Swift tips

This is a growing list of resources and tips I compile to make my developer life easier.


translatesAutoResizingMasksIntoConstraints = false is done by Interface Builder automatically. If working in code do it yourself.

Constants are essentially linear equations.

Content compressionresistance priority controls how the frame of a UIView will be calculated when AutoLayour contraints (one or more) describe its width/heigh as being smaller than its intrinsic content size.

Content hugging deals whether or not a view can be made larget than its intrinsic content size instead of managing wheter a view is made smaller than its intrinsic content size.


  • Layout guides create invisible boxes that exist for a layout purpose. Use them to replace dummy views that you might have created to represent spacers between views. They can also act as a black box containing a number of other views and controls.

How to deal with 'Unsatisfiable Layouts' errors?

  • Check if translatesAutoResizingMasksIntoConstraints is set to false on the views we're adding constraints to.
  • Review the priorities, content hugging and compression resistance of each constraint.
  • AutoLayout will try to get as close to the desired result as possible while still satisfying all other constraints.

Add identifiers to a constraint to see the associated Auto Layout messages.

To get a full list of the constraints responsible for positioning a view within its superview log the results of calling the UIView instance with constraintsAffectingLayout(for:).

view.constraintsAffectingLayout(for: .vertical)
view.constraintsAffectingLayout(for: .horizontal)

AutoLayout's Visual Format Language

Animate constraints using:

func animateConstraints() {

  // Force layout before animating

  UIView.animateWithDuration(0.4) {
    // Force layout inside the animation block

Background Operations

https://littlebitesofcocoa.com/76-background-fetch https://www.andyibanez.com/posts/modern-background-tasks-ios13/

<details> <summary>Codable </summary> Codable maps JSON into model types. Not only it can be used to work with JSON but it cna also decode a .plist file into Swift structs and vice versa.

https://www.donnywals.com/an-introduction-to-working-with-codable-in-swift/ https://www.donnywals.com/customizing-how-codable-objects-map-to-json-data/ https://www.donnywals.com/writing-custom-json-encoding-and-decoding-logic/ https://www.donnywals.com/splitting-a-json-object-into-an-enum-and-an-associated-object-with-codable/






Strong capturing is used as default by Swift. The closure will capture any external values that are used inside the closure and make sure they are neved destroyed.

Weak capturing. Weakly captured values aren't kept alive by the closure, so they might be destroyed and set to nil. Weakly captured values are always optional in Swift. This stops you assuming they are present when in fact they might not be.

Unowned capturing behaves more like implicitly unwrapped optionals. Like weak capturing, unowned capturing allows values to become nil at any point in the future. For example, [unowned self] means you want to reference self inside the closure but you don't want to own it. If self is nil inside a closure there's something else you should be looking for.

Closures themselves are copied because their captured data becomes shared amongst copies.

When to use each one? If you don't have any idea, start with weak and change accordingly only if you need to.

Strong reference cycles

<!-- **Copies of closures** -->

Accidental strong references




What's the difference between flatMap and map? They behave differently than Sequence.map and Sequence.flatMap. Publisher.map(_:) works like Sequence.map except it operates with publishers. Takes a closure that changes an element into another element. Publisher.flatMap(_:) transforms a publisher into a completely new publisher that produces elements of the same type. Use it whenever you want to reach into inner publisher to get its elements.

  • A Future is a placeholder for a value that doesn't exist yet.


Adding an exception breakpoint to the Breakpoint navigator will pause the execution whenever an exception is thrown. This will let you poke around the causes that produced it.

Conditional breakpoints pause execution whenever a certain condition is met.

On the debugging console use p or po to print out variables. Use eto evaluate an expression or modify values.

A property breakpoint is a breakpoint set on the line that contains a property definition. Xcode will pause execution anytime that property's value is changed. It will also display a stack trace of the function that caused the change.

Dependency Injection

Create some type or object while passing in some other type or object that the first type depends on to do its work.

It's useful around testing so you can pass a mock version that can behave in expected ways. Also, by not using shared instances and passing specific types we reduce the global/shared state in the codebase.

Design Patterns

  • Singletons are a design pattern describing globally accessible instances of objects. They should only be used when there is a shared global state that needs to be read/written. They are thread-safe and lazily initialized. Classes like URLSession, FileManager and UserDefaults use this design pattern.
class MySingleton {

    /// The initializer of global variables and static properties are executed lazily by default.
    static let shared = MySingleton()

    /// Private initializer, only `MySingleton` can create instances of itself
    private init() { }

Dynamic Type

Dynamic Type lets users specify their preferred text size. Apps that support this feature will adjust their text based on the user preferred text size.


