The .slint language reference

The Slint design markup language is used to describe graphical user interfaces:

  • Place and compose a tree of visual elements in a window using a textual representation.

  • Configure the appearance of elements via properties. For example a Text element has font and text properties, while a Rectangle element offers a background color.

  • Assign binding expressions to properties to automatically compute values that depend on other properties.

  • Group binding expressions together with named states and conditions.

  • Declare animations on properties and states to make the user interface feel alive.

  • Build your own re-usable components and share them in .slint module files.

  • Define data structures and models and access them from programming languages.

  • Build highly customized user interfaces with the builtin elements provided.

Slint also comes with a catalog of high-level widgets, that are written in the .slint language.

.slint files

The basic idea is that the .slint files contains one or several components. These components contain a tree of elements. Each declared component can be given a name and re-used under that name as an element later.

Below is an example of components and elements:


MyButton := Text {
    color: black;
    // ...
}

export MyApp := Window {
    preferred-width: 200px;
    preferred-height: 100px;
    Rectangle {
        width: 200px;
        height: 100px;
        background: green;
    }
    MyButton {
        text: "hello";
    }
    MyButton {
        x: 50px;
        text: "world";
    }
}

Here, both MyButton and MyApp are components. Window and Rectangle are built-in elements used by MyApp. MyApp also re-uses the MyButton component.

You can assign a name to the elements using the := syntax in front an element:

MyButton := Text {
    // ...
}

MyApp := Window {
    preferred-width: 200px;
    preferred-height: 100px;

    hello := MyButton {
        text: "hello";
    }
    world := MyButton {
        text: "world";
        x: 50px;
    }
}

The outermost element of a component is always accessible under the name root. The current element can be referred as self. The parent element can be referred as parent. These names are reserved and cannot be used as element names.

Container Components

When creating components, it may sometimes be useful to influence where child elements are placed when they are used. For example, imagine a component that draws a label above whatever element the user places inside:

MyApp := Window {

    BoxWithLabel {
        Text {
            // ...
        }
    }

    // ...
}

Such a BoxWithLabel could be implemented using a layout, but by default child elements like the Text element become children of the BoxWithLabel, when they would have to be somewhere else, inside the layout. For this purpose, you can change the default child placement by using the @children expression inside the element hierarchy of a component:

BoxWithLabel := GridLayout {
    Row {
        Text { text: "label text here"; }
    }
    Row {
        @children
    }
}

MyApp := Window {
    preferred-height: 100px;
    BoxWithLabel {
        Rectangle { background: blue; }
        Rectangle { background: yellow; }
    }
}

Comments

C-style comments are supported:

  • line comments: // means everything to the end of the line is commented.

  • block comments: /* .. */. Note that the blocks comments can be nested, so /* this is a /* single */ comment */

Identifiers

Identifiers can be composed of letter (a-zA-Z), of numbers (0-9), or of the underscore (_) or the dash (-). They cannot start with a number or a dash (but they can start with underscore) The underscores are normalized to dashes. Which means that these two identifiers are the same: foo_bar and foo-bar.

Properties

The elements can have properties. Built-in elements come with common properties such as color or dimensional properties. You can assign values or entire expressions to them:

Example := Window {
    // Simple expression: ends with a semi colon
    width: 42px;
    // or a code block (no semicolon needed)
    height: { 42px }
}

You can also declare your own properties. The properties declared at the top level of a component are public and can be accessed by the component using it as an element, or using the language bindings:

Example := Rectangle {
    // declare a property of type int with the name `my-property`
    property<int> my-property;

    // declare a property with a default value
    property<int> my-second-property: 42;
}

You can annotate the properties with a qualifier that specifies how the property can be read and written:

  • private: The property can only be accessed from within the component.

  • in: The property is an input. It can be set and modified by the user of this component, for example through bindings or by assignment in callbacks. The component can provide a default binding, but it cannot overwrite it by assignment

  • out: An output property that can only be set by the component. It is read-only for the users of the components.

  • in-out (the default): The property can be read and modified by everyone.

