Flex

Overview

The Flexbox (or Flex for short) is a subset of CSS Flexbox.

It can arrange items into rows or columns (tracks), handle wrapping, adjust the spacing between the items and tracks, handle grow to make the item(s) fill the remaining space with respect to min/max width and height.

To make an object flex container call lv_obj_set_layout(obj, LV_LAYOUT_FLEX).

Note that the flex layout feature of LVGL needs to be globally enabled with LV_USE_FLEX in lv_conf.h.

Terms

  • tracks: the rows or columns

  • main direction: row or column, the direction in which the items are placed

  • cross direction: perpendicular to the main direction

  • wrap: if there is no more space in the track a new track is started

  • grow: if set on an item it will grow to fill the remaining space on the track. The available space will be distributed among items respective to their grow value (larger value means more space)

  • gap: the space between the rows and columns or the items on a track

Simple interface

With the following functions you can set a Flex layout on any parent.

Flex flow

lv_obj_set_flex_flow(obj, flex_flow)

The possible values for flex_flow are:

  • LV_FLEX_FLOW_ROW: Place the children in a row without wrapping

  • LV_FLEX_FLOW_COLUMN: Place the children in a column without wrapping

  • LV_FLEX_FLOW_ROW_WRAP: Place the children in a row with wrapping

  • LV_FLEX_FLOW_COLUMN_WRAP: Place the children in a column with wrapping

  • LV_FLEX_FLOW_ROW_REVERSE: Place the children in a row without wrapping but in reversed order

  • LV_FLEX_FLOW_COLUMN_REVERSE: Place the children in a column without wrapping but in reversed order

  • LV_FLEX_FLOW_ROW_WRAP_REVERSE: Place the children in a row with wrapping but in reversed order

  • LV_FLEX_FLOW_COLUMN_WRAP_REVERSE: Place the children in a column with wrapping but in reversed order

Flex align

To manage the placement of the children use lv_obj_set_flex_align(obj, main_place, cross_place, track_cross_place)

  • main_place determines how to distribute the items in their track on the main axis. E.g. flush the items to the right on LV_FLEX_FLOW_ROW_WRAP. (It's called justify-content in CSS)

  • cross_place determines how to distribute the items in their track on the cross axis. E.g. if the items have different height place them to the bottom of the track. (It's called align-items in CSS)

  • track_cross_place determines how to distribute the tracks (It's called align-content in CSS)

The possible values are:

  • LV_FLEX_ALIGN_START: means left on a horizontally and top vertically. (default)

  • LV_FLEX_ALIGN_END: means right on a horizontally and bottom vertically

  • LV_FLEX_ALIGN_CENTER: simply center

  • LV_FLEX_ALIGN_SPACE_EVENLY: items are distributed so that the spacing between any two items (and the space to the edges) is equal. Does not apply to track_cross_place.

  • LV_FLEX_ALIGN_SPACE_AROUND: items are evenly distributed in the track with equal space around them. Note that visually the spaces aren't equal, since all the items have equal space on both sides. The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies. Not applies to track_cross_place.

  • LV_FLEX_ALIGN_SPACE_BETWEEN: items are evenly distributed in the track: first item is on the start line, last item on the end line. Not applies to track_cross_place.

Flex grow

Flex grow can be used to make one or more children fill the available space on the track. When more children have grow parameters, the available space will be distributed proportionally to the grow values. For example, there is 400 px remaining space and 4 objects with grow: - A with grow = 1 - B with grow = 1 - C with grow = 2

A and B will have 100 px size, and C will have 200 px size.

Flex grow can be set on a child with lv_obj_set_flex_grow(child, value). value needs to be > 1 or 0 to disable grow on the child.

Style interface

All the Flex-related values are style properties under the hood and you can use them similarly to any other style property. The following flex related style properties exist:

  • FLEX_FLOW

  • FLEX_MAIN_PLACE

  • FLEX_CROSS_PLACE

  • FLEX_TRACK_PLACE

  • FLEX_GROW

Internal padding

To modify the minimum space flexbox inserts between objects, the following properties can be set on the flex container style:

  • pad_row Sets the padding between the rows.

