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, where you can ask questions and respond with feedback. I will really glad to have you there! 😀
#27 Debugging: Breakpoints
#26 Debugging: Asserts
#25 Debugging: Log functions
#24 Update UIView content with animation
#23 Observe MOC changes
#22 Split String into words
#21 Comparing tuples
#20 How to detect that user stop typing
#19 Left/rigth text offset inside UITextField
#18 Common elements in two arrays
#17 Apply gradient to Navigation Bar
#16 Get next element of array
#15 Split array by chunks of given size
#14 Transparent/Opaque Navigation Bar
#13 Group objects by property
#12 Semicolons in Swift
#11 Fake AppDelegate
#10 Invoke didSet when property’s value is set inside init context
#9 Change type of items in array
#8 forEach and map execution order difference
#7 Testing settings
#6 Tips for writing error messages
#5 Profit to compiler
#4 Combinations of pure functions
#3 Enumerated iteration
#2 Easy way to hide Status Bar
#1 Safe way to return element at specified index
A breakpoint is a debugging tool that allows you to pause the execution of your program up to a certain moment. Creating pause points in your code can help you investigate your code.
While your app is paused, light green arrow that shows your current execution position can be moved. Just click and drag it somewhere else to have execution pick up from there – although Xcode will warn you that it might have unexpected results, so tread carefully!
Right-click on the breakpoint (the blue arrow marker) and choose Edit Breakpoint.
In the popup that appears you can set the condition. Execution will now pause only when your condition is true. You can use conditional breakpoints to execute debugger commands automatically – the Automatically continue checkbox is perfect for making your program continue uninterrupted while breakpoints silently trigger actions.
Also you can set Ignore times before stoping and actions like Debuger command, Log message, Sound, etc.
Shortcuts:
F6- Step Over.
Ctrl+Cmd+Y - Continue (continue executing my program until you hit another breakpoint)
In Xcode debug console you can use po to print what you need during pause.
assert() is debug-only check that will force your app to crash if specific condition is false.
assert(4 == 4, "Maths error") //OK
assert(3 == 2, "Maths error") //CrashAs you can see assert() takes two parameters:
- Something to check.
- Message to print out of the check fails.
If the check evaluates to false, your app will be forced to crash because you know it's not in a safe state, and you'll see the error message in the debug console.
If you don’t have a condition to evaluate, or don’t need to evaluate one, you can use assertionFailure() function.
precondition() is not debug-only check. It will crash your app even in release mode.
precondition(4 == 4, "Maths error") //OK
precondition(3 == 2, "Maths error") //CrashpreconditionFailure() works the same as assertionFailure(). With the same difference as above, it works for release builds.
fatalError(), like assertionFailure() and preconditionFailure() works for all optimisation levels in all build configurations.
fatalError("ERROR")More about asserts and optimisation levels you can find here
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Debuging 👨🔧 is one of the most importent aspects of programing👨💻. It should be in your skillbox anyway. It contains log functions, asserts, breakpoints and view debuging. Let's observe everyone of them just one by one. And here are log functions in the crosshairs 🔍.
We know print() as a variadic function. Function that accepts any number of parameters.
print("one", "two", "three", "four") //one two three fourBut it's variadic nature becomes much more useful when you use separator and terminator, its optional extra parameters. separator gives you opportunity to provide a string that should be placed between every item. It's "space" by default.
print("one", "two", "three", "four", separator: "-") //one-two-three-fourMeanwhile terminator is what should be placed after the last item.
It’s \n by default, which means "line break".
print("one", "two", "three", "four", terminator: " five") //one two three four fiveBack to Top
Really lightweight way 🎈 How to add content changing animation to UIView and it subclasses.
extension UIView {
func fadeTransition(_ duration: CFTimeInterval) {
let animation = CATransition()
animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
animation.type = kCATransitionFade
animation.duration = duration
layer.add(animation, forKey: kCATransitionFade)
}
}Just invoke 🧙♂️ fadeTransition(_ duration: CFTimeInterval) by your view before you will apply a change.
label.fadeTransition(1)
label.text = "Updated test content with animation"Back to Top
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)Back to Top
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}
}
}Back to Top
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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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
}
}Back to Top
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
}
}
}Back to Top
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) }
}Back to Top
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!)
}
}Back to Top
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
}
}Back to Top
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)])
})
}
}Back to Top
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)
}Back to Top
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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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
}Back to Top
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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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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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 compactNumbers = numbers.compactMap { Int($0) } // [1, 2, 3, 4]Back to Top
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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- Even if you don't write UI Tests, they still take considerable amount of time to run. Just skip it.
- 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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- Say what happened and why
- Suggest a next step
- 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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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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compactMap 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 // whiteBack to Top
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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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)") // trueBack to Top
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 nilBack to Top