Button := Rectangle {
    // This is meant to be set by the user of the component.
    in property <string> text;
    // This property is meant to be read by the user of the component.
    out property <bool> pressed;
    // This property is meant to both be changed by the user and the component itself.
    in-out property <bool> checked;

    // This property is internal to this component.
    private property <bool> has-mouse;
}

Bindings

The expression on the right of a binding is automatically re-evaluated when the expression changes.

In the following example, the text of the button is automatically changed when the button is pressed, because changing the counter property automatically changes the text.

import { Button } from "std-widgets.slint";
Example := Window {
    preferred-width: 50px;
    preferred-height: 50px;
    Button {
        property <int> counter: 3;
        clicked => { counter += 3 }
        text: counter * 2;
    }
}

The re-evaluation happens when the property is queried. Internally, a dependency will be registered for any property accessed while evaluating this binding. When the dependent properties are changed, all the dependent bindings are marked dirty. Callbacks in native code by default do not depend on any properties unless they query a property in the native code.

Two-way Bindings

Using the <=> syntax, one can create two ways binding between properties. These properties are now linked together. The right hand side of the <=> must be a reference to a property of the same type. The type can be omitted in a property declaration to have the type automatically inferred.

Example := Window {
    property<brush> rect-color <=> r.background;
    // it is allowed to omit the type to have it automatically inferred
    property rect-color2 <=> r.background;
    r:= Rectangle {
        width: parent.width;
        height: parent.height;
        background: blue;
    }
}

Types

All properties in elements have a type. The following types are supported:

Type Description
int Signed integral number.
float Signed, 32-bit floating point number. Numbers with a % suffix are automatically divided by 100, so for example 30% is the same as 0.30.
bool boolean whose value can be either true or false.
string UTF-8 encoded, reference counted string.
color RGB color with an alpha channel, with 8 bit precision for each channel. CSS color names as well as the hexadecimal color encodings are supported, such as #RRGGBBAA or #RGB.
brush A brush is a special type that can be either initialized from a color or a gradient specification. See the Colors Section for more information.
physical-length This is an amount of physical pixels. To convert from an integer to a length unit, one can simply multiply by 1px. Or to convert from a length to a float, one can divide by 1phx.
length The type used for x, y, width and height coordinates. Corresponds to a literal like 1px, 1pt, 1in, 1mm, or 1cm. It can be converted to and from length provided the binding is run in a context where there is an access to the device pixel ratio.
duration Type for the duration of animations. A suffix like ms (millisecond) or s (second) is used to indicate the precision.
angle Angle measurement, corresponds to a literal like 90deg, 1.2rad, 0.25turn
easing Property animation allow specifying an easing curve. Valid values are linear (values are interpolated linearly) and the four common cubiz-bezier functions known from CSS: ease, ease_in, ease_in_out, ease_out.
percent Signed, 32-bit floating point number that is interpreted as percentage. Literal number assigned to properties of this type must have a % suffix.
image A reference to an image, can be initialized with the @image-url("...") construct
relative-font-size Relative font size factor that is multiplied with the Window.default-font-size and can be converted to a length.

Please see the language specific API references how these types are mapped to the APIs of the different programming languages.

Structs

Anonymous structs type can be declared with curly braces: { identifier1: type2, identifier1: type2, } The trailing semicolon is optional. They can be initialized with a struct literal: { identifier1: expression1, identifier2: expression2  }

Example := Window {
    property<{name: string, score: int}> player: { name: "Foo", score: 100 };
    property<{a: int, }> foo: { a: 3 };
}

Custom named structures

It is possible to define a named struct using the struct keyword,

export struct Player := {
    name: string,
    score: int,
}

Example := Window {
    property<Player> player: { name: "Foo", score: 100 };
}

Arrays / Model

The type array is using square brackets for example [int] is an array of int. In the runtime, they are basically used as models for the for expression.

Example := Window {
    property<[int]> list-of-int: [1,2,3];
    property<[{a: int, b: string}]> list-of-structs: [{ a: 1, b: "hello" }, {a: 2, b: "world"}];
}
  • length: One can query the length of an array and model using the builtin .length property.

