Home > Uncategorized > Understanding #protocols in #swift

Understanding #protocols in #swift


Protocols are the Swift / Objective C name for Interfaces, as they would be known in .NET , C++ or Java. What they basically say, is that any object that is of this type will definitely handle a particular function call.

A really common thing to do in Swift is to pass data between two viewControllers. When you segue between them, then you get a reference to the destination viewController, and you can call methods and set properties on the destination viewController in order to pass data.

The issue is, that if you dismiss a view Controller, you are no longer seguing – and therefore have no reference to the destination view Controller in order to pass any data.

The trick is on the first view Controller, you set a reference to self (the first view controller) on the second view controller. Then, just before the second view controller dismisses itself, it can call methods and set properties on the reference that was set previously, in order to pass it’s data back.

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
let dest = segue.destination as! SecondViewController
dest.delegate = self
dest.textPassedViaSegue = “Hello World”

Now, this reference (delegate) can be of type FirstViewController, and that’s 100% fine. but you are then making the assumption that the only way to get to the SecondViewController is via the First one,  maybe, you may have a third view controller somewhere? – You are also perhaps exposing too much functionality when it’s not really needed.

So, instead of setting the reference to be of type FirstViewController, you could define a protocol (Interface) as follows;

protocol Callable
func calledFromOtherViewController(text : String);

And ensure that your first view controller implements this protocol (Interface) like so;

class FirstViewController: UIViewController, Callable {

func calledFromOtherViewController(text : String)

Now, the reference on the Second View Controller can be of type Callable?

// delegate can defined as type FirstViewController
// but this is less flexible, in the case that
// this could be returning to multiple possible view controllers
var delegate : Callable? = nil

To see an example of this in action, you can clone this project on GitHub


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