Extend existing protocols to implement another protocol with default implements

Question

Is it possible to add protocol compliance to a different protocol by way of an extension?

For instance we would like A to comply with B:

protocol A {   var a : UIView {get} }  protocol B {   var b : UIView {get} } 

I want to give a default implementation (compliance) of B to objects of type A

// This isn't possible extension A : B {   var b : UIView {     return self.a   } } 

The motivation being to reuse objects of A in cases where a B is required without creating my own "bridge"

class MyClass {   func myFunc(object : A) {     ...     ...     let view = object.a      ... do something with view ...      myFunc(object)      // would like to use an 'A' without creating a 'B'   }    func myFunc2(object : B) {     ...     ...     let view = object.b     ... do something with view ...    } } 

As a side note we can extend a class to implement a protocol

class C {   let C : UIView }  // this will work extension C : B {   var B : UIView {     return self.c   } } 

and protocols can give default implementations

extension A {   // a default implementation   var a : UIView {      return UIView()   } } 

Answer 1

When extending A, you could specify that the type also conforms to B:

extension A where Self: B {     var b : UIView {         return self.a     } } 

Then make your type conform to A and B, e.g.

struct MyStruct : A, B {     var a : UIView {         return UIView()     } } 

Due to the protocol extension, instances of MyStruct will be able to use a and b, even though only a was implemented in MyStruct:

let obj = MyStruct() obj.a obj.b 

Answer 2

You can make A inherits from B:

protocol A: B { var a: String { get } } protocol B    { var b: String { get } }  // Default implementation of property b extension A {     var b: String { get { return "PropertyB" } } }   class MyClass: A {     var a: String { get { return "PropertyA" } }      func printA(obj: A) {         print(obj.a)         printB(obj)     }      func printB(obj: B) {         print(obj.b)     } }  let obj = MyClass() obj.printA(obj) 

Since A inherits from B, every property in B is available in A.