  • array[index]: Individual elements of an array can be retrieved using the array[index] syntax.

Conversions

  • int can be converted implicitly to float and vice-versa

  • int and float can be converted implicitly to string

  • physical-length and length can be converted implicitly to each other only in context where the pixel ratio is known.

  • the units type (length, physical-length, duration, …) cannot be converted to numbers (float or int) but they can be divided by themselves to result in a number. Similarly, a number can be multiplied by one of these unit. The idea is that one would multiply by 1px or divide by 1px to do such conversions

  • The literal 0 can be converted to any of these types that have associated unit.

  • Struct types convert with another struct type if they have the same property names and their types can be converted. The source struct can have either missing properties, or extra properties. But not both.

  • Array generally do not convert between each other. But array literal can be converted if the type does convert.

  • String can be converted to float by using the to-float function. That function returns 0 if the string is not a valid number. you can check with is-float if the string contains a valid number

Example := Window {
    // ok: int converts to string
    property<{a: string, b: int}> prop1: {a: 12, b: 12 };
    // ok even if a is missing, it will just have the default value
    property<{a: string, b: int}> prop2: { b: 12 };
    // ok even if c is too many, it will be discarded
    property<{a: string, b: int}> prop3: { a: "x", b: 12, c: 42 };
    // ERROR: b is missing and c is extra, this does not compile, because it could be a typo.
    // property<{a: string, b: int}> prop4: { a: "x", c: 42 };

    property<string> xxx: "42.1";
    property<float> xxx1: xxx.to-float(); // 42.1
    property<bool> xxx2: xxx.is-float(); // true
}

Relative Lengths

Sometimes it is convenient to express the relationships of length properties in terms of relative percentages. For example the following inner blue rectangle has half the size of the outer green window:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    background: green;
    Rectangle {
        background: blue;
        width: parent.width * 50%;
        height: parent.height * 50%;
    }
}

This pattern of expressing the width or height in percent of the parent’s property with the same name is common. For convenience, a short-hand syntax exists for this scenario:

  • The property is width or height

  • A binding expression evaluates to a percentage.

If these conditions are met, then it is not necessary to specify the parent property, instead you can simply use the percentage. The earlier example then looks like this:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    background: green;
    Rectangle {
        background: blue;
        width: 50%;
        height: 50%;
    }
}

Callback

Components may declare callbacks, that allow it to communicate changes of state to the outside. Callbacks are emitted by “calling” them and you can react to callback emissions by declaring a handler using the => arrow syntax. The built-in TouchArea element comes with a clicked callback, that’s emitted when the user touches the rectangular area covered by the element, or clicks into it with the mouse. In the example below, the emission of that callback is forwarded to another custom callback (hello) by declaring a handler and emitting our custom callback:

Example := Rectangle {
    // declare a callback
    callback hello;

    area := TouchArea {
        // sets a handler with `=>`
        clicked => {
            // emit the callback
            root.hello()
        }
    }
}

It is also possible to add parameters to the callback.

Example := Rectangle {
    // declares a callback
    callback hello(int, string);
    hello(aa, bb) => { /* ... */ }
}

And return value.

Example := Rectangle {
    // declares a callback with a return value
    callback hello(int, int) -> int;
    hello(aa, bb) => { aa + bb }
}

Callback aliases

It is possible to declare callback aliases in a similar way to two-way bindings:

Example := Rectangle {
    callback clicked <=> area.clicked;
    area := TouchArea {}
}

Functions

You can declare helper functions with the function keyword. Functions are private by default, but can be made public with the public annotation.

Example := Rectangle {
    property <int> min;
    property <int> max;
    public function inbound(x: int) -> int {
        return Math.min(root.max, Math.max(root.min, x));
    }
}

Expressions

Expressions are a powerful way to declare relationships and connections in your user interface. They are typically used to combine basic arithmetic with access to properties of other elements. When these properties change, the expression is automatically re-evaluated and a new value is assigned to the property the expression is associated with:

Example := Rectangle {
    // declare a property of type int
    property<int> my-property;

    // This accesses the property
    width: root.my-property * 20px;

}

If something changes my-property, the width will be updated automatically.

