Positioning and Layout of Elements

All visual elements are shown in a window. Their position is stored in the x and y properties as coordinates relative to their parent element. The absolute position of an element in a window is calculated by adding the parent’s position to the element’s position. If the parent has a grandparent element, then that one is added as well. This calculation continues until the top-level element is reached.

The size of visual elements is stored in the width and height properties.

You can create an entire graphical user interface by placing the elements in two different ways:

Explicitly - by setting the x, y, width, and height properties.
Automatically - by using layout elements.

Explicit placement is great for static scenes with few elements. Layouts are suitable for complex user interfaces, because the geometric relationship between the elements is expressed in dedicated layout elements. This requires less effort to maintain and helps to create scalable user interfaces.

Explicit Placement

The following example places two rectangles into a window, a blue one and a green one that is a child of the blue:

// Explicit positioning
export component Example inherits Window {
    width: 200px;
    height: 200px;
    Rectangle {
        x: 100px;
        y: 70px;
        width: parent.width - self.x;
        height: parent.height - self.y;
        background: blue;
        Rectangle {
            x: 10px;
            y: 5px;
            width: 50px;
            height: 30px;
            background: green;
        }
    }
}

The position of both rectangles is fixed, as well as the size of the inner green one. The outer blue rectangle however has a size that’s automatically calculated using binding expressions for the width and height properties. The calculation results in the bottom left corner aligning with the corner of the window - it is updated whenever the width and height of the window changes.

When specifying explicit values for any of the geometric properties, Slint requires you to attach a unit to the number. You can choose between two different units:

Logical pixels, using the px unit suffix. This is the recommended unit.
Physical pixels, using the phx unit suffix

Logical pixels scale automatically with the device pixel ratio that your system is configured with. For example, on a modern High-DPI display the device pixel ratio can be 2, so every logical pixel occupies 2 physical pixels. On an older screen the user interface scales without any adaptations.

Additionally, the width and height properties can also be specified as a % percentage unit, which applies relative to the parent element. For example a width: 50% means half of the parent’s width.

The default values for x and y properties are set such that elements are centered within their parent. (Unless using the legacy syntax, in which case the default is 0, 0)

The default values for width and height depend on the type of element. Some elements are sized automatically based on their content, such as Image, Text, and most widgets. The following elements do not have content and therefore default to fill their parent element:

Rectangle
TouchArea
FocusScope
Flickable

Automatic Placement using Layouts

Slint comes with different layout elements that automatically calculate the position and size of their children:

VerticalLayout / HorizontalLayout: The children are placed along the vertical or horizontal axis.
GridLayout: The children are placed in a grid of columns and rows.

Layouts can also be nested, making it possible to create complex user interfaces.

You can tune the automatic placement using different constraints, to accommodate the design of your user interface. For example each element has a minimum and a maximum size. Set these explicitly using the following properties:

min-width
min-height
max-width
max-height

A layout element also affects the minimum and maximum size of its parent.

An element is considered to have a fixed size in a layout when the width and height is specified directly.

When there is extra space in a layout, elements can stretch along the layout axis. You can control this stretch factor between the element and its siblings with these properties:

horizontal-stretch
vertical-stretch

A value of 0 means that the element will not be stretched at all; unless all siblings also have a stretch factor of 0. Then all the elements will be equally stretched.

The default value of these constraint properties may depends on the content of the element. If the element does not set a x or a y property, these constraints are also automatically applied to the parent element. When using the legacy syntax, only the layout elements apply their constraints to the parent.

Common Properties on Layout Elements

All layout elements have the following properties in common:

spacing: This controls the spacing between the children.
padding: This specifies the padding within the layout, the space between the elements and the border of the layout.

For more fine grained control, the padding property can be split into properties for each side of the layout:

padding-left
padding-right
padding-top
padding-bottom

`VerticalLayout` and `HorizontalLayout`

The VerticalLayout and HorizontalLayout elements place elements in a column or row. By default, they will be stretched or shrunk so that they take the whole space, and their alignment can be adjusted.

