Styles Overview

Styles are used to set the appearance of Widgets. Styles in LVGL are heavily inspired by CSS.

Styles are used to set the appearance of Widgets. Styles in LVGL are heavily inspired by CSS. The concept in a nutshell is that a style is an lv_style_t variable which can hold properties like border width, font, text color and so on. It's similar to a class in CSS.

• Styles can be assigned to Widgets to change their appearance. Upon assignment, the target part (pseudo-element in CSS) and target state (pseudo-class in CSS) can be specified. For example one can add style_blue to the knob of a slider when it's in pressed state.
• The same style can be used by any number of Widgets.
• Styles can be cascaded which means multiple styles may be assigned to a Widget and each style can have different properties. Therefore, not all properties have to be specified in a style. LVGL will search for a property until a style defines it or use a default value if it's not specified by any of the styles. For example style_btn can result in a default gray button and style_btn_red can add only a background-color=red to overwrite the background color.
• The most recently added style has higher precedence. This means if a property is specified in two styles the newest style in the Widget will be used.
• Some properties (e.g. text color) can be inherited from a parent(s) if it's not specified in a Widget.
• Widgets can also have local styles with higher precedence than "normal" styles.
• Unlike CSS (where pseudo-classes describe different states, e.g. :focus), in LVGL a property is assigned to a given state.
• Transitions can be applied when the Widget changes state.

See the full list of style properties.

States

The Widgets can be in the combination of the following states:

• LV_STATE_DEFAULT: (0x0000) Normal, released state
• LV_STATE_CHECKED: (0x0001) Toggled or checked state
• LV_STATE_FOCUSED: (0x0002) Focused via keypad or encoder or clicked via touchpad/mouse
• LV_STATE_FOCUS_KEY: (0x0004) Focused via keypad or encoder but not via touchpad/mouse
• LV_STATE_EDITED: (0x0008) Edit by an encoder
• LV_STATE_HOVERED: (0x0010) Hovered by mouse
• LV_STATE_PRESSED: (0x0020) Being pressed
• LV_STATE_SCROLLED: (0x0040) Being scrolled
• LV_STATE_DISABLED: (0x0080) Disabled state
• LV_STATE_USER_1: (0x1000) Custom state
• LV_STATE_USER_2: (0x2000) Custom state
• LV_STATE_USER_3: (0x4000) Custom state
• LV_STATE_USER_4: (0x8000) Custom state

A Widget can be in a combination of states such as being focused and pressed at the same time. This is represented as LV_STATE_FOCUSED | LV_STATE_PRESSED.

A style can be added to any state or state combination. For example, setting a different background color for the default and pressed states. If a property is not defined in a state the best matching state's property will be used. Typically this means the property with LV_STATE_DEFAULT is used. If the property is not set even for the default state the default value will be used.

What "best matching" means

Each state's precedence equals its value in the list above — a higher value wins. When a property isn't set for the current state, the value from the highest-precedence matching state is used; if none matches, the default state's value (or the global default) applies.

For example, with bg_color white in LV_STATE_DEFAULT, gray in LV_STATE_PRESSED (0x0020) and red in LV_STATE_FOCUSED (0x0002):

• Pressed → gray (0x0020 beats default's 0x0000).
• Focused → red.
• Focused + pressed → gray (pressed outranks focused).
• LV_STATE_PRESSED | LV_STATE_FOCUSED set to rose → rose (0x0022 beats either alone).
• Checked → white (no checked value, so it falls back to default).

The ordering is intentional: a pressed Widget should still look pressed while focused. To cover every state, set the property in the default state. For combinations that are hard to enumerate, vary one part per state (e.g. background color for pressed/checked, border color for focused).

Cascading Styles

It's not required to set all the properties in one style. It's possible to add more styles to a Widget and have the latter added style modify or extend appearance. For example, create a general gray button style and create a new one for red buttons where only the new background color is set.

This is much like in CSS when used classes are listed like <div class=".btn .btn-red">.

Styles added later have precedence over ones set earlier. So in the gray/red button example above, the normal button style should be added first and the red style second. However, the precedence of the states are still taken into account. So let's examine the following case:

• the basic button style defines dark-gray color for the default state and light-gray color for the pressed state
• the red button style defines the background color as red only in the default state

In this case, when the button is released (it's in default state) it will be red because a perfect match is found in the most recently added style (red). When the button is pressed the light-gray color is a better match because it describes the current state perfectly, so the button will be light-gray.

Inheritance

Some properties (typically those related to text) can be inherited from the parent Widget's styles. Inheritance is applied only if the given property is not set in the Widget's styles (even in default state). In this case, if the property is inheritable, the property's value will be searched up the parent hierarchy until a Widget specifies a value for the property. The parents will use their own state to determine the value. So if a button is pressed, and the text color comes from a parent, the pressed text color will be used.

Parts

Widgets can be composed of parts which may each have their own styles.

The following predefined parts exist in LVGL:

• LV_PART_MAIN: (0x000000) A background like rectangle
• LV_PART_SCROLLBAR: (0x010000) The scrollbar(s)
• LV_PART_INDICATOR: (0x020000) Indicator, e.g. for slider, bar, switch, or the tick box of the checkbox
• LV_PART_KNOB: (0x030000) Like a handle to grab to adjust a value
• LV_PART_SELECTED: (0x040000) Indicate the currently selected option or section
• LV_PART_ITEMS: (0x050000) Used if the widget has multiple similar elements (e.g. table cells)
• LV_PART_CURSOR: (0x060000) Mark a specific place e.g. Text Area's or chart's cursor
• LV_PART_CUSTOM_FIRST: (0x080000) Custom part identifiers can be added starting from here.
• LV_PART_ANY: (0x0F0000) Special value can be used in some functions to target all parts.

For example a Slider has three parts:

• Main (background)
• Indicator
• Knob

This means all three parts of the slider can have their own styles. See later how to add styles to Widgets and parts.

Since LV_PART_MAIN and LV_STATE_DEFAULT both have zero values, you can simply pass 0 as the selector argument instead of LV_PART_MAIN | LV_STATE_DEFAULT as a shortcut when adding styles to an object.

Properties Requiring New Layers

Setting opa, blend_mode, transform_angle, or transform_zoom to a non-default value on the MAIN part makes LVGL snapshot the Widget and its children into an intermediate layer so the whole Widget is blended together.

If only opa and/or blend_mode is set, the layer is built from smaller chunks sized by these lv_conf.h options (transformations force a single full-size layer):

• LV_LAYER_SIMPLE_BUF_SIZE — target chunk buffer size in bytes.
• LV_LAYER_SIMPLE_FALLBACK_BUF_SIZE — used if the above can't be allocated.

Typical Background Properties

In documentation of widgets you will see sentences like "XY Widget uses the typical background style properties". These "typical background properties" are the properties being referred to:

• Background
• Border
• Outline
• Shadow
• Padding
• Width and height transformation
• X and Y translation

Effects

Examples that combine several style properties for a visual effect. Per-property examples are on the Style Properties page.

Conic gradient knob

Radial gradient background

Gradient button backgrounds

Modal overlay

Interactive transform