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Modify your native iOS app in real time.
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Traits - Modify your native iOS app properties in real time.

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What are Traits?

Traits allows you to modify the design and behavior of native iOS apps without having to restart them, even across the globe.

Wouldn't it be great if we could adjust designs of our native apps without having to sit back to Xcode, change code, recompile and navigate back to the screen we were at?

Traits is a library that provides you the ability to change many properties of your apps (not only design) without having to recompile the application, even via network across the globe.

Using it offers many benefits:

  • Immediate feedback
  • Ability to update running applications remotely
  • Requires almost no code changes in your app
  • Both Code and Interface Builder designs are supported
  • Can tweak more than just UI
  • Fully tested
  • Supports code injection and real-time programming


Article describing design and implementation details of this tool


1. Add the library into your project

2. Assign proper view identifiers either via code:

view.traitSpec = "article.containerView"

or via interface builder:

IBInspectable demo

3. [Optional] If you want live-reloading:

While running on simulator you can observe file on your desktop:


While running on device, you can observe remote URL:

TraitsProvider.setupRemoteDaemon(url: myFileURL)


Traits supports extending the library in real-time, without even having to recompile your project and without unsigning your Xcode 8.

Just install Injection app and code away:

Adding new Trait that is specific to your project is really simple:

  1. Create a new subclass of Trait
  2. Declares the target types that are supported.
  3. Provide apply function that returns a reversal closure
  4. Declare data mapping using ObjectMapper
  5. Compile. (or code inject as we support that as well)
final public class Constraint: Trait {
    private(set) var constant: CGFloat?

    open override class var restrictedTypes: [AnyClass]? { return [NSLayoutConstraint.self] }

    open override func apply(to target: Trait.Target, remove: inout RemoveClosure) throws {
        let target = target as! NSLayoutConstraint

        remove = { [weak target, constant = target.constant] in
            guard let target = target else { return }
            target.constant = constant

        if let constant = constant {
            target.constant = constant

    public init(constant: CGFloat) {
        self.constant = constant

    public required init?(map: Map) {
        super.init(map: map)

    open override func mapping(map: Map) {
        super.mapping(map: map)
        constant <- map["constant"]


This library requires a deployment target of iOS 8.0 or greater.


Traits is available through CocoaPods.

To install it, simply add the following line to your Podfile:

pod "Traits"


Created by Krzysztof Zabłocki (twitter | website) during Maker Week at The New York Times

Article describing design and implementation details of this tool


Contributions to Traits are welcomed and encouraged! Please see the Contributing guide.

A list of contributors is available through GitHub.

To give clarity of what is expected of our members, we adopted the code of conduct defined by the Contributor Covenant. This document is used across many open source communities, and we think it articulates my values well. For more, see the Code of Conduct.


Traits is available under the MIT license. See LICENSE for more information.


This tool is powered by

Thank you goes to:

  • Véronique Brossier and Chrys Wu from NYT because they helped me clarify expectations for this library.

Other Libraries / Tools

Make sure to check my other libraries and tools, especially:

  • Sourcery - Tool that introduces type-safe meta-programming for swift, allowing you to avoid boilerplate code.
  • KZPlayground - Powerful playgrounds for Swift and Objective-C.
  • KZFileWatchers - Daemon for observing local and remote file changes, used for building other developer tools (Traits uses it)

You can follow me on twitter for news/updates about other projects I am creating.

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