Error Handling




Chainable methods perform actions and return self so we can compose compossable code.

class ChainableTest {

  var number: Int?
  var numberStringified: String?

  func double(_ number: Int) -> Self {

    self.number = number * 2

    return self

  func toString(_ textToAdd: String) -> Self {

      if let number = number {
        numberStringified = "\(textToAdd) \(number)"

      return self

let result = ChainableTest()

Grand Central Dispatch

Also referred to as GCD.

One common use is to hop on to a background queue to do some work and then hop onto the main queue to update the UI



Lazy properties

  • lazy is great for times when we have expensive work to perform.

Local Notifications

Local Notifications a system notification without needing to implement a push server. Register the device to receive these notifictions under AppDelegate's didFinishLaunchingWithOptions: methos.

Remote Notifications

WWDC 2020 Session 10095

Push notifications do not require your app to be in the foreground. They are delivered regardless of the app state and if necessary your application will be launched.

  • Alert notifications: Visible notifications that can be interacted with and display new information. Doesn't require the application to be in the foreground, can be delivered with the app in the background. The appearance can be customizable
  • Background notifications: Allow your app to fetch data in the background upon receiving a background notification. System will launch the app if necessary and will give you runtime to perform your update. There are some limitations as the system limits the number of times you can update your app per day. Updates won't be performed if the app is on a constrained state such as low battery.

Set up remote push notifications

  1. Register for remote notifications in application(_:didFinishLaunchingWithOptions:). Will register the device for APNS and will send a token. It will identify your device and can be targeted to send specific notifications.
  1. Conform to UNUserNotificationCenterDelegate.
  2. Assign the AppDelegate as a delegate for the Notification Center. This will let your app be notified when an alert is received.
UNUserNotificationCenter.current().delegate = self
  1. Once you've registered to receive remote notifications you will receive a delegate call on either these two methods: application(_:didFailToRegisterForRemoteNotificationsWithError:) or application(_:didRegisterForRemoteNotificationsWithDeviceToken:).
  2. If registration suceeds you need to send the token to your server so it's able to send notifications later on. The token is delivered as a Data object and it needs to be converted to a String.
let tokenComponents = token.map { data in String(format: "%02.2hhx", data) }
let deviceTokenString = tokenComponents.string()
  1. Ask for permission so your app is able to display notifications using requestAuthorization(options:completionHandler:).

Notifications being sent to a device have a JSON payload specifying how to render the notification.

When a notification is being opened or interacted with userNotificationCenter(_:didReceive:withCompletionHandler:) will be called.


Generics are a way to specifying parts of your code can work with different types of objects of a given type but you don't want to hardcode one singlet type. It would be something like "this function accepts any object of some type and returns an object of that same type but I do not care what that type is".

func genericOperation<Type>(parameter: Type) -> Type {
    return parameter

The < > brackets inform the Swift compiler that you are writing a generic function. You can use any name you want for Type. Swift generics also let you only accept types that conform to a protocol.

func genericOperation<Type: SomeProtocol>(parameter: Type) -> Type {
    return parameter

Generics not only can be applied to functions but also types. For example this is a generic data source.

struct DataSource<ObjectType> {
  var objects = [ObjectType]()

  func objectAtIndex(index: Int) -> ObjectType {
    return objects[index]

So that property can later be used:

class SpaceshipsViewController : UITableViewController {
  var dataSource = DataSource<Spaceship>()

Gesture Recognizer


Begin a task or activity on one device and then continue it on another one.

Multipeer Connectivity

Allows devices to communicate with each other without the need for a central server.


Reachability.swift helps you deal with how "reachable" internet is at a given time.


Use URLComponents to compose URLs in a safer and more predictable way. You can also manually get all the components by instantiating URLComponents from a URL.


Low Data mode

It's an explicit user preference to minimize data usage. Background app refresh is disabled and discretionary tasks should be deferred. Your application should implement specific policies that react to these changes.

How can you implement this?

  • If your app is image-heavy reduce the quality of the images.
  • Reduce pre-fetching. Avoid fetching resources taht users won't need
  • Synchronize less often
  • Mark background tasks as discretionary, some of them might not need to be executed immediately.
allowsConstrainedNetworkAccess = false

Time Profiler

A common use is to detect under-performing functions that might be blocking the main thread.

