Reactant UI

Important Note

Reactant UI is currently a preview. However we’ll try to keep the number of API changes to a minimum.

What is Reactant UI

Reactant UI is an extension for Reactant allowing you to declare views and layout using XML. Don’t worry, there’s no runtime overhead, as all those declarations are precompiled into Swift. Reactant then uses the generated code to create your UI.


When we created Reactant, our primary goal was maximal compile-time safety. We devised an API for writing views using Reactant, but it was still constrained by Swift’s syntax.


Reactant UI is available through CocoaPods. To install it, add the following line to your application target in your Podfile:

pod 'ReactantUI'

This will add Reactant UI to your project, but we’re not done yet. Open your project in Xcode and open Build Phases and create a new Run Script phase (+ button in top left corner -> New Run Script Phase). Name the phase (for example Generate Reactant UI) and move it just above phase named Compile Sources (n items). Now add the following into the newly created phase.

pushd "$PODS_ROOT/ReactantUI"
env -i HOME="$HOME" PATH="$PATH" swift build
# Go into directory where you'll have `.ui.xml` files.
cd "$SRCROOT/Application/Source"
# Run reactant-ui generator and save the output to a single `.swift` file
"$PODS_ROOT/ReactantUI/.build/debug/reactant-ui" > "$SRCROOT/Application/Generated/GeneratedUI.swift"

We recommend you to download Atom editor and install linter-autocomplete-jing and **xml-common-schemata plugins. This combination will give you auto-complete support for UI XML files.


Reactant UI loads view declarations from XML files ending with .ui.xml. The location of these files is up to you, but we recommend putting them alongside your .swift view files. In Reactant architecture guide we made a very simple, single-screen application with one input and one textfield. Let’s now recreate the GreeterRootView using Reactant UI.

We begin with creating a new file named GreeterRootView.ui.xml, adding a root element called Component and setting namespace attributes to enable autocompletion.

<?xml version="1.0" encoding="UTF-8" ?>

NOTE: Once we release Reactant UI beta, we will freeze schema definitions and release plugin for Atom that will allow omission of xmlns:xsi and xsi:schemaLocation attributes. This will result in simpler declarations.

By default, the name of the file is used as the type name. However, you can override this behavior. To do so, add type attribute to the Component element with the name of your desired type.

You might want to create all .ui.xml at once before writing their Swift counterparts (especially if you use Live Reload). Unfortunately, Reactant UI in its current version doesn’t scan your Swift files for existing types and generates Swift extensions for every .ui.xml in your project. If you don’t have matching classes, you’ll get compile errors. The current workaround is setting a value of anonymous attribute to true on the Component element. Reactant UI then generates an empty class which you can then use in your code. Later on you would replace it with your own class and remove the anonymous attribute.

Last attribute to keep an eye on for Component element is rootView. When you set it to true, Reactant UI will automatically add the RootView protocol conformity and allow you to specify extend attribute. This attribute sets the RootView’s edgesForExtendedLayout which has similar behavior to UIViewController#edgesForExtendedLayout

Let’s go ahead and add the two children our GreeterRootView should have, a label and a text field.

<?xml version="1.0" encoding="UTF-8" ?>

        text="Hello World"
        layout:right="super" />

        placeholder="Name to greet"
        layout:below="greeting offset(10)"
        layout:height="50" />

Each view in the hierarchy has its own properties and we tried to keep same names they have in UIKit. In our example, we set the text attribute for Label element to setup a label with that text. The same applies to the textColor and placeholder attributes.

When you need to create a connection between your Swift code and the XML, set the field attribute. This attribute tells Reactant UI to use field with the specified name on the view. Make sure that field (property) has an internal visibility and that the type matches. We also recommend that the field is a constant (let property) because it limits mutability even further.

Last but not least, layout attributes. You will use these to declare AutoLayout constraints for your views. Each attribute from the layout namespace accepts a constraint declaration with the following format:

[constraintField = ][>=|<=|==] target[.targetAnchor] [modifier...][ @priority]

Parts of the declaration should be familiar to you if you use AutoLayout.

  • constraintField - makes the created constraint accessible in your Swift code
  • >=|<=|== - relation of the constraint
  • target - The other view to constraint to. Possible values are:
    • id:{name} - target a view using layout:id attribute of such view
    • {name} - target a view using field attribute of such view
    • super - parent container (equal to UIView#superview)
    • self - this view, mostly useful for constraining width and height
  • targetAnchor - an anchor on the target view to constraint to. Defaults
  • modifier
    • Supported modifiers
      • offset(value) - constant space between two views
      • inset(value) - constant space between parent and its child view (inside)
      • multiplied(by: value) - multiplier for the constraint
      • divided(by: value) - same function as a multiplier, but using it as 1/value
    • value is any Float number
  • priority
    • one of required, high, medium, low
    • any float ranging from 1 to 1000

To complete the example we need to write the Swift part of the GreeterRootView. We basically just remove the loadView and setupConstraints methods, make the greeting and nameField properties internal and remove the RootView protocol conformance. And this is what we’ll get:

import Reactant

final class GreeterRootView: ViewBase<(greeting: String, name: String), GreeterAction> {
    override var actions: [Observable<GreeterAction>] {
        return [

    let greeting = UILabel()
    let nameField = UITextField()

    override func update() {
        greeting.text = "Hello \(componentState)!"
        nameField.componentState = componentState

As you can see, when we use the XML to declare views, our Swift code will contain only the code that decides what should be done with componentState in the update() method and it produces actions for owners to handle. At the same time the XML gives us much cleaner UI code as you can see the view’s position in the layout and its properties at one place.

This split to XML also allowed us to implement a live UI reloading. It’s something that will save quite a lot of development time as you can see every change in the UI without recompilation and redeployment.

Learn more about Reactant UI’s Live Reload capabilities