A small Objective-C-like simulation in Swift.

ObjcSimulator.swift

//
//  main.swift
//
//  Created by Tanner Bennett on 10/19/17.
//  Copyright © 2017 Tanner Bennett. All rights reserved.
//

import Foundation

// For shorthand casting
prefix operator ~
@inline(__always) public prefix func ~<T,U>(thing: T) -> U {
    return thing as! U
}

typealias SEL = String
typealias IMP = (_ self: Foo, _ _cmd: SEL, _ args: Any) -> Any

/// Dynamically calls a method on a `Foo` instance given a method name (like objc_msgSend)
func msgSend<T>(_ target: Foo, _ _cmd: SEL, _ args: Any = ()) -> T {
    let imp = Foo.methodLookup[_cmd]!
    return ~imp(target, _cmd, args)
}

/// Convenience for `Void`
@inline(__always) func msgSend(_ target: Foo, _ _cmd: SEL, _ args: Any = ()) {
    // Using `Any` instead of `Void` ensures that the correct msgSend is called
    let _: Any = msgSend(target, _cmd, args)
}

struct Foo {
    let x: Int

    static let methodLookup: [SEL: IMP] = [
        "bar::": Foo.bar,
        "baz": Foo.baz
    ]

    static var bar: IMP = { (this: Foo, _cmd: SEL, args: Any) -> Any in
        // Layer of indirection with actual type signature
        func bar$(_ this: Foo, _ _cmd: SEL, arg1: Int, arg2: Int) -> Int {
            print("calling \(_cmd) on \(this) with \(arg1) and \(arg2)")
            return 5
        }

        // Cast out arguments, call method
        let args = (args as! (Int, Int))
        return bar$(this, _cmd, arg1: args.0, arg2: args.1)
    }

    static var baz: IMP = { (this: Foo, _cmd: SEL, args: Any) -> Any in
        // Doesn't return anything, still works
        func baz$(_ this: Foo, _ _cmd: SEL) {
            print("calling \(_cmd) on \(this)")
        }

        baz$(this, _cmd)
        return ()
    }
}

// Prints: calling bar:: on Foo(x: 2) with 2 and 3
let bar: Int = msgSend(Foo(x: 2), "bar::", (2, 3))
// Prints: 5
print(bar)

// Prints: calling baz on Foo(x: 2)
msgSend(Foo(x: 2), "baz")