  • pad_column Sets the padding between the columns.

These can for example be used if you don't want any padding between your objects: lv_style_set_pad_column(&row_container_style, 0)

Other features

RTL

If the base direction of the container is set the LV_BASE_DIR_RTL the meaning of LV_FLEX_ALIGN_START and LV_FLEX_ALIGN_END is swapped on ROW layouts. I.e. START will mean right.

The items on ROW layouts, and tracks of COLUMN layouts will be placed from right to left.

New track

You can force Flex to put an item into a new line with lv_obj_add_flag(child, LV_OBJ_FLAG_FLEX_IN_NEW_TRACK).

Example

A simple row and a column layout with flexbox

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

/**
 * A simple row and a column layout with flexbox
 */
void lv_example_flex_1(void)
{
    /*Create a container with ROW flex direction*/
    lv_obj_t * cont_row = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont_row, 300, 75);
    lv_obj_align(cont_row, LV_ALIGN_TOP_MID, 0, 5);
    lv_obj_set_flex_flow(cont_row, LV_FLEX_FLOW_ROW);

    /*Create a container with COLUMN flex direction*/
    lv_obj_t * cont_col = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont_col, 200, 150);
    lv_obj_align_to(cont_col, cont_row, LV_ALIGN_OUT_BOTTOM_MID, 0, 5);
    lv_obj_set_flex_flow(cont_col, LV_FLEX_FLOW_COLUMN);

    uint32_t i;
    for(i = 0; i < 10; i++) {
        lv_obj_t * obj;
        lv_obj_t * label;

        /*Add items to the row*/
        obj = lv_button_create(cont_row);
        lv_obj_set_size(obj, 100, LV_PCT(100));

        label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "Item: %"LV_PRIu32"", i);
        lv_obj_center(label);

        /*Add items to the column*/
        obj = lv_button_create(cont_col);
        lv_obj_set_size(obj, LV_PCT(100), LV_SIZE_CONTENT);

        label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "Item: %"LV_PRIu32, i);
        lv_obj_center(label);
    }
}

#endif

MicroPython code  

 View on GitHub MicroPython Simulator
#
# A simple row and a column layout with flexbox
#

# Create a container with ROW flex direction
cont_row = lv.obj(lv.screen_active())
cont_row.set_size(300, 75)
cont_row.align(lv.ALIGN.TOP_MID, 0, 5)
cont_row.set_flex_flow(lv.FLEX_FLOW.ROW)

# Create a container with COLUMN flex direction
cont_col = lv.obj(lv.screen_active())
cont_col.set_size(200, 150)
cont_col.align_to(cont_row, lv.ALIGN.OUT_BOTTOM_MID, 0, 5)
cont_col.set_flex_flow(lv.FLEX_FLOW.COLUMN)

for i in range(10):
    # Add items to the row
    obj = lv.button(cont_row)
    obj.set_size(100, lv.pct(100))

    label = lv.label(obj)
    label.set_text("Item: {:d}".format(i))
    label.center()

    # Add items to the column
    obj = lv.button(cont_col)
    obj.set_size(lv.pct(100), lv.SIZE_CONTENT)

    label = lv.label(obj)
    label.set_text("Item: {:d}".format(i))
    label.center()

Arrange items in rows with wrap and even spacing

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

/**
 * Arrange items in rows with wrap and place the items to get even space around them.
 */
void lv_example_flex_2(void)
{
    static lv_style_t style;
    lv_style_init(&style);
    lv_style_set_flex_flow(&style, LV_FLEX_FLOW_ROW_WRAP);
    lv_style_set_flex_main_place(&style, LV_FLEX_ALIGN_SPACE_EVENLY);
    lv_style_set_layout(&style, LV_LAYOUT_FLEX);

    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 300, 220);
    lv_obj_center(cont);
    lv_obj_add_style(cont, &style, 0);

    uint32_t i;
    for(i = 0; i < 8; i++) {
        lv_obj_t * obj = lv_obj_create(cont);
        lv_obj_set_size(obj, 70, LV_SIZE_CONTENT);
        lv_obj_add_flag(obj, LV_OBJ_FLAG_CHECKABLE);

        lv_obj_t * label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "%"LV_PRIu32, i);
        lv_obj_center(label);
    }
}