Arithmetic in expression with numbers works like in most programming language with the operators *, +, -, /:

Example := Rectangle {
    property <int> p: 1 * 2 + 3 * 4; // same as (1 * 2) + (3 * 4)
}

+ can also be applied with strings to mean concatenation.

There are also the operators && and || for logical and and or between booleans. Comparisons of values of the same types can be done with ==, !=, >, <, => and <=.

You can access properties by addressing the associated element, followed by a . and the property name:

Example := Rectangle {
    foo := Rectangle {
        x: 42px;
    }
    x: foo.x;
}

The ternary operator ... ? ... : ... is also supported, like in C or JavaScript:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    Rectangle {
        touch := TouchArea {}
        background: touch.pressed ? #111 : #eee;
        border-width: 5px;
        border-color: !touch.enabled ? #888
            : touch.pressed ? #aaa
            : #555;
    }
}

Strings

Strings can be used with surrounding quotes: "foo".

Some character can be escaped with slashes (\)

Escape Result
\" "
\\ \
\n new line
\u{xxx} where xxx is an hexadecimal number, this expand to the unicode character represented by this number
\{expression} the expression is evaluated and inserted here

Anything else after a \ is an error.

(TODO: translations: tr!"Hello")

Example := Text {
    text: "hello";
}

Colors and Brushes

Color literals follow the syntax of CSS:

Example := Window {
    background: blue;
    property<color> c1: #ffaaff;
    property<brush> b2: Colors.red;
}

In addition to plain colors, many elements have properties that are of type brush instead of color. A brush is a type that can be either a color or gradient. The brush is then used to fill an element or draw the outline.

CSS Color names are only in scope in expressions of type color or brush. Otherwise, you can access colors from the Colors namespace.

Methods

All colors and brushes have methods that can be called on them:

  • brighter(factor: float) -> Brush

    Returns a new color that is derived from this color but has its brightness increased by the specified factor. For example if the factor is 0.5 (or for example 50%) the returned color is 50% brighter. Negative factors decrease the brightness.

  • darker(factor: float) -> Brush

    Returns a new color that is derived from this color but has its brightness decreased by the specified factor. For example if the factor is .5 (or for example 50%) the returned color is 50% darker. Negative factors increase the brightness.

Linear Gradients

Gradients allow creating smooth colorful surfaces. They are specified using an angle and a series of color stops. The colors will be linearly interpolated between the stops, aligned to an imaginary line that is rotated by the specified angle. This is called a linear gradient and is specified using the @linear-gradient macro with the following signature:

@linear-gradient(angle, color percentage, color percentage, ...)

The first parameter to the macro is an angle (see Types). The gradient line’s starting point will be rotated by the specified value.

Following the initial angle is one or multiple color stops, describe as a space separated pair of a color value and a percentage. The color specifies which value the linear color interpolation should reach at the specified percentage along the axis of the gradient.

The following example shows a rectangle that’s filled with a linear gradient that starts with a light blue color, interpolates to a very light shade in the center and finishes with an orange tone:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    Rectangle {
        background: @linear-gradient(90deg, #3f87a6 0%, #ebf8e1 50%, #f69d3c 100%);
    }
}

Radial Gradients

Linear gradiants are like real gradiant but the colors is interpolated in a circle instead of along a line. To describe a readial gradiant, use the @radial-gradient macro with the following signature:

@radial-gradient(circle, color percentage, color percentage, ...)

The first parameter to the macro is always circle because only circular radients are supported. The syntax is otherwise based on the CSS radial-gradient function.

Example:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;
    Rectangle {
        background: @radial-gradient(circle, #f00 0%, #0f0 50%, #00f 100%);
    }
}

Images

The image type is a reference to an image. It be initialized with the @image-url("...") construct. The URL within the @image-url function need to be known at compile time, and it is looked up relative to the file. In addition, it will also be looked in the include path specified to load .slint files via import.

It is possible to access the width and height of an image.

