Syntax

Naming conventions

  • A standard XML syntax is used.

  • Lowercase letters with _ separation are used for attribute names.

  • Tag names follow the usual variable-name rules: they must start with a letter or '_' and the rest of the name may be comprised of letters, '_' and digits.

  • The LVGL API is followed as much as possible, e.g., align="center", bg_color="0xff0000".

  • For colors, all these syntaxes are supported (similar to CSS colors): 0x112233, #112233, 112233, 0x123, #123, 123. Note that like CSS, 0x123, #123 and 123 all mean #112233.

  • params can be referenced with $.

  • consts can be referenced with #.

  • styles can be attached to states and/or parts like this: styles="style1 style2:pressed style3:focused:scrollbar".

  • Local styles (i.e. styles that are stored within the Component and thus not shared by any other Components) can be used like this: <lv_label style_text_color="0xff0000" style_text_color:checked="0x00ff00"/>.

Types

All of the types can be used as API property types, but only a subset of them can be used as constant and Subject types.

Simple types

The following simple built-in types are supported:

bool:

a true or false.

int:

Integer number in the range of roughly -2B to 2B by default. (Same as int32_t in C.)

px:

Simple pixel units. The unit px can be omitted.

%:

Percentage. % must be appended to the value as the unit. (Means the same as lv_pct().)

content:

Means LV_SIZE_CONTENT.

string:

Simple \0-terminated string. When multiple strings are used in a property or string array, ' should be used. E.g. foo="'a' 'b'".

color:

A color stored as 24-bit RGB (lv_color_t).

opa:

Opacity value in the range of 0 to 255 or 0 to 100%. Like CSS, percentage values must be be followed by '%'.

lv_obj:

Pointer to a Widget (lv_obj_t*).

screen:

Pointer to a screen (also lv_obj_t*).

time_ms:

Means some time in milliseconds unit

deg_0.1:

Degrees with 0.1 resolution

scale_1/256:

Scale/Zoom, where 256 means 100%, 128 means 50%, 512 means 200% etc.

Name-based types

In XML files, fonts, images, styles, etc., are not used by pointer but by string names. For example, a style is defined like <style name="red" bg_color="0xff0000"/>. Later, they can be referenced by their names.

This means that the actual values need to be bound to the names when the UI is loaded from XML, otherwise, LVGL will not know what a name means.

Most of these connections are done automatically (e.g., for styles, fonts, images, animations, gradients, etc.) when the components and globals.xmls are registered.

However, others need to be connected manually. For example the event callbacks where the callback itself is available only in the code, or fonts or images if they are referring to data stored in memory.

To learn more about how to register the name-data pairs visit Loading XML Files at Runtime.

Arrays

An array of any type can be defined in three ways:

int[count]:

An integer array. The number of elements will be passed as a separate parameter.

string[NULL]:

An array terminated with a NULL element.

string[]:

No NULL termination and no count parameter. Used when the number of elements is not known or delivered via another mechanism, such as via another setting.

Enums

<enumdef> can be used in the <api> tags to create custom enums for Widgets. This is not supported for Components.

For example:

<api>
    <enumdef name="my_widget_mode" help="Possible modes" help-zh="Chinese help">
        <enum name="normal" help="Normal mode" help-zh="Normal mode in Chinese" value="0x10"/>
        <enum name="inverted" help="Inverted mode"/>
    </enumdef>

    <prop name="mode" help="help"type="enum:my_widget_mode" help="help"/>
</api>

When used as a type, a + suffix means multiple values can be selected and ORed. For example: type="axis+". In this case, the options should be separated by |, for example: axis=primary_x|secondary_y.

Compound types

Types can be compound, meaning multiple options/types are possible. For example, for width: type="px|%|content".

Limiting accepted values

Not supported yet; the examples below illustrate the planned syntax.

It is also possible to limit the possible options the user can select from an enum. For example:

  • Enums: type="dir(top bottom)"

  • Colors: type="color(0xff0000 0x00ff00 0x0000ff)"

  • Strings: type="string('Ok' 'Cancel')