#endif

MicroPython code  

 View on GitHub MicroPython Simulator
#
# Arrange items in rows with wrap and place the items to get even space around them.
#
style = lv.style_t()
style.init()
style.set_flex_flow(lv.FLEX_FLOW.ROW_WRAP)
style.set_flex_main_place(lv.FLEX_ALIGN.SPACE_EVENLY)
style.set_layout(lv.LAYOUT.FLEX)

cont = lv.obj(lv.screen_active())
cont.set_size(300, 220)
cont.center()
cont.add_style(style, 0)

for i in range(8):
    obj = lv.obj(cont)
    obj.set_size(70, lv.SIZE_CONTENT)

    label = lv.label(obj)
    label.set_text("{:d}".format(i))
    label.center()

Demonstrate flex grow

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

/**
 * Demonstrate flex grow.
 */
void lv_example_flex_3(void)
{
    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 300, 220);
    lv_obj_center(cont);
    lv_obj_set_flex_flow(cont, LV_FLEX_FLOW_ROW);

    lv_obj_t * obj;
    obj = lv_obj_create(cont);
    lv_obj_set_size(obj, 40, 40);           /*Fix size*/

    obj = lv_obj_create(cont);
    lv_obj_set_height(obj, 40);
    lv_obj_set_flex_grow(obj, 1);           /*1 portion from the free space*/

    obj = lv_obj_create(cont);
    lv_obj_set_height(obj, 40);
    lv_obj_set_flex_grow(obj, 2);           /*2 portion from the free space*/

    obj = lv_obj_create(cont);
    lv_obj_set_size(obj, 40, 40);           /*Fix size. It is flushed to the right by the "grow" items*/
}

#endif

MicroPython code  

 View on GitHub MicroPython Simulator
#
# Demonstrate flex grow.
#

cont = lv.obj(lv.screen_active())
cont.set_size(300, 220)
cont.center()
cont.set_flex_flow(lv.FLEX_FLOW.ROW)

obj = lv.obj(cont)
obj.set_size(40, 40)           # Fix size

obj = lv.obj(cont)
obj.set_height(40)
obj.set_flex_grow(1)           # 1 portion from the free space

obj = lv.obj(cont)
obj.set_height(40)
obj.set_flex_grow(2)           # 2 portion from the free space

obj = lv.obj(cont)
obj.set_size(40, 40)           # Fix size. It is flushed to the right by the "grow" items

Demonstrate flex grow.

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

/**
 * Reverse the order of flex items
 */
void lv_example_flex_4(void)
{

    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 300, 220);
    lv_obj_center(cont);
    lv_obj_set_flex_flow(cont, LV_FLEX_FLOW_COLUMN_REVERSE);

    uint32_t i;
    for(i = 0; i < 6; i++) {
        lv_obj_t * obj = lv_obj_create(cont);
        lv_obj_set_size(obj, 100, 50);

        lv_obj_t * label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "Item: %"LV_PRIu32, i);
        lv_obj_center(label);
    }
}

#endif

MicroPython code  

 View on GitHub MicroPython Simulator
#
# Reverse the order of flex items
#
cont = lv.obj(lv.screen_active())
cont.set_size(300, 220)
cont.center()
cont.set_flex_flow(lv.FLEX_FLOW.COLUMN_REVERSE)

for i  in range(6):
    obj = lv.obj(cont)
    obj.set_size(100, 50)

    label = lv.label(obj)
    label.set_text("Item: " + str(i))
    label.center()

Demonstrate column and row gap style properties

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

static void row_gap_anim(void * obj, int32_t v)
{
    lv_obj_set_style_pad_row(obj, v, 0);
}

static void column_gap_anim(void * obj, int32_t v)
{
    lv_obj_set_style_pad_column(obj, v, 0);
}

