When is the destination view controller initialized during a segue? - ios

Question

I'm attempting to write a protocol and a custom UIStoryboardSegue class that will allow me to easily implement custom transitions in my UIViewControllers:
public protocol TransitionController
{
var transitionDurationIn: CFTimeInterval { get }
var transitionDurationOut: CFTimeInterval { get }
func prepareTransitionIn()
func prepareTransitionOut()
func performTransitionIn(finished: #escaping () -> Void)
func performTransitionOut(finished: #escaping () -> Void)
}
class JFTransitionControllerSegue: UIStoryboardSegue {
override func perform() {
let defaultTransitionDuration : CFTimeInterval = 1.5
if let dvc = self.destination as? TransitionController {
dvc.prepareTransitionIn()
}
else {
// Default transition
self.destination.view.alpha = 0
}
if let svc = self.source as? TransitionController {
svc.prepareTransitionOut()
svc.performTransitionOut(){ () in
if let dvc = self.destination as? TransitionController {
dvc.performTransitionIn(){ () in
self.source.present(self.destination, animated: false, completion: nil)
}
}
else {
// Default transition for the destination controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.destination.view.alpha = 1
}) { (Finished) in
self.source.present(self.destination, animated: false, completion: nil)
}
}
}
}
else
{
// Default transition for the source controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.source.view.alpha = 0
}) { (Finished) in
if let dvc = self.destination as? TransitionController {
dvc.performTransitionIn(){ () in
self.source.present(self.destination, animated: false, completion: nil)
}
}
else {
// Default transition for the destination controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.destination.view.alpha = 1
}) { (Finished) in
self.source.present(self.destination, animated: false, completion: nil)
}
}
}
}
}
}
class TestController: UIViewController, TransitionController {
#IBOutlet weak var form_username: UITextField!
// MARK: - TransitionController Protocol
var transitionDurationIn : CFTimeInterval {return 1.0}
var transitionDurationOut : CFTimeInterval {return 1.0}
func prepareTransitionIn()
{
//self.view.alpha = 0 // no fade in if you uncomment
form_username.alpha = 0 // nil
}
func prepareTransitionOut()
{
self.view.alpha = 1 // works
}
func performTransitionIn(finished: #escaping () -> Void)
{
UIView.animate(withDuration: self.transitionDurationIn, animations: {
//self.view.alpha = 1 // no fade in if you uncomment
self.form_username.alpha = 1 // nil, crashes
}) { (Finished) in
finished()
}
}
func performTransitionOut(finished: #escaping () -> Void)
{
UIView.animate(withDuration: self.transitionDurationOut, animations: {
self.view.alpha = 0 // fades out correctly
}) { (Finished) in
finished()
}
}
}
Basically, you just implement the protocol in any UIViewController you want, then make a segue of class JFTransitionControllerSegue. In the performTransitionIn function, you can just do something like UIView.animate and change the alpha or whatever you like. The problem I'm having is that the destination segue simply pops up instead of transitioning in properly. From what I can tell while debugging it isn't fully initialized - IBOutlet variables are nil, but the controller itself isn't. Is this a bad design pattern, or am I just missing something simple?

Answer 1

A view controller being initialised is one event. It's view being loaded is another.
The view property of a view controller is loaded lazily and the outlets are built and connected at that point. That's why viewDidLoad() is a thing.
If you want the view to be ready for you, you can call loadViewIfNeeded() on the view controller first.

Answer 2

Misdiagnosed the problem...the destination controller was loaded, but I had forgotten to add the destination controller's view to the window in the Segue class:
class JFTransitionControllerSegue: UIStoryboardSegue {
override func perform() {
let defaultTransitionDuration : CFTimeInterval = 1.5
if let dvc = self.destination as? TransitionController {
dvc.prepareTransitionIn()
}
else {
// Default transition
self.destination.view.alpha = 0
}
if let svc = self.source as? TransitionController {
svc.prepareTransitionOut()
svc.performTransitionOut(){ () in
UIApplication.shared.keyWindow?.insertSubview(self.destination.view, aboveSubview: self.source.view)
if let dvc = self.destination as? TransitionController {
dvc.performTransitionIn(){ () in
self.source.present(self.destination, animated: false, completion: nil)
}
}
else {
// Default transition for the destination controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.destination.view.alpha = 1
}) { (Finished) in
self.source.present(self.destination, animated: false, completion: nil)
}
}
}
}
else
{
// Default transition for the source controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.source.view.alpha = 0
}) { (Finished) in
UIApplication.shared.keyWindow?.insertSubview(self.destination.view, aboveSubview: self.source.view)
if let dvc = self.destination as? TransitionController {
dvc.performTransitionIn(){ () in
self.source.present(self.destination, animated: false, completion: nil)
}
}
else {
// Default transition for the destination controller
UIView.animate(withDuration: defaultTransitionDuration, animations: {
self.destination.view.alpha = 1
}) { (Finished) in
self.source.present(self.destination, animated: false, completion: nil)
}
}
}
}
}
}