Example := Window {
    preferred-width: 150px;
    preferred-height: 50px;

    property <image> some_image: @image-url("https://slint-ui.com/logo/slint-logo-full-light.svg");

    Text {
        text: "The image is " + some_image.width + "x" + some_image.height;
    }
}

Arrays/Structs

Arrays are currently only supported in for expressions. [1, 2, 3] is an array of integers. All the types in the array have to be of the same type. It is useful to have arrays of struct. An struct is between curly braces: { a: 12, b: "hello"}.

Statements

Inside callback handlers, more complicated statements are allowed:

Assignment:

clicked => { some-property = 42; }

Self-assignment with += -= *= /=

clicked => { some-property += 42; }

Calling a callback

clicked => { root.some-callback(); }

Conditional statements

clicked => {
    if (condition) {
        foo = 42;
    } else if (other-condition) {
        bar = 28;
    } else {
        foo = 4;
    }
}

Empty expression

clicked => { }
// or
clicked => { ; }

Repetition

The for-in syntax can be used to repeat an element.

The syntax look like this: for name[index] in model : id := Element { ... }

The model can be of the following type:

  • an integer, in which case the element will be repeated that amount of time

  • an array type or a model declared natively, in which case the element will be instantiated for each element in the array or model.

The name will be available for lookup within the element and is going to be like a pseudo-property set to the value of the model. The index is optional and will be set to the index of this element in the model. The id is also optional.

Examples

Example := Window {
    preferred-width: 300px;
    preferred-height: 100px;
    for my-color[index] in [ #e11, #1a2, #23d ]: Rectangle {
        height: 100px;
        width: 60px;
        x: width * index;
        background: my-color;
    }
}
Example := Window {
    preferred-width: 50px;
    preferred-height: 50px;
    property <[{foo: string, col: color}]> model: [
        {foo: "abc", col: #f00 },
        {foo: "def", col: #00f },
    ];
    VerticalLayout {
        for data in root.model: my-repeated-text := Text {
            color: data.col;
            text: data.foo;
        }
    }
}

Conditional element

Similar to for, the if construct can instantiate element only if a given condition is true. The syntax is if condition : id := Element { ... }

Example := Window {
    preferred-width: 50px;
    preferred-height: 50px;
    if area.pressed : foo := Rectangle { background: blue; }
    if !area.pressed : Rectangle { background: red; }
    area := TouchArea {}
}

Animations

Simple animation that animates a property can be declared with animate like this:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    background: area.pressed ? blue : red;
    animate background {
        duration: 250ms;
    }

    area := TouchArea {}
}

This will animate the color property for 100ms when it changes.

Animation can be configured with the following parameter:

  • delay: the amount of time to wait before starting the animation

  • duration: the amount of time it takes for the animation to complete

  • iteration-count: The number of times a animation should run. A negative value specifies infinite reruns. Fractual values are possible.

  • easing: can be linear, ease, ease-in, ease-out, ease-in-out, cubic-bezier(a, b, c, d) as in CSS

It is also possible to animate several properties with the same animation:

animate x, y { duration: 100ms; }

is the same as

animate x { duration: 100ms; }
animate y { duration: 100ms; }

States

The states statement allow to declare states like this:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    text := Text { text: "hello"; }
    property<bool> pressed;
    property<bool> is-enabled;

    states [
        disabled when !is-enabled : {
            background: gray; // same as root.background: gray;
            text.color: white;
        }
        down when pressed : {
            background: blue;
        }
    ]
}

In that example, when the is-enabled property is set to false, the disabled state will be entered This will change the color of the Rectangle and of the Text.

Transitions

Complex animations can be declared on state transitions:

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    text := Text { text: "hello"; }
    property<bool> pressed;
    property<bool> is-enabled;

    states [
        disabled when !is-enabled : {
            background: gray; // same as root.background: gray;
            text.color: white;
            out {
                animate * { duration: 800ms; }
            }
        }
        down when pressed : {
            background: blue;
            in {
                animate background { duration: 300ms; }
            }
        }
    ]
}

Global Singletons

Declare a global singleton with global Name := { /* .. properties or callbacks .. */ } when you want to make properties and callbacks available throughout the entire project. Access them using Name.property.