The following example places the blue and yellow rectangle in a row and evenly stretched across the 200 logical pixels of width:

// Stretch by default
export component Example inherits Window {
    width: 200px;
    height: 200px;
    HorizontalLayout {
        Rectangle { background: blue; min-width: 20px; }
        Rectangle { background: yellow; min-width: 30px; }
    }
}

The example below, on the other hand, specifies that the rectangles shell be aligned to the start of the layout (the visual left). That results in no stretching but instead the rectangles retain their specified minimum width:

// Unless an alignment is specified
export component Example inherits Window {
    width: 200px;
    height: 200px;
    HorizontalLayout {
        alignment: start;
        Rectangle { background: blue; min-width: 20px; }
        Rectangle { background: yellow; min-width: 30px; }
    }
}

The example below nests two layouts for a more complex scene:

export component Example inherits Window {
    width: 200px;
    height: 200px;
    HorizontalLayout {
        // Side panel
        Rectangle { background: green; width: 10px; }

        VerticalLayout {
            padding: 0px;
            //toolbar
            Rectangle { background: blue; height: 7px; }

            Rectangle {
                border-color: red; border-width: 2px;
                HorizontalLayout {
                    Rectangle { border-color: blue; border-width: 2px; }
                    Rectangle { border-color: green; border-width: 2px; }
                }
            }
            Rectangle {
                border-color: orange; border-width: 2px;
                HorizontalLayout {
                    Rectangle { border-color: black; border-width: 2px; }
                    Rectangle { border-color: pink; border-width: 2px; }
                }
            }
        }
    }
}

Alignment

Each elements is sized according to their width or height is specified, otherwise it is set to the minimum size which is set with the min-width or min-height property, or the minimum size of an inner layout, whatever is bigger. Then, the elements are placed according to the alignment. The size of elements is bigger than the minimum size only if the alignment is stretch

This example show the different alignment possibilities

export component Example inherits Window {
    width: 300px;
    height: 200px;
    VerticalLayout {
        HorizontalLayout {
            alignment: stretch;
            Text { text: "stretch (default)"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: start;
            Text { text: "start"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: end;
            Text { text: "end"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: start;
            Text { text: "start"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: center;
            Text { text: "center"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: space-between;
            Text { text: "space-between"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
        HorizontalLayout {
            alignment: space-around;
            Text { text: "space-around"; }
            Rectangle { background: blue; min-width: 20px; }
            Rectangle { background: yellow; min-width: 30px; }
        }
    }
}

Stretch algorithm

When the alignment is set to stretch (the default), the elements are sized to their minimum size, then the extra space is shared amongst element proportional to their stretch factor set with the horizontal-stretch and vertical-stretch properties. The stretched size will not exceed the maximum size. The stretch factor is a floating point number. The elements that have a default content size usually defaults to 0 while elements that default to the size of their parents defaults to 1. An element of a stretch factor of 0 will keep its minimum size, unless all the other elements also have a stretch factor of 0 or reached their maximum size.

Examples:

export component Example inherits Window {
    width: 300px;
    height: 200px;
    VerticalLayout {
        // Same stretch factor (1 by default): the size is divided equally
        HorizontalLayout {
            Rectangle { background: blue; }
            Rectangle { background: yellow;}
            Rectangle { background: green;}
        }
        // Elements with a bigger min-width are given a bigger size before they expand
        HorizontalLayout {
            Rectangle { background: cyan; min-width: 100px;}
            Rectangle { background: magenta; min-width: 50px;}
            Rectangle { background: gold;}
        }
        // Stretch factor twice as big:  grows twice as much
        HorizontalLayout {
            Rectangle { background: navy; horizontal-stretch: 2;}
            Rectangle { background: gray; }
        }
        // All elements not having a maximum width have a stretch factor of 0 so they grow
        HorizontalLayout {
            Rectangle { background: red; max-width: 20px; }
            Rectangle { background: orange; horizontal-stretch: 0; }
            Rectangle { background: pink; horizontal-stretch: 0; }
        }
    }
}

`for`

The VerticalLayout and Horizontal layout may also contain for or if expressions, and it does what one expect

export component Example inherits Window {
    width: 200px;
    height: 50px;
    HorizontalLayout {
        Rectangle { background: green; }
        for t in [ "Hello", "World", "!" ] : Text {
            text: t;
        }
        Rectangle { background: blue; }
    }
}

GridLayout

The GridLayout lays the element in a grid. Each element gains the properties row, col, rowspan, and colspan. One can either use a Row sub-element, or set the row property explicitly. These properties must be statically known at compile time, so it is not possible to use arithmetic or depends on properties. As of now, the use of for or if is not allowed in a grid layout.

This example use the Row element

export component Foo inherits Window {
    width: 200px;
    height: 200px;
    GridLayout {
        spacing: 5px;
        Row {
            Rectangle { background: red; }
            Rectangle { background: blue; }
        }
        Row {
            Rectangle { background: yellow; }
            Rectangle { background: green; }
        }
    }
}

This example use the col and row property

export component Foo inherits Window {
    width: 200px;
    height: 150px;
    GridLayout {
        spacing: 0px;
        Rectangle { background: red; }
        Rectangle { background: blue; }
        Rectangle { background: yellow; row: 1; }
        Rectangle { background: green; }
        Rectangle { background: black; col: 2; row: 0; }
    }
}