Swift 3 : Switch ViewControllers programmatically

I am creating an application which shows a PageVC: UIPageViewController as intro and guide to app. After navigating through the intro, a Next Button leads to "SettingsVC" which is used to store default settings for the app. Settings have to be chosen by the user initially although can be changed later.

PageVC ---> SettingsVC

A Save and a Cancel button on the SettingsVC leads to the MainVC of the app.

A button on MainVC leads to SettingsVC.

SettingsVC <---> MainVC

The app would work as follows:

if isFirstLaunch{
    instantiate PageVC
}else{
    instantiate MainVC
}

in PageVC

nextButtonPressedinPageVC{
        instantiate SettingsVC
}

in SettingsVC

if saveButtonPressed && cameFromPageVC{
    instantiate MainVC
}
if cancelButtonPressed && cameFromPageVC {
    do Nothing
}
if saveButtonPressed && cameFromMainVC{
    dismiss currentVC 
}
if cancelButtonPressed && cameFromMainVC {
    dismiss currentVC
}

in MainVC

if settingsButtonPressedinMainVC {
        instantiate SettingsVC
}

I have made sure that if it is application's first launch, PageVC will be instantiated else MainVC will be instantiated.

How can I move between the viewControllers without a possible memory leak i.e. where to performSegue and where to dismiss current VC?

Please include code for reference.

Solution

There are many ways to do this, here is one that I find very straightforward because you can do most of the work in your Storyboard: Think of your MainVC as the rootViewController and the other two as accessory views that will only temporarily be shown. The MainVC should always be your entry point, so set it as the initial VC in your Storyboard.

The other two should be displayed modally so that you can easily return to the MainVC by dismissing them, no matter how you opened them in the first place.

To do this, draw a segue from your MainVC button to the PageVC and name it "showPageVC". From the Next button in your PageVC, draw another segue to the SettingsVC. Now you need some code to handle the dismiss actions: put this snippet in your MainVC:

@IBAction func unwindToMain(segue:UIStoryboardSegue) {
}

This function is just a marker, so it doesn't need a body. It just enables you to create a unwind segue back to MainVC: For each of the buttons in SettingsVC, hold Ctrl and draw from the button to the right exit icon in the header of the SettingsVC storyboard scene and choose unwindToMain in the tiny black popup.

Now you only have to implement the logic to decide if you want to show the PageVC or not in viewDidAppear() of the MainVC. So the whole code would look something like this:

class MainVC: UIViewController {

    var didDisplayPageVC = false

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        if (didDisplayPageVC == false) {
            performSegue(withIdentifier: "showPageVC", sender: self)
            didDisplayPageVC = true
        }
    }

    @IBAction func unwindToMain(segue:UIStoryboardSegue) {
    }

}

The rest is in the storyboard. If this little proof-of-concept is working, you can go and configure the segues (you might want to remove the animation) etc.