.sorted().last // white

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#5 Profit to compiler

Do you know that using map gives profit to the compiler: it's now clear we want to apply some code to every item in an array, then like in for loop we could have break on halfway through.

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#6 Tips for writing error messages

1. Say what happened and why
2. Suggest a next step
3. Find the right tone (If it’s a stressful or serious issue, then a silly tone would be inappropriate)

Common​ ​Types​ ​of​ ​Error​ ​Messages

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#7 Testing settings

1. Even if you don't write UI Tests, they still take considerable amount of time to run. Just skip it.
2. Enable code coverage stats in Xcode, it helps to find which method was tested, not tested, partly tested. But don’t pay too much attention to the percentage 😊.

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#8 forEach and map execution order difference

Execution order is interesting difference between forEach and map: forEach is guaranteed to go through array elements in its sequence, while map is free to go in any order.

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#9 Change type of items in array

Two ways of changing type of items in array and obvious difference between them 🧐👨‍💻

let numbers = ["1", "2", "3", "4", "notInt"]
let mapNumbers = numbers.map { Int($0) }  // [Optional(1), Optional(2), Optional(3), Optional(4), nil]
let flatNumbers = numbers.flatMap { Int($0) } // [1, 2, 3, 4]

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#10 Invoke didSet when property’s value is set inside init context

Apple's docs specify that: "Property observers are only called when the property’s value is set outside of initialization context."

defer can change situation 😊

class AA {
    var propertyAA: String! {
        didSet {
            print("Function: \(#function)")
        }
    }

    init(propertyAA: String) {
        self.propertyAA = propertyAA
    }
}

class BB {
    var propertyBB: String! {
        didSet {
            print("Function: \(#function)")
        }
    }

    init(propertyBB: String) {
        defer {
            self.propertyBB = propertyBB
        }
    }
}

let aa = AA(propertyAA: "aa")
let bb = BB(propertyBB: "bb")

Result:

Function: propertyBB

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#11 Fake AppDelegate

Unit testing shouldn’t have any side effects. While running tests, Xcode firstly launches app and thus having the side effect of executing any code we may have in our App Delegate and initial View Controller. Fake AppDelegate in your main.swift to prevent it.

You can find main.swift file here

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#12 Semicolons in Swift

Do you need semicolons in Swift ? Short answer is NO, but you can use them and it will give you interesting opportunity. Semicolons enable you to join related components into a single line.

func sum(a: Int, b: Int) -> Int {
    let sum = a + b; return sum
}

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#13 Group objects by property

One more useful extension 🔨💻 Gives you opportunity to group objects by property 👨‍💻🧐

extension Sequence {
    func group<GroupingType: Hashable>(by key: (Iterator.Element) -> GroupingType) -> [[Iterator.Element]] {
        var groups: [GroupingType: [Iterator.Element]] = [:]
        var groupsOrder: [GroupingType] = []
        forEach { element in
            let key = key(element)
            if case nil = groups[key]?.append(element) {
                groups[key] = [element]
                groupsOrder.append(key)
            }
        }
        return groupsOrder.map { groups[$0]! }
    }
}

Usage:

struct Person {
    var name: String
    var age: Int
}

let mike = Person(name: "Mike", age: 18)
let john = Person(name: "John", age: 18)
let bob = Person(name: "Bob", age: 56)
let jake = Person(name: "Jake", age: 56)
let roman = Person(name: "Roman", age: 25)

let persons = [mike, john, bob, jake, roman]

let groupedPersons = persons.group { $0.age }

for persons in groupedPersons {
    print(persons.map { $0.name })
}

Result:

["Mike", "John"]
["Bob", "Jake"]
["Roman"]

Also in-box alternative

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#14 Transparent/Opaque Navigation Bar

Scene with UIImageView on top looks stylish if navigation bar is transparent. Easy way how to make navigation bar transparent or opaque.

func transparentNavigationBar() {
    self.setBackgroundImage(UIImage(), for: .default)
    self.shadowImage = UIImage()
}

func opaqueNavigationBar() {
    self.shadowImage = nil
    self.setBackgroundImage(nil, for: .default)
}

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#15 Split array by chunks of given size

Great extension to split array by chunks of given size

extension Array {
    func chunk(_ chunkSize: Int) -> [[Element]] {
        return stride(from: 0, to: self.count, by: chunkSize).map({ (startIndex) -> [Element] in
            let endIndex = (startIndex.advanced(by: chunkSize) > self.count) ? self.count-startIndex : chunkSize
            return Array(self[startIndex..<startIndex.advanced(by: endIndex)])
        })
    }
}

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#16 Get next element of array

Easy way how to get next element of array

extension Array where Element: Hashable {
    func after(item: Element) -> Element? {
        if let index = self.index(of: item), index + 1 < self.count {
            return self[index + 1]
        }
        return nil
    }
}

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#17 Apply gradient to Navigation Bar

