How passing a protocol as parameter in Swift

Question

In Objective-C, I know how passing a protocol as parameter:

- (void)MyMethod:(Protocol *)myparameter

But in Swift there is no more Protocol type.

How can I pass a protocol as parameter without knowing which is ?

Answer 1

In one of your comments you say:

"I want create a method which return an array of type of class which implements a desired protocol."

Have you tried something like the following:

//notice the use of @objc here
@objc protocol AlertProtocol
{
    func getMyName()->String
}

class Class1 : AlertProtocol
{
     let name = "Object 1"
     func getMyName() -> String
    {
        return name
    }
}

class Class2 : AlertProtocol
{
    let name = "Object 2"
    func getMyName() -> String
    {
        return name
    }
}

//borrowing from and refactoring siLo's answer
func classesConformingToProtocol(proto:Protocol) -> [AnyClass]
{
    let availableClasses : [AnyClass] = [ Class1.self, Class2.self ]

    var conformingClasses = Array<AnyClass>()

    for myClass : AnyClass in availableClasses
    {
        if myClass.conforms(to: proto)
        {
            conformingClasses.append(myClass)
        }
    }

    return conformingClasses
}

Then use the above structure like this:

let classes = classesConformingToProtocol(AlertProtocol.self)

The tricky part that does the work is the "@objc" that exposes the protocol to the objective c runtime and allows us to pass any "Protocol Type" as a parameter.

Probably at some point in the future we will be able to do this in a "pure" Swift way.

Answer 2

Here is what I have tried:

@objc protocol Walker
{
    func walk()
}

@objc protocol Runner
{
    func run()
}

@objc class Zombie : Walker
{
    func walk () { println("Brains...") }
}

@objc class Survivor : Runner
{
    func run() { println("Aaaah, zombies!") }
}

func classesConformingToProtocol(proto:Protocol) -> AnyClass[]
{
    let availableClasses : AnyClass[] = [ Zombie.self, Survivor.self ]

    var conformingClasses = Array<AnyClass>()

    for myClass : AnyClass in availableClasses
    {
        if myClass.conformsToProtocol(proto)
        {
            conformingClasses.append(myClass)
        }
    }

    return conformingClasses
}

// This does not work
let walkers = classesConformingToProtocol(Walker.self)
let runners = classesConformingToProtocol(Runner.self)

I have been unable to convert Swift's Metatype information into a Protocol object.