Check if class conforms to protocol

Question

I am trying to make a simple Dependency Injection system for our app in swift, for 2 day now. I'm flexible to whatever solution but I would like something so I can say "give me a instance of something that conforms to this protocol" and the actual type returned can be whatever as long as it conforms to the said protocol. I have tried a great many thing including generics but managed to figure out that that can not(?) really work so now I'm down to the bare basics, something like this:

protocol AProtocol {

}

class AClass: AProtocol {

} 

class MyDiThing {
    public static func objectConformingTo(aProtocol: Any) -> Any? {
        // And here I want to do something like
        if AClass is aProtocol {
            return AClass()
        }
        return nil
    }
}

// The calling code ..
let aObject = MyDIThing.objectConformingTo(AProtocol)

It's not beautiful, I know, but right now i'm not that picky about performance/bad code as long as it solves the decoupling problem (and preferably can be contained in the MyDIThing class). If this is impossible in swift I'm open to other solutions. I have used similar solutions with objective-c with good success, just having a dictionary with keys being NSStringFromProtocol and values being the class, subscripting the dictionary with the inbound protocol and instantiating the class. Super simple. In swift it feels impossible!

Answer 1

The comment given by nhgrif is the correct answer for Swift 2. You should use an optional binding:

if let aObjectWithAProtocol = aObject as? AProtocol {
    // object conforms to protocol
    someObject.someFunction(aObjectWithAProtocol)
} else {
    // object does not conform to protocol
}

This if let something = obj as? type statement is called optional binding and checks if the object can be type-casted to the given type/class/protocol/....

If so, you can use the optional (as?) or force unwrap (as!) the object.

Answer 2

If you import obj-c then you can do something like you used to.

Otherwise, it's hard because protocols don't exist in the same way. Consider a registration based system for your factory. Each of your classes would register themselves by supplying a function or closure that can be called to return a new instance of that class, and the registration is against a string or some other type of identifier. This is where it would be good to have a protocol type, but in obj-c you were really doing the same thing with a string conversion. You could register against anything that is Equatable to keep things very generic.

Answer 3

This function will perfectly meet your requirement:

bool _swift_typeIsTargetType(id sourceType, id targetType)

It can check whether a swift type is the target type (is same type or subclass, or conforms to the target protocol), works like is operator in swift. But it's for a type, not for an instance:

let sourceType: Any.Type = type(of: self)
let targetType: Any.Type = AnyProtocol.self
let result = _swift_typeIsTargetType(sourceType, targetType)

It uses private APIs in libswiftCore.dylib, See swift source code Casting.cpp:

bool _conformsToProtocols(const OpaqueValue *value, const Metadata *type, const ExistentialTypeMetadata *existentialType, const WitnessTable **conformances)

Optional binding is not same. It can also check conformance, but you can't dynamically give the type to check:

let targetType: Any.Type = AnyProtocol.self
//error: use of undeclared type: 'targetType'
if let _ = aObject as? targetType {
    // object conforms to protocol
} else {
    // object does not conform to protocol
}

This function is used in ZIKRouter. It's a module router and also a Dependency Injection framework. It uses protocol to discover modules and inject dependencies. You can try it if you want to do something like 'finding a module conforming to some protocol'.