AnyObject vs Any vs NSObject vs id — the difference

Middle PRO

Four levels of "anything"

• Any — any Swift type: struct, enum, class, function, Optional, tuple. An existential container; to do anything you need as?.
• AnyObject — any class instance (reference type). A class-bound protocol; used for weak references to "something", for identity ===, in generic constraints T: AnyObject. On Apple platforms AnyObject supports @objc calls through the optional syntax: obj.perform(...), (obj as AnyObject).description.
• AnyClass — AnyObject.Type, "any class as a value" (a metatype): let cls: AnyClass = UIView.self, NSClassFromString.
• NSObject — the concrete root class of Foundation. Provides isa, KVC/KVO, isEqual/hash, description, NSCoding, #selector, compatibility with Cocoa APIs. Not to be confused with the protocol NSObjectProtocol (NSObject in Obj-C), which describes the same contract for NSProxy and AnyObject existentials.
• id (Obj-C) — a pointer to any Obj-C object. Imported into Swift as Any, not AnyObject (SE-0116, Swift 3): an id can hold a String, Int, array — Swift bridges them to NSString, NSNumber, NSArray automatically, so the only honest type is Any.

import Foundation

final class Box: NSObject {
    @objc dynamic var value = 1
}

let anything: [Any] = [1, "two", Box(), { (x: Int) in x }]
let objects: [AnyObject] = [Box(), NSString(string: "s")]
let cls: AnyClass = Box.self
let legacy: Any = NSNumber(value: 3)   // this is how id arrives from Obj-C

func describe(_ v: Any) -> String {
    switch v {
    case let n as Int: return "Int \(n)"
    case let s as String: return "String \(s)"
    case let o as NSObject: return "NSObject \(o)"
    default: return "other"
    }
}
print(describe(legacy))   // Int 3 — NSNumber bridges to Int
_ = (anything, objects, cls)