For example, this can be useful for a common color palette:

global Palette := {
    property<color> primary: blue;
    property<color> secondary: green;
}

Example := Rectangle {
    background: Palette.primary;
    border-color: Palette.secondary;
    border-width: 2px;
}

A global can be declared in another module file, and imported from many files.

Access properties and callbacks from globals in native code by marking them as exported in the file that exports your main application component. In the above example it is sufficient to directly export the Logic global:

export global Logic := {
    property <int> the-value;
    callback magic-operation(int) -> int;
}
// ...

It’s also possible to export globals from other files:

import { Logic as MathLogic } from "math.slint";
export { MathLogic } // known as "MathLogic" when using native APIs to access globals
Usage from Rust
slint::slint!{
export global Logic := {
    property <int> the-value;
    callback magic-operation(int) -> int;
}

export App := Window {
    // ...
}
}

fn main() {
    let app = App::new();
    app.global::<Logic>().on_magic_operation(|value| {
        eprintln!("magic operation input: {}", value);
        value * 2
    });
    app.global::<Logic>().set_the_value(42);
    // ...
}
Usage from C++
#include "app.h"

fn main() {
    auto app = App::create();
    app->global<Logic>().on_magic_operation([](int value) -> int {
        return value * 2;
    });
    app->global<Logic>().set_the_value(42);
    // ...
}

It is possible to re-expose a callback or properties from a global using the two way binding syntax.

global Logic := {
    property <int> the-value;
    callback magic-operation(int) -> int;
}

SomeComponent := Text {
    // use the global in any component
    text: "The magic value is:" + Logic.magic-operation(42);
}

export MainWindow := Window {
    // re-expose the global properties such that the native code
    // can access or modify them
    property the-value <=> Logic.the-value;
    callback magic-operation <=> Logic.magic-operation;

    SomeComponent {}
}

Modules

Components declared in a .slint file can be shared with components in other .slint files, by means of exporting and importing them. By default, everything declared in a .slint file is private, but it can be made accessible from the outside using the export keyword:

ButtonHelper := Rectangle {
    // ...
}

Button := Rectangle {
    // ...
    ButtonHelper {
        // ...
    }
}

export { Button }

In the above example, Button is usable from other .slint files, but ButtonHelper isn’t.

It’s also possible to change the name just for the purpose of exporting, without affecting its internal use:

Button := Rectangle {
    // ...
}

export { Button as ColorButton }

In the above example, Button is not accessible from the outside, but instead it is available under the name ColorButton.

For convenience, a third way of exporting a component is to declare it exported right away:

export Button := Rectangle {
    // ...
}

Similarly, components exported from other files can be accessed by importing them:

import { Button } from "./button.slint";

App := Rectangle {
    // ...
    Button {
        // ...
    }
}

In the event that two files export a type under the same name, then you have the option of assigning a different name at import time:

import { Button } from "./button.slint";
import { Button as CoolButton } from "../other_theme/button.slint";

App := Rectangle {
    // ...
    CoolButton {} // from other_theme/button.slint
    Button {} // from button.slint
}

Elements, globals and structs can be exported and imported.

Focus Handling

Certain elements such as TextInput accept not only input from the mouse/finger but also key events originating from (virtual) keyboards. In order for an item to receive these events, it must have the focus. This is visible through the has-focus property.

You can manually activate the focus on an element by calling focus():

import { Button } from "std-widgets.slint";

App := Window {
    VerticalLayout {
        alignment: start;
        Button {
            text: "press me";
            clicked => { input.focus(); }
        }
        input := TextInput {
            text: "I am a text input field";
        }
    }
}

If you have wrapped the TextInput in a component, then you can forward such a focus activation using the forward-focus property to refer to the element that should receive it:

import { Button } from "std-widgets.slint";

LabeledInput := GridLayout {
    forward-focus: input;
    Row {
        Text {
            text: "Input Label:";
        }
        input := TextInput {}
    }
}

App := Window {
    GridLayout {
        Button {
            text: "press me";
            clicked => { label.focus(); }
        }
        label := LabeledInput {
        }
    }
}

If you use the forward-focus property on a Window, then the specified element will receive the focus the very first time the window receives the focus - it becomes the initial focus element.

