Finding All Types That Conform to a Protocol

In Swift, can you find all types in a module that adhere to a specific protocol?

I don't believe Swift currently has a 'native' (not dependant on the Objective-C runtime) API for doing this kind of reflection.

However, if you're on an Apple platform (and therefore have Obj-C interoperability), you could get a list of all classes registered with the Obj-C runtime, and then filter those that conform to a given protocol. This will work for Swift classes (even those that don't inherit from NSObject) because under the hood Swift classes are built on top of Obj-C classes (when there's Obj-C interop).

Because this will only work for classes (not structures or enumerations), you may want to restrict your protocol such that only classes can conform to it:

protocol Favorite : class {}

You can then do the following, using objc_copyClassList:

import Foundation

protocol Animal {}
protocol Vehicle {}
protocol Favorite : class {}

class Dog : Animal {}
class Cat : Animal, Favorite {}
class Car : Vehicle {}
class Bicycle : Vehicle, Favorite {}

/// Invokes a given closure with a buffer containing all metaclasses known to the Obj-C
/// runtime. The buffer is only valid for the duration of the closure call.
func withAllClasses<R>(
  _ body: (UnsafeBufferPointer<AnyClass>) throws -> R
) rethrows -> R {

  var count: UInt32 = 0
  let classListPtr = objc_copyClassList(&count)
  defer {
    free(UnsafeMutableRawPointer(classListPtr))
  }
  let classListBuffer = UnsafeBufferPointer(
    start: classListPtr, count: Int(count)
  )

  return try body(classListBuffer)
}
//                               .flatMap in Swift < 4.1
let classes = withAllClasses { $0.compactMap { $0 as? Favorite.Type } }
print(classes) // [Bicycle, Cat]

Here we're calling compactMap(_:) on the UnsafeBufferPointer to get back an array of metatypes that represent types conforming to Favorite (i.e those that can be cast to the existential metatype Favorite.Type).

Checking if Any.Type conforms to a protocol in Swift

To check the pure Swift code, you can do:

protocol MySwiftProtocol: AnyObject {
}

class MySwiftClass: MySwiftProtocol {
}

if MySwiftClass.self as? MySwiftProtocol.Type != nil {
    print("conforms")
} else {
    print("does not conform")
}

or more simply:

if MySwiftClass.self is MySwiftProtocol.Type {

How to list all classes conforming to protocol in Swift?

Since you're using the Objective-C runtime to get the type introspection you need to add @objc to your code in this manner:

@objc protocol Animal {
  func speak()
}

class Cat:Animal {
  @objc func speak() {
    print("meow")
  }
}

class Dog: Animal {
  @objc func speak() {
    print("Av Av!")
  }
}

class Horse: Animal {
  @objc func speak() {
    print("Hurrrr")
  }
}

Note that this kind of type introspection might be very slow.

How do I get list of all classes conform to certain protocol in Xcode 4?

Use the Objective-C runtime functions.

• objc_getClassList to get the list of Classes
• class_getSuperclass or the -superclass method to walk the superclass chain
• class_conformsToProtocol or the -conformsToProtocol: method to check if a class conforms to a protocol

How do I determine what protocols a class conforms to in swift?

I don't believe you can do it without leaning on the Objective-C runtime. Import ObjectiveC and use the class_copyProtocolList() function.

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