Common Widget Features

Scrolling

In LVGL scrolling works very intuitively: if a Widget is outside its parent content area (the size without padding), the parent becomes scrollable and scrollbar(s) will appear. That's it.

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Overview

In LVGL scrolling works very intuitively: if a Widget is outside its parent content area (the size without padding), the parent becomes scrollable and scrollbar(s) will appear. That's it.

Any Widget can be scrollable including base widget, lv_image, lv_button, lv_meter, etc

The Widget can either be scrolled horizontally or vertically in one stroke; diagonal scrolling is not possible.

Scrollbar

Mode

Scrollbars are displayed according to the configured scrollbar-mode. The following modes are available:

lv_obj_set_scrollbar_mode(widget, LV_SCROLLBAR_MODE_...) sets the scrollbar mode on a Widget.

Styling

A Scrollbar is a dedicated part of a Widget, called LV_PART_SCROLLBAR. For example, a scrollbar can turn to red like this:

 
static lv_style_t style_red;
lv_style_init(&style_red);
lv_style_set_bg_color(&style_red, lv_color_red());

...

lv_obj_add_style(widget, &style_red, LV_PART_SCROLLBAR);

A Widget goes to the LV_STATE_SCROLLED state while it's being scrolled. This allows adding different styles to the Widget that will be effective while it is being scrolled. For example, this code makes the scrollbar blue while the Widget is being scrolled:

 
static lv_style_t style_blue;
lv_style_init(&style_blue);
lv_style_set_bg_color(&style_blue, lv_color_blue());

...

lv_obj_add_style(widget, &style_blue, LV_STATE_SCROLLED | LV_PART_SCROLLBAR);

If the base direction of the LV_PART_SCROLLBAR is RTL (LV_BASE_DIR_RTL) the vertical scrollbar will be placed on the left. Note that, the base_dir style property is inherited. Therefore, it can be set directly on the LV_PART_SCROLLBAR part of a Widget, or on the Widget's LV_PART_MAIN part, or that of any of its parents, to make a scrollbar inherit the base direction.

pad_left/right/top/bottom sets the spacing around the scrollbars, width sets the scrollbar's width and length sets the scrollbar's length: If length is not set or left at 0 the scrollbar's length will be set automatically according to the length of the content.

 
static lv_style_t style_scrollbar;
lv_style_init(&style_scrollbar);
lv_style_set_pad_left(&style_scrollbar, 2);
lv_style_set_pad_right(&style_scrollbar, 2);
lv_style_set_pad_top(&style_scrollbar, 2);
lv_style_set_pad_bottom(&style_scrollbar, 2);
lv_style_set_width(&style_scrollbar, 10);
lv_style_set_length(&style_scrollbar, 50);

...

lv_obj_add_style(widget, &style_scrollbar, LV_PART_SCROLLBAR);

The minimum length of the scrollbar is fixed to 10, while its maximum length is limited by the Widget's height or width, depending on whether the scrollbar is vertical or horizontal. Any length value set outside of these limits will automatically result in a length fixed to either limit.

Scrolling Events

The following events are emitted as part of scrolling:

  • LV_EVENT_SCROLL_BEGIN: Signals that scrolling has begun. The event parameter is NULL or an lv_anim_t * with a scroll animation descriptor that can be modified if required.
  • LV_EVENT_SCROLL_END: Signals that scrolling has ended.
  • LV_EVENT_SCROLL: Signals that the scrolling position changed; triggered on every position change.

Features of Scrolling

Besides, managing "normal" scrolling there are many interesting and useful additional features.

Scrollable

It is possible to make a Widget non-scrollable with lv_obj_remove_flag(widget, LV_OBJ_FLAG_SCROLLABLE).

Non-scrollable Widgets can still propagate the scrolling (chain) to their parents.

The direction in which scrolling happens can be controlled by lv_obj_set_scroll_dir(widget, LV_DIR_...).

The following values can be used for the direction:

OR-ed values are also possible. E.g. LV_DIR_TOP | LV_DIR_LEFT.

Scroll chaining

If a Widget can't be scrolled further (e.g. its content has reached the bottom-most position), additional scrolling is propagated to its parent. If the parent can be scrolled in that direction than it will be scrolled instead. It continues propagating up the Widget's parent hierarchy up to the Screen.

The propagation on scrolling is called "scroll chaining" and it can be enabled/disabled with LV_OBJ_FLAG_SCROLL_CHAIN_HOR/VER flag. If chaining is disabled the propagation stops on the Widget and the parent(s) won't be scrolled.

Scroll momentum

When the user scrolls a Widget and releases it, LVGL can emulate inertial momentum for the scrolling. It's like the Widget was "thrown" and scrolling slows down smoothly.

Scroll momentum can be enabled/disabled with the LV_OBJ_FLAG_SCROLL_MOMENTUM flag.

Elastic scroll

Normally a Widget can't be scrolled past the extremities of its content. That is, the top side of the content can't be below the top side of the Widget, and vice versa for the bottom side.