Builtin functions

  • debug(string) -> string

The debug function take a string as an argument and prints it

  • animation-tick() -> duration: This function returns a monotonically increasing time, which can be used for animations. Calling this function from a binding will constantly re-evaluate the binding. It can be used like so: x: 1000px + sin(animation-tick() / 1s * 360deg) * 100px; or y: 20px * mod(animation-tick(), 2s) / 2s

Example := Window {
    preferred-width: 100px;
    preferred-height: 100px;

    Rectangle {
        background: red;
        height: 50px;
        width: parent.width * mod(animation-tick(), 2s) / 2s;
    }

    Rectangle {
        background: blue;
        height: 50px;
        y: 50px;
        width: parent.width * abs(sin(360deg * animation-tick() / 3s));
    }
}

Builtin callbacks

Every element implicitly declares an init callback. You can assign a code block to it that will be invoked when the element is instantiated and after all properties are initialized with the value of their final binding. The order of invocation is from inside to outside. The following example will print “first”, then “second”, and then “third”:

MyButton := Rectangle {
    property <string> text: "Initial";
    init => {
        // If `text` is queried here, it will have the value "Hello".
        debug("first");
    }
}

MyCheckBox := Rectangle {
    init => { debug("second"); }
}

MyWindow := Window {
    MyButton {
        text: "Hello";
        init => { debug("third"); }
    }
    MyCheckBox {
    }
}

Do not use this callback to initialize properties, because this violates the the declarative principle. Avoid using this callback, unless you need it, for example, in order to notify some native code:

global SystemService := {
    // This callback can be implemented in native code using the Slint API
    callback ensure_service_running();
}

MySystemButton := Rectangle {
    init => {
        SystemService.ensure_service_running();
    }
    // ...
}

Math namespace

These functions are available both in the global scope and in the Math namespace.

  • min, max

Return the arguments with the minimum (or maximum) value. All arguments must be of the same numeric type

  • mod(T, T) -> T

Perform a modulo operation, where T is some numeric type.

  • abs(float) -> float

Return the absolute value.

  • round(float) -> int

Return the value rounded to the nearest integer

  • ceil(float) -> int, floor(float) -> int

Return the ceiling or floor

  • sin(angle) -> float, cos(angle) -> float, tan(angle) -> float, asin(float) -> angle, acos(float) -> angle, atan(float) -> angle

The trigonometry function. Note that the should be typed with deg or rad unit (for example cos(90deg) or sin(slider.value * 1deg)).

  • sqrt(float) -> float

Square root

  • pow(float, float) -> float

Return the value of the first value raised to the second

  • log(float, float) -> float

Return the log of the first value with a base of the second value

Colors namespace

These functions are available both in the global scope, and in the Colors namespace.

  • rgb(int, int, int) -> color, rgba(int, int, int, float) -> color

Return the color as in CSS. Like in CSS, these two functions are actually aliases that can take three or four parameters.

The first 3 parameters can be either number between 0 and 255, or a percentage with a % unit. The fourth value, if present, is an alpha value between 0 and 1.

Unlike in CSS, the commas are mandatory.

Font Handling

Elements such as Text and TextInput can render text and allow customizing the appearance of the text through different properties. The properties prefixed with font-, such as font-family, font-size and font-weight affect the choice of font used for rendering to the screen. If any of these properties is not specified, the default-font- values in the surrounding Window element apply, such as default-font-family.

The fonts chosen for rendering are automatically picked up from the system. It is also possible to include custom fonts in your design. A custom font must be a TrueType font (.ttf) or a TrueType font collection (.ttc). You can select a custom font with the import statement: import "./my_custom_font.ttf" in a .slint file. This instructions the Slint compiler to include the font and makes the font families globally available for use with font-family properties.

For example:

import "./NotoSans-Regular.ttf";

Example := Window {
    default-font-family: "Noto Sans";

    Text {
        text: "Hello World";
    }
}