Latest 0.5.0
License MIT
Platforms ios 8.0, osx 10.9, watchos 2.0, tvos 9.0

Build Status
Swift 3.1

RuntimeKit is a Swift library for accessing the Objective-C runtime.
In addition to providing wrappers around features like method swizzling or associated values, it also provides some type safety.

As of right now, RuntimeKit supports method swizzling, replacing a methods implementation with a block, setting associated objects, looking up an object’s properties, adding new methods to existing classes and creating new classes and protocols. More features will be added over time.


CocoaPods (recommended)

pod 'RuntimeKit'

Build from source

Just copy everything in RuntimeKit/Sources into your Xcode project


  1. Method swizzling
  2. Associated objects
  3. Creating new classes
  4. Creating new protocols
  5. Performing ObjC Selectors w/ some additional type safety

Method Swizzling

Switching two method’s implementations

extension NSDate {
    func xxx_addingTimeInterval(_ ti: TimeInterval) -> NSDate {
        print("will add (ti)")
        return self.xxx_addingTimeInterval(ti)

try! NSDate.swizzle(#selector(NSDate.addingTimeInterval(_:)), with: #selector(NSDate.xxx_addingTimeInterval(_:)))

NSDate().addingTimeInterval(100) // prints "will add 100"

Replacing a method’s implementation with a block

let systemFontOfSize_block: @convention(block) (UIFont, Selector, CGFloat) -> UIFont = { (self, sel, size) in
    return UIFont(name: "Avenir-Book", size: size)!

try! UIFont.replace(#selector(UIFont.systemFont(ofSize:)), withBlock: systemFontOfSize_block, methodType: .class)

let systemFont = UIFont.systemFont(ofSize: 15) // -> <UICTFont: 0x7fddc5703150> font-family: "Avenir-Book"; font-weight: normal; font-style: normal; font-size: 15.00pt>

Associated objects

extension AssociatedObjectKeys {
    static let name = AssociatedObjectKey<String>("name")
    static let age  = AssociatedObjectKey<Int>("age")

class Person: NSObject {}

let me = Person()
me.setAssociatedObject("Lukas", forKey: .name)
me.setAssociatedObject(18, forKey: .age)

let name = me.getAssociatedObject(forKey: .name) // Optional("Lukas")
let age  = me.getAssociatedObject(forKey: .age)  // Optional(18)

Creating new classes

let LKGreeter = try! Runtime.createClass("LKGreeter")

let greetMethod_block: @convention(block) (NSObject, Selector, String) -> String = { (_self, _sel, name) in
    return "Hello, (name)"

try! LKGreeter.addMethod("greet", implementation: greetMethod_block, methodType: .class, returnType: .object, argumentTypes: [.object, .selector, .object])

LKGreeter.perform("greet", with: "Lukas").takeRetainedValue() // result: "Hello, Lukas"

Creating new protocols

let methods = [
    ObjCMethodDescription("greet", returnType: .object, argumentTypes: [.object, .selector, .object], methodType: .class, isRequired: true)

let GreeterProtocol = try! Runtime.createProtocol("Greeter", methods: methods)
let LKGreeter = try! Runtime.createClass("LKGreeter", superclass: NSObject.self, protocols: [GreeterProtocol])

Performing Objective-C Selectors with some additional type safety

let formatBlock: @convention(block) (NSDate, Selector, String) -> String = { (_self, _sel, format) in
    let formatter = DateFormatter()
    formatter.dateFormat = format
    return formatter.string(from: _self as Date)
try! NSDate.addMethod("customFormat:", implementation: formatBlock, methodType: .instance, returnType: .object, argumentTypes: [.object, .selector, .object])

extension ObjCMethodCallRequests {
    static let customFormat = ObjCMethodCallRequest<String>("customFormat:")

let now = NSDate()
let formatString: String = try! now.perform(.customFormat, "EEEE MMM d, yyyy") // -> "Saturday Apr 1, 2017"

Latest podspec

    "name": "RuntimeKit",
    "version": "0.5.0",
    "summary": "Swift wrapper around the Objective-C runtime APIs",
    "description": "RuntimeKit is a Swift library for accessing the Objective-C runtime.nIn addition to providing wrappers around features like method swizzling, adding new methods to classes, creating new classes or protocols, or associated values, it also provides more type safety.",
    "homepage": "",
    "license": {
        "type": "MIT",
        "file": "License"
    "authors": {
        "Lukas Kollmer": "[email protected]"
    "source": {
        "git": "",
        "tag": "0.5.0"
    "social_media_url": "",
    "platforms": {
        "ios": "8.0",
        "osx": "10.9",
        "watchos": "2.0",
        "tvos": "9.0"
    "source_files": "RuntimeKit/Sources/**/*",
    "pushed_with_swift_version": "3.0"

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