lvgl/docs/overview/style.md

.. include:: /header.rst 
:github_url: |github_link_base|/overview/style.md

Styles

Styles are used to set the appearance of the objects. Styles in lvgl are heavily inspired by CSS. The concept in nutshell is as follows:

• A style is an lv_style_t variable which can hold properties like border width, text color and so on. It's similar to a class in CSS.
• Styles can be assigned to objects to change their appearance. During the assignment the target part (pseudo element in CSS) and target state (pseudo class) 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 objects.
• Styles can be cascaded which means multiple styles can be assigned to an object and each style can have different properties. Therefore not all properties have to be specified in style. LVLG will look for a property until a style defines it or use a default if it's not spefied 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.
• Later added styles have higher precedence. It means if a property is specified in two styles the later added will be used.
• Some properties (e.g. text color) can be inherited from the parent(s) if it's not specified in the object.
• Objects can have local styles that have 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 object changes state.

States

The objects 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 (not supported now)
• 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

The combination states the object can be focused and pressed at the same time. This is represented as LV_STATE_FOCUSED | LV_STATE_PRESSED.

The style can be added to any state and state combination. For example, setting a different background color for default and pressed state. 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. (See later)

But what does the "best matching state's property" really mean? States have a precedence which is shown by their value (see in the above list). A higher value means higher precedence. To determine which state's property to use let's take an example. Imagine the background color is defined like this:

• LV_STATE_DEFAULT: white
• LV_STATE_PRESSED: gray
• LV_STATE_FOCUSED: red

1. By the default the object is in default state, so it's a simple case: the property is perfectly defined in the object's current state as white.
2. When the object is pressed there are 2 related properties: default with white (default is related to every state) and pressed with gray. The pressed state has 0x0020 precedence which is higher than the default state's 0x0000 precedence, so gray color will be used.
3. When the object is focused the same thing happens as in pressed state and red color will be used. (Focused state has higher precedence than default state).
4. When the object is focused and pressed both gray and red would work, but the pressed state has higher precedence than focused so gray color will be used.
5. It's possible to set e.g rose color for LV_STATE_PRESSED | LV_STATE_FOCUSED. In this case, this combined state has 0x0020 + 0x0002 = 0x0022 precedence, which is higher than the pressed state's precedence so rose color would be used.
6. When the object is in checked state there is no property to set the background color for this state. So for lack of a better option, the object remains white from the default state's property.

Some practical notes:

• The precedence (value) of states is quite intuitive and it's something the user would expect naturally. E.g. if an object is focused the user will still want to see if it's pressed, therefore pressed state has a higher precedence. If the focused state had a higher precedence it would overwrite the pressed color.
• If you want to set a property for all states (e.g. red background color) just set it for the default state. If the object can't find a property for its current state it will fall back to the default state's property.
• Use ORed states to describe the properties for complex cases. (E.g. pressed + checked + focused)
• It might be a good idea to use different style elements for different states. For example, finding background colors for released, pressed, checked + pressed, focused, focused + pressed, focused + pressed + checked, etc states is quite difficult. Instead, for example, use the background color for pressed and checked states and indicate the focused state with a different border color.

Cascading styles

It's not required to set all the properties in one style. It's possible to add more styles to an object and let the later added style to modify or extend appearance. For example, create a general gray button style and create a new 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 coming from states are still taken into account. So let's examine the following case:

• the basic button style defines dark-gray color for default state and light-gray color 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 that are related to texts) can be inherited from the parent object's styles. Inheritance is applied only if the given property is not set in the object's styles (even in default state). In this case, if the property is inheritable, the property's value will be searched in the parents too until an object specifies a value for the property. The parents will use their own state to detemine the value. So if a button is pressed, and the text color comes from here, the pressed text color will be used.

Parts

Objects can have parts which can have their own styles.

The following predefined parts exist in LVGL:

• LV_PART_MAIN A background like rectangle*/
• LV_PART_SCROLLBAR The scrollbar(s)
• LV_PART_INDICATOR Indicator, e.g. for slider, bar, switch, or the tick box of the checkbox
• LV_PART_KNOB Like a handle to grab to adjust the value*/
• LV_PART_SELECTED Indicate the currently selected option or section
• LV_PART_ITEMS Used if the widget has multiple similar elements (e.g. table cells)*/
• LV_PART_TICKS Ticks on scales e.g. for a chart or meter
• LV_PART_CURSOR Mark a specific place e.g. text area's or chart's cursor
• LV_PART_CUSTOM_FIRST Custom parts can be added from here.

