How to Cast a Metaclass Object to a Protocol Type in Swift

Can I cast a metaclass object to a protocol type in Swift?

As of Xcode 7 beta 2 and Swift 2 it has been fixed. You can now write:

for type in types {
    if let fooType = type as? Foo.Type {
        // in Swift 2 you have to explicitly call the initializer of metatypes
        let obj = fooType.init(foo: "special snowflake string")
    }
}

Or if you only want type as type Foo.Type you can use for case

for case let type as Foo.Type in types {
    let obj = type.init(foo: "special snowflake string")
}

Forced to cast, even if protocol requires given type

Adopting the protocol Fooable tells the compiler that this particular UIViewController responds to foo(), no less no more.

In reverse conclusion Fooable does not become UIViewController necessarily.

The constraint Self: UIViewController is just another information for the compiler to complain at compile time if the affected class is not UIViewController

In your case when annotating SampleViewController to Fooable the compiler knows only that SampleViewController responds to foo(). It does not know that the type is actually a subclass of UIViewController.

So do not annotate a concrete class to a protocol if you want to access properties of the concrete class.

However you could add the show method and other common properties / methods to the protocol

protocol Fooable: class where Self: UIViewController {
    func foo()
    func show(_ vc: Fooable, sender: Any?)
}

then you can use Fooable because the compiler knows that the type adopting the protocol responds to the method.

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A suitable practice to annotate a type to a protocol is for example when you are going to create a heterogenous but restricted collection type

let array : [CustomStringConvertible] = ["Foo", 1, false]
array.forEach{ print("\($0)")}

The code prints the three items using the description property which all items respond to. The compiler recognizes the three items as types which have a description property, not as String, Int and Bool.

Update:

In Swift 5 support of superclass constrained protocols is implemented.

Could not cast value of type - initialising with protocol (Swift)

Make variable presenter lazy or initialize it after viewcontroller initialization. This error might be because its trying to type cast viewcontroller object before getting initalized.

private lazy var presenter: WirelessSpeakersPresenter = {
        let p = WirelessSpeakersPresenter(view: self as! WirelessSpeakersView)
        return p
    }()

Instantiate class from protocol type

I usually do that by defining a generic method. Try this:

class func createClass<T: CreatableClass>(classType: T.Type) -> CreatableClass {
    return classType()
}

Update

A possible workaround is to pass a closure creating a class instance, rather than passing its type:

class ClassCreator {
    class func createClass(instantiator: () -> CreatableClass) -> (CreatableClass, CreatableClass.Type) {
        let instance = instantiator()
        let classType = instance.dynamicType

        return (instance, classType)
    }
}

let ret = ClassCreator.createClass { ExampleClass() }

The advantage in this case is that you can store the closure in a dictionary for example, and create more instances on demand by just knowing the key (which is something in 1:1 relationship with the class name).

I used that method in a tiny dependency injection framework I developed months ago, which I realized it works only for @objc-compatible classes only though, making it not usable for my needs...

Type cast an instance of a class

Type cast operator (as? or as!) is compile time operator. Type of any variable in Swift can't be dynamic as you intend to achieve with your cast.

You can't easily avoid if let, guard etc. In your your case you can do an explicit cast (as!), if you know that view controller at particular index never changes. But it is not recommended approach, usually you should tend to avoid using explicit casting.

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