You can make an existing type conform to a protocol you define later in Swift.

retroactiveProtocols.swift

class OriginalClass {
	var fooProp: Int = 15
	
	func sayHello(_ string: String) {
		print("Hello \(string) \(fooProp)")
	}
}

protocol MockProtocol {
	var fooProp: Int { get }
	
	func sayHello(_ string: String)
}

extension OriginalClass: MockProtocol {}

class MockClass: MockProtocol {
	var fooProp: Int = 42

	func sayHello(_ string: String) {
		print("MOCKED Hello \(string) \(fooProp)")
	}
}

let original: MockProtocol = OriginalClass()
let mocked: MockProtocol = MockClass()

original.sayHello("Roger")
mocked.sayHello("Gracie")