Declare array of classes that conform to a protocol

Question

Lets say I have created this protocol and a couple of classes

import UIKit

protocol ControllerConstructorProtocol {
    class func construct() -> UIViewC         


        

Answer 1

"array of classes that conform to a protocol" can be declared like Array<TheProtocol.Type>.

You can:

var array: Array<ControllerConstructorProtocol.Type> = [
    MyConstructor.self,
    MyOtherConstructor.self,
]

But...,

    array[0].construct()
//  ^ error: accessing members of protocol type value 'ControllerConstructorProtocol.Type' is unimplemented

Calling method on the item is "unimplemented".

As of now, you have to declare the protocol as @objc, and call the method via AnyClass. Moreover, for some reasons, we cannot directly cast array[0] to AnyClass, instead, we have to cast it to Any, then AnyClass.

@objc protocol ControllerConstructorProtocol {
    class func construct() -> UIViewController?
}

var array: Array<ControllerConstructorProtocol.Type> = [
    MyConstructor.self,
    MyOtherConstructor.self,
]

let vc = (array[0] as Any as AnyClass).construct()

Note: Casting problem was fixed in Swift 1.2 / Xcode 6.3. But "unimplemented" is "unimplmented" :(

---

Just random ideas:

It's depends on your actual use-case, but in this particular case, array of ()-> UIViewController? closures is sufficient:

var array: [() -> UIViewController?] = [
    MyConstructor.construct,
    MyOtherConstructor.construct,
]

let vc = array[0]()

If you have several methods, you might want to use type-erased wrapper of the protocol.

protocol ControllerConstructorProtocol {
    class func construct() -> UIViewController?
    class func whoami() -> String
}

struct ControllerConstructorWrapper {
    private let _construct: () -> UIViewController?
    private let _whoami: () -> String
    init<T: ControllerConstructorProtocol>(_ t:T.Type) {
        _construct = { t.construct() }
        _whoami = { t.whoami() }
    }
    func construct() -> UIViewController? { return _construct() }
    func whoami() -> String { return _whoami() }
}

var array: [ControllerConstructorWrapper] = [
    ControllerConstructorWrapper(MyConstructor),
    ControllerConstructorWrapper(MyOtherConstructor),
]

let who = array[0].whoami()
let vc = array[0].construct()

Answer 2

If you really want to sotre classes only (that conforms to protocol) then you can do it the following way (in Swift 3):

If you want to create a new instance with the protocol type you have to add init() to protocol declaration:

protocol SomeProtocol: ConformsToOtherProtocolIfNeeded {
    init(...) { ... }
    func someFunction(...) { ... }
}

class Class1: SomeProtocol {
    init(...) { ... }
    func someFunction(...) { ... }
}

class Class2: SomeProtocol {
    init(...) { ... }
    func someFunction(...) { ... }
}

declare array (as above):

var someProtocols: Array<SomeProtocol.Type> = [
    Class1.self,
    Class2.self,
]

and if you want to use someFunction, you have to create an instance, because the elements in the array not an instance. Example:

for sp in someProtocols {
    let instance = sp.init()
    instance.someFunction()
}

If you want to compare the type of class, you also have to create an instance. (So you cannot use item of array directly (sp).)

Examples:

if type(of: instance) == type(of: otherInstance) { ... }
if instance is SomeProtocol { ... }
if instance is Class1 { ... }

Answer 3

Not sure if I got your question completely, but why not this way:

var array: [ControllerConstructorProtocol] = [MyConstructor(), MyOtherConstructor()]

Answer 4

Try this:

var myConst = MyConstructor()
var myOthConst = MyOtherConstructor()

var array:[AnyObject] = [
    myConst,
    myOthConst
]

for a in array
{
    if a is MyConstructor
    {
        println("a is type of MyConstructor")
        (a as! MyConstructor).myMethod()
    }
    else if a is MyOtherConstructor
    {
        println("a is type of MyOtherConstructor")
        (a as! MyOtherConstructor).myMethod()
    }
}

Another solution, although not that pretty...