However, with LV_OBJ_FLAG_SCROLL_ELASTIC a fancy effect is added when the user "over-scrolls" the content. The scrolling slows down, and the content can be scrolled inside the Widget. When the Widget is released the content scrolled in it is animated back to the closest valid position.

Snapping

The children of a Widget can be snapped according to specific rules when scrolling ends. Children can be made snappable individually with the LV_OBJ_FLAG_SNAPPABLE flag.

A Widget can align snapped children in four ways:

Snap alignment is set with lv_obj_set_scroll_snap_x(widget, LV_SCROLL_SNAP_...) and lv_obj_set_scroll_snap_y(widget, LV_SCROLL_SNAP_...).

This is what happens under the hood:

  1. user scrolls and releases a Widget;
  2. LVGL calculates where the scroll would end considering scroll momentum;
  3. LVGL finds the nearest scroll point;
  4. LVGL scrolls to the snap point with an animation.

Scroll one

The "scroll one" feature tells LVGL to allow scrolling only one snappable child at a time. This requires making the children snappable and setting scroll snap alignment to something other than LV_SCROLL_SNAP_NONE.

This feature can be enabled by the LV_OBJ_FLAG_SCROLL_ONE flag.

Scroll on focus

Imagine that there are a lot of Widgets in a group that are on a scrollable Widget. Pressing the "Tab" button moves focus to the next Widget but it might be outside the visible area of the scrollable Widget. If the "scroll on focus" feature is enabled LVGL will automatically scroll Widgets to bring the child Widget with focus into view. The scrolling happens recursively therefore even nested scrollable Widgets are handled properly. The Widget will be scrolled into view even if it is on a different page of a tabview.

Scrolling Programmatically

The following API functions allow programmatic scrolling of Widgets:

  • lv_obj_scroll_by(widget, x, y, LV_ANIM_ON/OFF) scroll by x and y values
  • lv_obj_scroll_to(widget, x, y, LV_ANIM_ON/OFF) scroll to bring the given coordinate to the top left corner
  • lv_obj_scroll_to_x(widget, x, LV_ANIM_ON/OFF) scroll to bring the given coordinate to the left side
  • lv_obj_scroll_to_y(widget, y, LV_ANIM_ON/OFF) scroll to bring the given coordinate to the top side

From time to time you may need to retrieve the scroll position of a scrollable Widget, either to restore it later, or to dynamically display some elements according to its current scroll position. Here is an example to illustrate how to combine scroll event and store the scroll-top position.

 
static int scroll_value = 0;

static void store_scroll_top_value_event_cb(lv_event_t* e) {
  lv_obj_t * scr = lv_event_get_target(e);
  scroll_value = lv_obj_get_scroll_top(scr);
  printf("%d pixels are scrolled above top edge of display.\n", scroll_value);
}

lv_obj_t * scr = lv_obj_create(NULL);
lv_obj_add_event_cb(scr, store_scroll_top_value_event_cb, LV_EVENT_SCROLL, NULL);

Scroll coordinates can be retrieved from different axes with these functions:

Setting scroll position can be done with these functions:

Self Size

Self size is a property of a Widget. Normally, the user shouldn't use this parameter but if a custom widget is created it might be useful.

In short, self size establishes the size of a Widget's content. To understand it better take the example of a table. Let's say it has 10 rows each with 50 px height. So the total height of the content is 500 px. In other words the "self height" is 500 px. If the user sets only 200 px height for the table LVGL will see that the self size is larger and make the table scrollable.

This means not only the children can make a Widget scrollable but a larger self size will as well.

LVGL uses the LV_EVENT_GET_SELF_SIZE event to get the self size of a Widget. Here is an example to see how to handle the event:

 
if(event_code == LV_EVENT_GET_SELF_SIZE) {
    lv_point_t * p = lv_event_get_param(e);

    /* If x or y < 0 then it doesn't need to be calculated now. */
    if(p->x >= 0) {
        p->x = 200; /* Set or calculate self width. */
    }

    if(p->y >= 0) {
        p->y = 50;  /* Set or calculate self height. */
    }
}

Examples

Nested scrolling

lv_example_scroll_1.c

Snapping

lv_example_scroll_2.c

Floating button

lv_example_scroll_3.c

Styling the scrollbars

lv_example_scroll_4.c

Right to left scrolling

lv_example_scroll_5.c

Translate on scroll

lv_example_scroll_6.c

Infinite scrolling

lv_example_scroll_7.c

Circular scrolling

lv_example_scroll_8.c

Scrolling Properties

lv_example_scroll_9.c

API

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OverviewScrollbarModeStylingScrolling EventsFeatures of ScrollingScrollableScroll chainingScroll momentumElastic scrollSnappingScroll oneScroll on focusScrolling ProgrammaticallySelf SizeExamplesNested scrollingSnappingFloating buttonStyling the scrollbarsRight to left scrollingTranslate on scrollInfinite scrollingCircular scrollingScrolling PropertiesAPI