Swift protocol to only implemented by specific classes

Question

I want to create a protocol which is only adopted by a specific class and its subClassses in swift. I know i can use protocol extensions like this

  protocol PeopleProtocol: class {
   }

   extension PeopleProtocol where Self: People {
   }

But the method that will go in my protocol will be an init method which will be implemented by a class or its subClasess and will return only some specific type of objects.

some thing like this.

protocol PeopleProtocol: class {
   init() -> People
}

or i can do some thing like this

extension PeopleProtocol where Self : People {
   init()
}

But there are two problems,

In the first approach if i put an init method in the protocol it don't allow me to put a return statement there like -> People in the first approach.
In the second approach i have to provide a function body in the protocol extensions, so this thing will be out of question, as i don't know what specific type to return for this general implementation.

So any suggestions how i can call an init method and do either:

Let the protocol (not protocol extension) to be implemented by only specific classe and its subClasses.
Or return an instance of a certain from protocol extension method without giving its body.

True, thats why i want to add a restriction at first place that this method will only be implemented by specific class type. — Madu, Mar 24 '17 at 21:28
sorry didn't get you, what do you men by **use `Self` in the body rather than the super class the protocol is restricted to**. — Madu, Mar 24 '17 at 21:37

Alain T. · Accepted Answer · 2018-07-22T21:36:06.247

You could add a required method that you only extend for the appropriate classes.

for example:

protocol PeopleProtocol
{
  var conformsToPeopleProtocol:Bool { get }
}

extension PeopleProtocol where Self:People
{
  var conformsToPeopleProtocol:Bool {return true}
}

class People  
{}

class Neighbours:People 
{}

extension Neighbours:PeopleProtocol // this works 
{}

class Doctors:People,PeopleProtocol // this also works
{}

class Dogs:PeopleProtocol // this will not compile
{}

This could easily be circumvented by a programmer who would want to, but at least it will let the compiler warn you if you try to apply the protocol to other classes.

Swift protocol to only implemented by specific classes

1 Answers1