/**
 * Demonstrate the effect of column and row gap style properties
 */
void lv_example_flex_5(void)
{
    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 300, 220);
    lv_obj_center(cont);
    lv_obj_set_flex_flow(cont, LV_FLEX_FLOW_ROW_WRAP);

    uint32_t i;
    for(i = 0; i < 9; i++) {
        lv_obj_t * obj = lv_obj_create(cont);
        lv_obj_set_size(obj, 70, LV_SIZE_CONTENT);

        lv_obj_t * label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "%"LV_PRIu32, i);
        lv_obj_center(label);
    }

    lv_anim_t a;
    lv_anim_init(&a);
    lv_anim_set_var(&a, cont);
    lv_anim_set_values(&a, 0, 10);
    lv_anim_set_repeat_count(&a, LV_ANIM_REPEAT_INFINITE);

    lv_anim_set_exec_cb(&a, row_gap_anim);
    lv_anim_set_time(&a, 500);
    lv_anim_set_playback_time(&a, 500);
    lv_anim_start(&a);

    lv_anim_set_exec_cb(&a, column_gap_anim);
    lv_anim_set_time(&a, 3000);
    lv_anim_set_playback_time(&a, 3000);
    lv_anim_start(&a);
}

#endif

MicroPython code  

 View on GitHub MicroPython Simulator
def row_gap_anim(obj, v):
    obj.set_style_pad_row(v, 0)


def column_gap_anim(obj, v):
    obj.set_style_pad_column(v, 0)

#
# Demonstrate the effect of column and row gap style properties
#

cont = lv.obj(lv.screen_active())
cont.set_size(300, 220)
cont.center()
cont.set_flex_flow(lv.FLEX_FLOW.ROW_WRAP)

for i in range(9):
    obj = lv.obj(cont)
    obj.set_size(70, lv.SIZE_CONTENT)

    label = lv.label(obj)
    label.set_text(str(i))
    label.center()

a_row = lv.anim_t()
a_row.init()
a_row.set_var(cont)
a_row.set_values(0, 10)
a_row.set_repeat_count(lv.ANIM_REPEAT_INFINITE)

a_row.set_time(500)
a_row.set_playback_time(500)
a_row.set_custom_exec_cb(lambda a,val: row_gap_anim(cont,val))
lv.anim_t.start(a_row)

a_col = lv.anim_t()
a_col.init()
a_col.set_var(cont)
a_col.set_values(0, 10)
a_col.set_repeat_count(lv.ANIM_REPEAT_INFINITE)

a_col.set_time(3000)
a_col.set_playback_time(3000)
a_col.set_custom_exec_cb(lambda a,val: column_gap_anim(cont,val))

lv.anim_t.start(a_col)

RTL base direction changes order of the items

C code  

 View on GitHub
#include "../../lv_examples.h"
#if LV_USE_FLEX && LV_BUILD_EXAMPLES

/**
 * RTL base direction changes order of the items.
 * Also demonstrate how horizontal scrolling works with RTL.
 */
void lv_example_flex_6(void)
{
    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_style_base_dir(cont, LV_BASE_DIR_RTL, 0);
    lv_obj_set_size(cont, 300, 220);
    lv_obj_center(cont);
    lv_obj_set_flex_flow(cont, LV_FLEX_FLOW_ROW_WRAP);

    uint32_t i;
    for(i = 0; i < 20; i++) {
        lv_obj_t * obj = lv_obj_create(cont);
        lv_obj_set_size(obj, 70, LV_SIZE_CONTENT);

        lv_obj_t * label = lv_label_create(obj);
        lv_label_set_text_fmt(label, "%"LV_PRIu32, i);
        lv_obj_center(label);
    }
}
#endif

MicroPython code  

 View on GitHub MicroPython Simulator
#
# RTL base direction changes order of the items.
# Also demonstrate how horizontal scrolling works with RTL.
#

cont = lv.obj(lv.screen_active())
cont.set_style_base_dir(lv.BASE_DIR.RTL,0)
cont.set_size(300, 220)
cont.center()
cont.set_flex_flow(lv.FLEX_FLOW.ROW_WRAP)

for i in range(20):
    obj = lv.obj(cont)
    obj.set_size(70, lv.SIZE_CONTENT)

    label = lv.label(obj)
    label.set_text(str(i))
    label.center()

API

lv_flex.h