UX Reactive Expressions

UX Markup offers a simple, powerful and extensible reactive expression syntax that can be used on any property on any object. This document explains how it works.

Note that UX expressions is an experimental feature and is subject to change. If you're using this in UX markup, please keep an eye on the changelog when updating. If you're using this feature from Uno, be aware that several large API changes are planned.

Quick Summary

UX Markup's Reactive Expressions allow you to combine literals, resources and all supported bindings into expressions using operators and functions. These expressions update on a push-based, automatic basis if any of the bindings produce a new value.

In the following example, a slider controls the position of a rectangle.

<Rectangle X="spring({Property slider.Value})*30% + 50%" Alignment="TopLeft" Width="32" Height="32" />
<Slider ux:Name="slider" Minimum="-1" Maximum="1" />

Note in particular the expression for the X-offset of the rectangle:

spring({Property slider.Value})*30% + 50%

This means that the rectangle will be offset by 50%, plus a value that ranges between -30% and 30% through a spring physics function.


Bindings are objects that listen for changes in the environment and produces a new value that feeds into the UX expression. All binding types can be used as part of expressions:

  • Data bindings ({path.to.data})
  • Property bindings ({Property someObject.SomeProperty} or {Property SomeProperty})
  • Resource bindings ({Resource resourceKey})

Note that inline expressions ({= expression}) also use the curly brace syntax, yet these are technically not bindings.


UX expressions can add (+), subtract (-), multiply (*) and divide (/) any scalar or vector value, with or without units (such as % and px).

<Panel Width="{foo} * 100% + 40%" />
<Slider Margin="{foo}" />

Arithmetic can only be performed between values of the same unit, or between one value with unit and one value without unit. You can not do 10% + 10px, for example. This will give a runtime error.


Vectors (up to 4 components) can be constructed using the comma , operator:

<Panel Margin="10, {spacing}, 10, {spacing} / 2" />

Vectors will also be automatically expanded to the appropriate size from a shorter vector, according to the following rules: X -> XXXX, XY -> XYXY and XYZ -> XYZ1.

<Panel Margin="{spacing}" />

Strings and text

String properties (such as <Text Value="") are parsed differently from other properties. A string property is treated as a literal string, except for curly braced operands.

<Text>Hello, {username}!</Text>

Or, equivalently:

<Text Value="Hello, {username}" />

To embed a computed expression inside a string value, you can use the {= expression} syntax, for example:

<Text>Hello, {= toLower({username})}!</Text>


Note: The current set of functions are experimental and will change. Keep an eye on future changelogs.

These functions are currently available:


  • x(element) - returns the X coordinate of an element's position, relative to its parent.
  • y(element) - returns the Y coordinate of an element's position, relative to its parent.
  • width(element) - returns the computed width resulting from layout, in points, of another UX element.
  • height(element) - returns the computed height resulting from layout, in points, of another UX element.


  • toLower(s) - returns a lowercase version of the given string.
  • toUpper(s) - returns the uppercase version of the given string.


  • min(a, b) - returns the minimum of two numeric values (component by component if vector).
  • max(a, b) - returns the maximum of two numeric values (component by component if vector).
  • abs(value) - returns the absolute value of the input number
  • mod(a, b) - returns the remainder after division of a with b
  • fract(value) - returns the fractional part of the input number by removing the integer digits
  • trunc(value) - returns the integer part of the input number by removing any fractional digits
  • odd(value) - returns true if the rounded value is odd, false otherwise
  • even(value) - returns true if the rounded value is even, false otherwise
  • sign(value) - returns the sign of the input number, indicating whether the number is positive, negative or zero
  • round(value) - returns the value of the input number rounded to the nearest integer
  • ceil(value) - returns the smallest integer greater than or equal to the input number
  • floor(value) - returns the largest integer less than or equal to the input number
  • sqrt(value) - returns the square root of the input number
  • pow(a, b) - returns a raised to the power of b
  • exp(value) - returns a number representing e raised to power of value, where e is Euler's number
  • exp2(value) - returns 2 raised to power of value
  • log(value) - returns the natural logarithm (base e) of the input number
  • log2(value) - returns the base 2 logarithm of the input number
  • sin(radians) - returns the sine of the input angle given in radians
  • cos(radians) - returns the cosine of the input angle given in radians
  • tan(radians) - returns the tangent of the input angle given in radians
  • asin(value) - returns the arcsine (in radians) of the input number
  • acos(value) - returns the arccosine (in radians) of the input number
  • atan(value) - returns the arctangent (in radians) of the input number
  • atan2(y, x) - returns the arctangent of the quotient of the arguments
  • degreesToRadians(degrees) - returns the value in radians for the input in degrees
  • radiansToDegrees(radians) - returns the value in degrees for the input in radians


  • attract(value, config) - animates the change in a value by using an AttractorConfig to define the animation style
<AttractorConfig Unit="Points" Easing="SinusoidalInOut" Duration="0.3" ux:Global="asPoints" />
    <Translation X="attract({xOffset}, asPoints)" />
  • alternate(value, groupSize) - alternate between true/false value for ranges of integers
  • spring(value) - performs a spring physics simulation that chases the given value.
  • lerp(from, to, step) - returns the linear interpolation between two values
  • clamp(value, min, max) - returns the value restricted by the range specified
  • index() - returns the index of this item inside Each.Items
  • offsetIndex() - returns the index of this item inside Each.Items, relative to Each.Offset
    var items = [1,2,3,4,5,6,7,8,9,10];
    module.exports = {
        items: items
<StackPanel ItemSpacing="8" Margin="8">
    <Each Items="{items}" Offset="2" Limit="4">
        <Panel ux:Name="item" Height="48">
            <Text Value="value: {}; index: {= index(item)}; offsetIndex: {= offsetIndex(item)}" Alignment="CenterLeft" />

Note: Avoid using layout functions (like width(element), height(element), x(element) or y(element)) as input to layout parameters (like Width, Height, Margin or Padding) . Doing this can cause unwanted or glitchy behavior:

<Panel Width="10" Height="width(this)" />