For example a Slider has three parts:

• Background
• Indiactor
• Knob

It means the all three parts of the slider can have their own styles. See later how to add style styles to objects and parts.

Initialize styles and set/get properties

Styles are stored in lv_style_t variables. Style variables should be static, global or dynamically allocated. In other words they can not be local variables in functions which are destroyed when the function exists. Before using a style it should be initialized with lv_style_init(&my_style). After initializing the style properties can be set or added to it.

Property set functions looks like this: lv_style_set_<property_name>(&style, <value>); For example:

static lv_style_t style_btn;
lv_style_init(&style_btn);
lv_style_set_bg_color(&style_btn, lv_color_grey());
lv_style_set_bg_opa(&style_btn, LV_OPA_50);
lv_style_set_border_width(&style_btn, 2);
lv_style_set_border_color(&style_btn, lv_color_black());

static lv_style_t style_btn_red;
lv_style_init(&style_btn_red);
lv_style_set_bg_color(&style_btn_red, lv_color_red());
lv_style_set_bg_opa(&style_btn_red, LV_OPA_COVER);

To remove a property use:

lv_style_remove_prop(&style, LV_STYLE_BG_COLOR);

To get a property's value from a style:

lv_style_value_t v;
lv_res_t res = lv_style_rget_prop(&style, LV_STYLE_BG_COLOR, &v);
if(res == LV_RES_OK) {	/*Found*/
	do_something(v.color);
}

lv_style_value_t has 3 fields:

• num for integer, boolean and opacity properties
• color for color properties
• ptr for pointer properties

To reset a style (free all its data) use

lv_style_reset(&style);

Add and remove styles to a widget

A style on its own is not that useful, it needs to be assigned to an object to take effect.

Add styles

To add a style to an object use lv_obj_add_style(obj, &style, <selector>). <selector> is an OR-ed value of parts and state to which the style should be added. Some examples:

• LV_PART_MAIN | LV_STATE_DEFAULT
• LV_STATE_PRESSED: The main part in pressed state. LV_PART_MAIN can be omitted
• LV_PART_SCROLLBAR: The scrollbar part in the default state. LV_STATE_DEFAULT can be omitted.
• LV_PART_SCROLLBAR | LV_STATE_SCROLLED: The scrollbar part when the object is being scrolled
• 0 Same as LV_PART_MAIN | LV_STATE_DEFAULT.
• LV_PART_INDICATOR | LV_STATE_PRESSED | LV_STATE_CHECKED The indicator part when the object is pressed and checked at the same time.

Using lv_obj_add_style:

lv_obj_add_style(btn, &style_btn, 0);      				  /*Default button style*/
lv_obj_add_style(btn, &btn_red, LV_STATE_PRESSED);  /*Overwrite only a some colors to red when pressed*/

Remove styles

To remove all styles from an object use lv_obj_remove_style_all(obj).

To remove specific styles use lv_obj_remove_style(obj, style, selector). This function will remove style only if the selector matches with the selector used in lv_obj_add_style. style can be NULL to check only the selector and remove all matching styles. The selector can use the LV_STATE_ANY and LV_PART_ANY values to remove the style with any state or part.

Report style changes

If a style which is already assigned to object changes (i.e. a property is added or changed) the objects using that style should be notified. There are 3 options to do this:

1. If you know that the changed properties can be applied by a simple redraw (e.g. color or opacity changes) just call lv_obj_invalidate(obj) or lv_obj_invalideate(lv_scr_act()).
2. If more complex style properties were changed or added, and you know which object(s) are affected by that style call lv_obj_refresh_style(obj, part, property). To refresh all parts and properties use lv_obj_refresh_style(obj, LV_PART_ANY, LV_STYLE_PROP_ANY).
3. To make LVGL check all objects to see whether they use the style and refresh them when needed call lv_obj_report_style_change(&style). If style is NULL all objects will be notified about the style change.

