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 NUL-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*).

point:

A point with x and y values (lv_point_t).

arglist:

List all parameters as arguments. Supports only int and string. E.g. foo="1 23 'hello' 'a'".

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 wouldn't know what a name means.

Most of these connections are done automatically (e.g., for styles, fonts, images, animations, gradients, etc.), but others need to be connected manually (e.g., event callbacks where the callback itself is available only in the code).

For fonts and images, the connections are created automatically if the source is a file. If the font or image is compiled into the application (as a C array), the user needs to specify which variable a given name refers to.

To create these connections, functions like

  • lv_xml_register_image(&ctx, name, pointer)

  • lv_xml_register_font(&ctx, name, pointer)

  • lv_xml_register_event_cb(&ctx, name, callback)

  • etc.

can be used. Later, a pointer to the object can be retrieved by

  • lv_xml_get_image(&ctx, name)

  • lv_xml_get_font(&ctx, name)

  • lv_xml_get_event_cb(&ctx, name)

  • etc.

style:

Name of a style. lv_xml_get_style_by_name(&ctx, name) returns an lv_style_t*.

font:

Name of a font. lv_xml_get_font(&ctx, name) returns an lv_font_t*.

image:

Name of an image. lv_xml_get_image(&ctx, name) returns an const void*, which can be lv_image_dsc_t* or a NUL-terminated string path to a file.

animation:

Name of an animation descriptor. lv_xml_get_anim(&ctx, name) returns an lv_anim_t*.

subject:

Name of a Subject. lv_xml_get_subject(&ctx, name) returns an lv_subject_t*.

grad:

Name of a gradient. lv_xml_get_grad(&ctx, name) returns an lv_grad_dsc_t*.

event_cb:

Name of an event callback. lv_xml_get_event_cb(&ctx, name) returns an lv_event_cb_t.

Arrays

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

int[N]:

An integer array with N elements.

string[...NULL]:

An array terminated with a NULL element. NULL can be replaced by any value, e.g., grid_template_last.

string[5]:

An array that must have exactly 5 elements.

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 a function parameter.

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

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')

These are checked in the UI Editor, and if an invalid option is selected, it will be highlighted as an error.