Turn all the call tree options. This will clean up the collected data



Multiple/Chained optional unwrapping. You can even use unwrapped references from earlier in the statements in later parts of the statement.

if let URL = NSURL(string: someURL),
   let data = NSData(contentsOfURL: URL),
   let ship = Spaceship(data: data) {
    // use 'ship'


A pointer is a variable that stores the memory address of an object. Swift's pointers can be broken into the following types:

  • A buffer

  • A mutable type allows us to mutate the memory referenced by that pointer. An immutable type provides read-only access to the referenced memory.

  • A raw type contains uninitialized and untyped data. Raw pointers must be bound to a certain type and value before we can use them. They can be reinterpreted to several different types. Typed pointers have a generic parameters, which is the type of the value being pointed to. Useful when we don't know what kind of data we point to.

  • An unsafe type does not have some of Swift's safety features. It's possible to violate memory, access unallocated memory or interpret memory as wrong type by means of unsafe pointes.

  • A managed type has automatic memory management. An unmanaged type makes you partially responsible for the object's lifecycle.

| C Variables | Swift Pointers | |:----------------------------:|:-------------------------:| | const int * someInt | UnsafePointer<Int> | | int * anotherInt | UnsafeMutablePointer<Int> | | struct Spaceship * spaceship | COpaquePointer |

How about void pointers in C?

void someFunction(const void *someVariable);
/// Create a regular `Int` value in Swift
var myVariable = 31
/// Pass the `Int` into the `withUnsafePointer` function along with a closure.
/// We will receive an `UnsafePointer` of our original `Int`.
withUnsafePointer(&myVariable) { (p: UnsafePointer<Int>) in
	/// Convert the pointer to a void pointer using `unsafeBitCast`
	let voidP = unsafeBitCast(p, UnsafePointer<Void>.self)

Calling functions with pointer parameters

Apple has a nice guide on this.


Protocol extensions allow you to add new functions to any class that implements a protocol.


SwiftUI makes use of property wrappers to understand how we create and store data in our views. They wrap behavior along with properties.

All property wrappers conform to DynamicProperty protocol.

This is a good rule of thumb to know what property wrapper to use.

  • Displaying a value type (struct or enum)
    • Won't change over time, use a regular property
    • Used to read/write from it?
      • If you're passing the value rom outside use Binding.
      • To store local view state use State. If you're doing something with a gesture on a View use GestureState.
  • Displaying a reference type (class)
    • Creating the object locally in your view, use StateObject
    • Passed as a parameter, use ObservedObject
    • Passed through the environment, EnvironmentObject.

These property wrappers are not the only ones. You also have other types for specific purposes.

  • AppStorage to read and write data using UserDefaults.
  • Environment, to read value from a SwiftUI View environment.
  • FetchRequest, reading from a Core Data store.
  • Published, automatically announces changes to properties. Constrained to use it on classes only. Subscribers receive the new value before it's actually set on the property.
  • ScaledMetric, scales a numeric value.
  • SceneStorage, reads and writes data to the current scene.

Save data when your app quits, need to know when your app is going to be quit

For SwiftUI lifecycle

struct MySwiftUIApp: App {
    @Environment(\.scenePhase) private var scenePhase

    var body: some Scene {
        WindowGroup {
        .onChange(of: scenePhase) { phase in
            if phase == .background {
                // Save here

AppDelegate Life Cycle

func applicationDidEnterBackground(_ application: UIApplication) {
    // Save here


UIKit View Lifecycle

  • viewDidLoad() is called whenever we programatically create the interface. Performs additional initialization on views.
  • loadView() creates the view that the controller manages. Should only be used when you programatically instantiate your view controllers. When using Storyboards this is the method that will load your nib and attach it to the view but when instantiating ViewControllers manually all this method does is create an empty UIView.

UI Testing

Pass parameters into a UI Test so the device knows it's running a UI test.

let app = XCUIApplication()
app.launchArguments = [ "IS_UI_TESTING" ]

Universal Links

They tie the content in your app to the content on a website. Once it's configured iOS will be able to launch your app when a user taps alink somewhere, rather than opening our site in Safari.


Debugging certain sizes of a widget

  • From the widget's Xcode scheme add an environment variable for _XCWidgetFamily.
  • Set the value to small, medium or large and the widget will boot to that size.


  • Scheme enable zombie objects


Keyboard Shortcuts

  • Command + Shift + O: Open a file inside Xcode. This will allow you to work faster when changing files without needing to reach from your mouse.


These are a collection of blogs, links, books, videos that helped me learn different topics. Buying the books linked below from their respective authors help them contribute to the community.