Get a property's value on an object

To get a final value of property - considering cascading, inheritance, local styles and transitions (see below) - get functions like this can be used: lv_obj_get_style_<property_name>(obj, <part>). These functions uses the object's current state and if no better candidate returns a default value.   For example:

lv_color_t color = lv_obj_get_style_bg_color(btn, LV_PART_MAIN);

Local styles

Besides "normal" styles, the objects can store local styles too. This concept is similar to inline styles in CSS (e.g. <div style="color:red">) with some modification.

So local styles are like normal styles but they can't be shared among other objects. If used, local styles are allocated automatically, and freed when the object is deleted. They are useful to add local customization to the object.

Unlike in CSS, in LVGL local styles can be assigned to states (pseudo-classes) and parts (pseudo-elements).

To set a local property use functions like lv_obj_set_style_local_<property_name>(obj, <value>, <selector>);   For example:

lv_obj_set_style_local_bg_color(slider, lv_color_red(), LV_PART_INDICATOR | LV_STATE_FOCUSED);

Properties

For the full list of style properties click here.

Typical background properties

In the documentation of the widgets you will see sentences like "The widget use the typical background properties". The "typical background properties" are the ones related to:

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

Transitions

By default, when an object changes state (e.g. it's pressed) the new properties from the new state are set immediately. However, with transitions it's possible to play an animation on state change. For example, on pressing a button its background color can be animated to the pressed color over 300 ms.

The parameters of the transitions are stored in the styles. It's possible to set

• the time of the transition
• the delay before starting the transition
• the animation path (also known as timing or easing function)
• the properties to animate

The transition properties can be defined for each state. For example, setting 500 ms transition time in default state will mean that when the object goes to the default state a 500 ms transition time will be applied. Setting 100 ms transition time in the pressed state will mean a 100 ms transition time when going to pressed state. So this example configuration will result in going to pressed state quickly and then going back to default slowly.

To describe a transition an lv_transition_dsc_t variable needs to initialized and added to a style:

/*Only its pointer is saved so must static, global or dynamically allocated */
static const lv_style_prop_t trans_props[] = {
											LV_STYLE_BG_OPA, LV_STYLE_BG_COLOR,
											0, /*End marker*/
};

static lv_style_transition_dsc_t trans1;
lv_style_transition_dsc_init(&trans1, trans_props, lv_anim_path_ease_out, duration_ms, delay_ms);

lv_style_set_transition(&style1, &trans1);

Color filter

TODO

Themes

Themes are a collection of styles. If there is an active theme LVGL applies it on every created widget. This will give a default appearance to the UI which can then be modified by adding further styles.

Every display can have a different theme. For example you could have a colorful theme on a TFT and monochrome theme on a secondary monochrome display.

To set a theme for a display, 2 steps are required:

1. Initialize a theme
2. Assign the initialized theme to a display.

Theme initialization functions can have different prototype. This example shows how to set the "default" theme:

lv_theme_t * th = lv_theme_default_init(display,  /*Use the DPI, size, etc from this display*/ 
                                        LV_COLOR_PALETTE_BLUE, LV_COLOR_PALETTE_CYAN,   /*Primary and secondary palette*/
                                        false,    /*Light or dark mode*/ 
                                        &lv_font_montserrat_10, &lv_font_montserrat_14, &lv_font_montserrat_18); /*Small, normal, large fonts*/
                                        
lv_disp_set_theme(display, th); /*Assign the theme to the display*/

The themes can be enabled in lv_conf.h. If the default theme is enabled by LV_USE_THEME_DEFAULT 1 LVGL automatically initializes and sets it when a display is created.

Extending themes

Built-in themes can be extended. If a custom theme is created a parent theme can be selected. The parent theme's styles will be added before the custom theme's styles. Any number of themes can be chained this way. E.g. default theme -> custom theme -> dark theme.

lv_theme_set_parent(new_theme, base_theme) extends the base_theme with the new_theme.

There is an example for it below.

Examples

.. include:: ../../examples/styles/index.rst

API

.. doxygenfile:: lv_style.h
  :project: lvgl

.. doxygenfile:: lv_theme.h
  :project: lvgl

.. doxygenfile:: lv_obj_style_gen.h
  :project: lvgl
  
.. doxygenfile:: lv_style_gen.h
  :project: lvgl