Common Widget FeaturesStyles

Styles Overview

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

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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.

For the full list of style properties click here.

States

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

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. (See later)

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.

What does the "best matching state's property" 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 Widget is in the default state, so it's a simple case: the property is perfectly defined in the Widget's current state as white.
  2. When the Widget 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 Widget has focus 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 Widget has focus 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 Widget 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 Widget 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. Example: if a Widget has focus 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 Widget 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 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

If the opa, blend_mode, transform_angle, or transform_zoom properties are set to a non-default value LVGL creates a snapshot of the widget and 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:

  • LV_LAYER_SIMPLE_BUF_SIZE: [bytes] the optimal target buffer size. LVGL will try to allocate this size of memory.
  • LV_LAYER_SIMPLE_FALLBACK_BUF_SIZE: [bytes] used if LV_LAYER_SIMPLE_BUF_SIZE couldn't be allocated.

If transformation properties were also used the layer cannot be rendered in chunks

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

Examples

Size styles

lv_example_style_1.c

Background styles

lv_example_style_2.c

Border styles

lv_example_style_3.c

Outline styles

lv_example_style_4.c

Shadow styles

lv_example_style_5.c

Image styles

lv_example_style_6.c

Arc styles

lv_example_style_7.c

Text styles

lv_example_style_8.c

Line styles

lv_example_style_9.c

Drop shadow styles

lv_example_style_10.c

Transition

lv_example_style_11.c

Using multiple styles

lv_example_style_12.c

Local styles

lv_example_style_13.c

Add styles to parts and states

lv_example_style_14.c

Extending the current theme

lv_example_style_15.c

Opacity and Transformations

lv_example_style_16.c

Metallic knob with conic gradient

lv_example_style_17.c

Radial gradient as background

lv_example_style_18.c

Gradients for button background

lv_example_style_19.c

Test between recolor style or full background modal

lv_example_style_20.c

Transform style

lv_example_style_21.c

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Styles

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Style Sheets

Styles are stored in lv_style_t variables. Style variables should be static, global or dynamically allocated.

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StatesWhat does the "best matching state's property" mean?Cascading StylesInheritancePartsProperties Requiring New LayersTypical Background PropertiesExamplesSize stylesBackground stylesBorder stylesOutline stylesShadow stylesImage stylesArc stylesText stylesLine stylesDrop shadow stylesTransitionUsing multiple stylesLocal stylesAdd styles to parts and statesExtending the current themeOpacity and TransformationsMetallic knob with conic gradientRadial gradient as backgroundGradients for button backgroundTest between recolor style or full background modalTransform style