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Styles

Styles are used to set the appearance of objects. Styles in lvgl are heavily inspired by CSS. The concept in a nutshell is as follows: - A style is an :cpp:type:`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. Upon 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 may be assigned to an object 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 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 object will be used.
  • Some properties (e.g. text color) can be inherited from a parent(s) if it's not specified in an object.
  • Objects 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 object changes state.

States

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

An object can be in a combination of states such as being focused and pressed at the same time. This is represented as :cpp:expr:`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 :cpp:enumerator:`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:

  1. Initially the object is in the 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 :cpp:expr:`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 the 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 the 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 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 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 until an object 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 here, the pressed text color will be used.

Parts

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

The following predefined parts exist in LVGL:

For example a :ref:`Slider <lv_slider>` has three parts:

  • Background
  • Indicator
  • Knob

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

Initialize styles and set/get properties

Styles are stored in :cpp:type:`lv_style_t` variables. Style variables should be static, global or dynamically allocated. In other words they cannot be local variables in functions which are destroyed when the function exits. Before using a style it should be initialized with :cpp:expr:`lv_style_init(&my_style)`. After initializing a style, properties can be added or changed.

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_hex(0x115588));
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_plaette_main(LV_PALETTE_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_result_t res = lv_style_get_prop(&style, LV_STYLE_BG_COLOR, &v);
if(res == LV_RESULT_OK) {  /*Found*/
    do_something(v.color);
}

:cpp:union:`lv_style_value_t` has 3 fields:

To reset a style (free all its data) use:

lv_style_reset(&style);

Styles can be built as const too to save RAM:

const lv_style_const_prop_t style1_props[] = {
   LV_STYLE_CONST_WIDTH(50),
   LV_STYLE_CONST_HEIGHT(50),
   LV_STYLE_CONST_PROPS_END
};

LV_STYLE_CONST_INIT(style1, style1_props);

Later const style can be used like any other style but (obviously) new properties can not be added.

Add and remove styles to a widget

A style on its own is not that useful. It must 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:

Using :cpp:func:`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 some colors to red when pressed*/

Replace styles

To replace a specific style of an object use :cpp:expr:`lv_obj_replace_style(obj, old_style, new_style, selector)`. This function will only replace old_style with new_style if the selector matches the selector used in lv_obj_add_style. Both styles, i.e. old_style and new_style, must not be NULL (for adding and removing separate functions exist). If the combination of old_style and selector exists multiple times in obj's styles, all occurrences will be replaced. The return value of the function indicates whether at least one successful replacement took place.

Using :cpp:func:`lv_obj_replace_style`:

lv_obj_add_style(btn, &style_btn, 0);                      /*Add a button style*/
lv_obj_replace_style(btn, &style_btn, &new_style_btn, 0);  /*Replace the button style with a different one*/

Remove styles

To remove all styles from an object use :cpp:expr:`lv_obj_remove_style_all(obj)`.

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

Report style changes

If a style which is already assigned to an 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 :cpp:expr:`lv_obj_invalidate(obj)` or :cpp:expr:`lv_obj_invalidate(lv_screen_active())`.
  2. If more complex style properties were changed or added, and you know which object(s) are affected by that style call :cpp:expr:`lv_obj_refresh_style(obj, part, property)`. To refresh all parts and properties use :cpp:expr:`lv_obj_refresh_style(obj, LV_PART_ANY, LV_STYLE_PROP_ANY)`.
  3. To make LVGL check all objects to see if they use a style and refresh them when needed, call :cpp:expr:`lv_obj_report_style_change(&style)`. If style is NULL all objects will be notified about a 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)
  • property get functions like this can be used: lv_obj_get_style_<property_name>(obj, <part>). These functions use the object's current state and if no better candidate exists they return a default value. For example:
lv_color_t color = lv_obj_get_style_bg_color(btn, LV_PART_MAIN);

Local styles

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

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 an object.

Unlike in CSS, 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_<property_name>(obj, <value>, <selector>); For example:

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

Properties

For the full list of style properties click :ref:`here <style_properties>`.

Typical background properties

In the documentation of the widgets you will see sentences like "The widget uses the typical background properties". These "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 the timing or easing function)
  • the properties to animate

The transition properties can be defined for each state. For example, setting a 500 ms transition time in the default state means that when the object goes to the default state a 500 ms transition time is applied. Setting a 100 ms transition time in the pressed state causes a 100 ms transition when going to the pressed state. This example configuration results in going to the pressed state quickly and then going back to default slowly.

To describe a transition an :cpp:struct:`lv_transition_dsc_t` variable needs to be 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);

Opacity, Blend modes and Transformations

If the opa, blend_mode, transform_angle, or transform_zoom properties are set to their non-default value LVGL creates a snapshot about the widget and all its children in order to blend the whole widget with the set opacity, blend mode and transformation properties.

These properties have this effect only on the MAIN part of the widget.

The created snapshot is called "intermediate layer" or simply "layer". If only opa and/or blend_mode is set to a non-default value LVGL can build the layer from smaller chunks. The size of these chunks can be configured by the following properties in lv_conf.h:

If transformation properties were also used the layer can not be rendered in chunks, but one larger memory needs to be allocated. The required memory depends on the angle, zoom and pivot parameters, and the size of the area to redraw, but it's never larger than the size of the widget (including the extra draw size used for shadow, outline, etc).

If the widget can fully cover the area to redraw, LVGL creates an RGB layer (which is faster to render and uses less memory). If the opposite case ARGB rendering needs to be used. A widget might not cover its area if it has radius, bg_opa != 255, has shadow, outline, etc.

The click area of the widget is also transformed accordingly.

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, two steps are required:

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

Theme initialization functions can have different prototypes. 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_display_set_theme(display, th); /*Assign the theme to the display*/

The included themes are enabled in lv_conf.h. If the default theme is enabled by :c:macro:`LV_USE_THEME_DEFAULT` 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.

:cpp:expr:`lv_theme_set_parent(new_theme, base_theme)` extends the base_theme with the new_theme.

There is an example for it below.

Examples

API