Better dependency injection for Storyboards in iOS13

If you have ever used a Storyboard you know that dependency injection does not work very well with it.

Existing implementation (Pre iOS13) #

The only way to set or inject something to view controller is through the func prepare(for segue: UIStoryboardSegue, sender: Any?) method.

func  prepare ( for segue :  UIStoryboardSegue , sender :  Any ?) {
if let viewController = segue.destination as ? DetailViewController {
viewController.detail = "injected detail"

The view controller passing along in segue object is already initialized with init?(coder: NSCoder). This implementation causes us some limitations.

Properties must be variable #

Constant (let) is not possible since constant must be assigned a value during the initialization period which is out of our control.

This resulting in we have to declare constant with implicitly unwrapped optional (var with !), which might cause confusion whether this property is mean to be changed or not.

Properties need to be public #

Since you can't inject anything at initialization time, you have to expose every property you want to be set or modify.

Improvement (iOS13) #

With the coming of Xcode 11 (iOS13 and macOOS10.15), we got new Interface Builder attribute, @IBSegueAction. You apply the @IBSegueAction attribute to method declarations in a view controller to dictate that method is responsible for creating segue's destination view controller (destinationViewController of the segue object passed to prepare(for:sender:).

An @IBSegueAction method takes up to three parameters: a coder, the sender, and the segue’s identifier. The first parameter is required, and the other parameters can be omitted from your method’s signature if desired. The NSCoder must be passed through to the destination view controller’s initializer, to ensure it’s customized with values configured in Storyboard. The method returns a view controller that matches the destination controller type defined in the storyboard, or nil to cause a destination controller to be initialized with the standard init(coder:) method. If you know you don’t need to return nil, the return type can be non-optional.

With this new power, we can have a custom initializer with any required values. Let's see some examples.

Example #

In Swift, add the @IBSegueAction attribute:

func makeDogController(coder: NSCoder, sender: Any?, segueIdentifier: String?) -> ViewController? {
coder: coder,
petName: self.selectedPetName, type: .dog

In Objective-C, add IBSegueAction in front of the return type:

- (IBSegueAction ViewController *)makeDogController:(NSCoder *)coder
segueIdentifier:(NSString *)segueIdentifier
return [PetController initWithCoder:coder

Where does PetController(coder: coder, petName: self.selectedPetName, type: .dog) coming from? #

There is no magic here, you have to create your own initializer. Just make sure you called init(coder:) in your initializer.

class PetController: UIViewController {

let petName: String
let type: String

init?(coder: NSCoder, petName: String, type: String) {
self.petName = petName
self.type = type

super.init(coder: coder)

required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")

Shortened form #

Since the only required parameter is the coder: NSCoder you can just create @IBSegueAction like this.

func makeDogController(coder: NSCoder) -> ViewController? {
coder: coder,
petName: self.selectedPetName, type: .dog

Binding #

The way we binding segue in Interface Builder with this @IBSegueAction is the same process as other attributes e.g. @IBAction and @IBOutlet. Just control + drag from segue in Interface Builder to the desired method.

Let's see it in action.

  1. Create action segue between 2 view controllers like you always did

Presentation segue

  1. There are 2 ways to bind this segue to code.

2.1 Click on created segue and control + drag to an existing @IBSegueAction method or empty space to create a new one.
Drag from segue

2.2 Click on created segue then goes to Connections inspector and drag from instantiation segue action.
Drag from connections inspector

Initialize Storyboard-back view controller in code #

Apple also adds 2 pair of UIStoryboard methods to create a view controller with custom initializer.


func instantiateInitialViewController<ViewController>(creator: ((NSCoder) -> ViewController?)? = nil) -> ViewController? where ViewController : UIViewController



func instantiateViewController<ViewController>(identifier: String, creator: ((NSCoder) -> ViewController?)? = nil) -> ViewController where ViewController : UIViewController

Using it like this.

let storyboard = UIStoryboard(name: "Main", bundle: nil)
storyboard.instantiateInitialViewController { (coder) -> ViewController? in
return PetController(
coder: coder,
petName: self.selectedPetName, type: .dog

storyboard.instantiateViewController(identifier: "Test") { (coder) -> ViewController? in
return PetController(
coder: coder,
petName: self.selectedPetName, type: .dog

Limitations #

You can use this on any action segues e.g. show, show detail, and present modally including embed segue, but not on relationship segue like navigation controller root view. Not sure whether this is a bug or by design.

You can use @IBSegueAction with relationship segue by binding relationship segue to a presentation view controller (The view controller that presented navigation controller).

It only works on iOS13 and macOS10.15, no backward compatibility for older OS.

Conclusion #

This is a welcome change for Storyboard users. It makes a view controller designed to use with Storyboard more reasonable and semantically correct.

References #

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