Gradient 🏳️‍🌈 on Navigation Bar is really good looking, but not very easy to implement 🧐🔨👨‍💻 Works with iOS 11 largeTitle navigation bar too 👌

struct GradientComponents {
    var colors: [CGColor]
    var locations: [NSNumber]
    var startPoint: CGPoint
    var endPoint: CGPoint
}

extension UINavigationBar {

    func applyNavigationBarGradient(with components: GradientComponents) {

        let size = CGSize(width: UIScreen.main.bounds.size.width, height: 1)
        let gradient = CAGradientLayer()
        gradient.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)

        gradient.colors = components.colors
        gradient.locations = components.locations
        gradient.startPoint = components.startPoint
        gradient.endPoint = components.endPoint

        UIGraphicsBeginImageContext(gradient.bounds.size)
        gradient.render(in: UIGraphicsGetCurrentContext()!)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        self.barTintColor = UIColor(patternImage: image!)
    }
}

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#18 Common elements in two arrays

I'm not huge fan of custom operators 😐 because they are intuitively obvious only to their authors, but I've created one which gives you opportunity to get common elements in two arrays whos elements implement Equatable protocol 🔨🧐💻

infix operator &
func  &<T : Equatable>(lhs: [T], rhs: [T]) -> [T] {
    return lhs.filter { rhs.contains($0) }
}

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#19 Left/rigth text offset inside UITextField

Clear way of adding left\right text offset inside UItextField 🔨🧐💻 Also, because of @IBInspectable it could be easily editable in Interface Builder’s inspector panel.

@IBDesignable
extension UITextField {

    @IBInspectable var leftPaddingWidth: CGFloat {
        get {
            return leftView!.frame.size.width
        }
        set {
            let paddingView = UIView(frame: CGRect(x: 0, y: 0, width: newValue, height: frame.size.height))
            leftView = paddingView
            leftViewMode = .always
        }
    }

    @IBInspectable var rigthPaddingWidth: CGFloat {
        get {
            return rightView!.frame.size.width
        }
        set {
            let paddingView = UIView(frame: CGRect(x: 0, y: 0, width: newValue, height: frame.size.height))
            rightView = paddingView
            rightViewMode = .always
        }
    }
}

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#20 How to detect that user stop typing

Painless way ( NO to timers from now ⛔️ ) how to detect that user stop typing text in text field ⌨️ Could be usefull for lifetime search 🔍

class TestViewController: UIViewController {

    @objc func searchBarDidEndTyping(_ textField: UISearchBar) {
        print("User finsihed typing text in search bar")
    }

    @objc func textFieldDidEndTyping(_ textField: UITextField) {
        print("User finished typing text in text field")
    }
}

extension TestViewController: UISearchBarDelegate {
    func searchBar(_ searchBar: UISearchBar, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
        NSObject.cancelPreviousPerformRequests(withTarget: self, selector: #selector(searchBarDidEndTyping), object: searchBar)
        self.perform(#selector(searchBarDidEndTyping), with: searchBar, afterDelay: 0.5)
        return true
    }
}

extension TestViewController: UITextFieldDelegate {
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        NSObject.cancelPreviousPerformRequests(withTarget: self, selector: #selector(textFieldDidEndTyping), object: textField)
        self.perform(#selector(textFieldDidEndTyping), with: textField, afterDelay: 0.5)
        return true
    }
}

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#21 Comparing tuples

I discovered strange behavior of tuples during comparing 🤪. Comparison cares only about types and ignores labels 😦. So result can be unexpected. Be careful ⚠️.

let car = (model: "Tesla", producer: "USA")
let company = (name: "Tesla", country: "USA")
if car == company {
    print("Equal")
} else {
    print("Not equal")
}

Printed result will be: Equal

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#22 Split String into words

Default ways of splitting ✂️ String don't work perfect sometimes, because of punctuation characters and other "wreckers" 🐛. Here is extension for splitting ✂️ String into words 💻🧐👌.

extension String {
    var words: [String] {
        return components(separatedBy: .punctuationCharacters)
            .joined()
            .components(separatedBy: .whitespaces)
            .filter{!$0.isEmpty}
    }
}

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#23 Observe MOC changes

Next code snippet 📃 I use to keep eye on changes that take place in the managed object context. Useful thing to know what's going on, what was added, updated ( what specific values were changed ) or deleted 📥📝📤

func changeNotification(_ notification: Notification) {
    guard let userInfo = notification.userInfo else { return }

    if let inserts = userInfo[NSInsertedObjectsKey] as? Set<NSManagedObject>, inserts.count > 0 {
        print("--- INSERTS ---")
        print(inserts)
        print("+++++++++++++++")
    }

    if let updates = userInfo[NSUpdatedObjectsKey] as? Set<NSManagedObject>, updates.count > 0 {
        print("--- UPDATES ---")
        for update in updates {
            print(update.changedValues())
        }
        print("+++++++++++++++")
    }

    if let deletes = userInfo[NSDeletedObjectsKey] as? Set<NSManagedObject>, deletes.count > 0 {
        print("--- DELETES ---")
        print(deletes)
        print("+++++++++++++++")
    }
}

NotificationCenter.default.addObserver(self, selector: #selector(self.changeNotification(_:)), name: .NSManagedObjectContextObjectsDidChange, object: moc)

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